Self Assembling

download Self Assembling

of 13

Transcript of Self Assembling

  • 8/3/2019 Self Assembling

    1/13

    DOI: 10.1002/asia.201000592

    Self-Assembled Gels for Biomedical Applications

    Warren Ty Truong,*[a] Yingying Su,[b] Joris T. Meijer,[a] Pall Thordarson,[a] andFilip Braet[b]

    30 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3042

    FOCUS REVIEWS

    http://www.interscience.wiley.com/
  • 8/3/2019 Self Assembling

    2/13

    Abstract: Natural and synthetic gel-like materials have

    featured heavily in the development of biomaterials for

    wound healing and other tissue-engineering purposes.

    More recently, molecular gels have been designed and

    tailored for the same purpose. When mixed with, or con-

    jugated to therapeutic drugs or bioactive molecules, thesematerials hold great promise for treating/curing life-

    threatening and degenerative diseases, such as cancer, os-

    teoarthritis, and neural injuries. This focus review explores

    the latest advances in this field and concentrates on self-

    assembled gels formed under aqueous conditions (i.e.,

    self-assembled hydrogels), and critically compares their

    performance within different biomedical applications, in-

    cluding three-dimensional cell-culture studies, drug deliv-

    ery, and tissue engineering. Although stability and toxicity

    issues still need to be addressed in more detail, it is clear

    from the work reviewed here that self-assembled gelshave a bright future as novel biomaterials.

    Keywords: biomaterials gels medicinal chemistry self-

    assembly solgel processes

    Introduction

    Since antiquity, humanity has used materials to replace parts

    of the body. Initially, naturally available materials like wood

    were used. With advances in modern technology, these ma-terials were superseded by synthetic polymers, ceramics, and

    metal alloys, which provided better performance, increased

    functionality, and enhanced reproducibility (in terms of

    their properties) than their naturally derived components.[1]

    The field of biomaterials has progressed from crude sutures

    constructed from plant or animal gut, to the more-advanced

    sutures used these days for wound closure, to the advent of

    contact lenses and drug-infused wafers[2] that are implanted

    within the body.

    Currently, there is a pressing need for better biomimetic

    materials, especially those that can mimic the extracellular

    matrix (ECM), which itself is a biological gel-like material.[3]

    The interest in ECM biomimetic materials arises from the

    fact that, if designed correctly, ECM biomimetic materials

    could, for instance, stimulate a particular type of cell growth

    or differentiation of stem cells. To fully realize the potential

    that any ECM biomimetic material may have requires a

    proper understanding of their structurefunction relation-

    ship. For self-assembled gels, this will always be difficult

    without a proper initial understanding of how these struc-

    tures are formed in the first place.

    Over the years, natural materials, such as reconstituted

    collagen, chitosan,[4] and other naturally derived polymeric

    gels, have featured heavily in the development of new bio-

    materials for wound healing and other tissue engineering.

    [5]

    On the basis of successful outcomes of these classes of natu-

    rally based polymeric gels, numerous chemically engineered

    polymeric gels have been designed and tailored for the same

    purpose in recent times.[6]

    More recently, self-assembling gels (Figure 1) made from

    low-molecular-mass organic gelators (LMOGs) have

    sparked interest in the field of biomaterials because of theirlow immunogenicity and cytotoxicity to tissues and cells.[7]

    Self-assembled gels (also known as supramolecular or physi-

    cal gels) comprised of LMOGs would theoretically be better

    suited to the field of biomaterials relative to polymer bioma-

    terials because their scaffold of nanofiber networks are on

    the same order of magnitude as found in the ECM, thereby

    providing a pseudo in-vivo environment for cell migration,

    growth, and differentiation. Furthermore, many small-mole-

    cule gelators (LMOGs), being derived from biocompatible

    components and held together by noncovalent forces, de-

    grade more-easily than the more prevalent polymer gels.[8]

    [a] W. T. Truong, Dr. J. T. Meijer, Dr. P. Thordarson

    School of Chemistry

    The University of New South Wales

    NSW 2052 (Australia)

    [b] Y. Su, Prof. F. Braet

    Australian Key Centre for Microscopy and Microanalysis

    The University to Sydney

    NSW 2006 (Australia)

    Fax: (+61)2-9351-7682

    Figure 1. Gels can be either made up of polymeric (below the horizontal

    line) or self-assembled gel fibers (above the horizontal line). The solvent

    that makes up the majority of the gels (by weight) can either be water

    (left: hydrogels) or organic solvent (right: organogels). Self-assembled

    gels are also known as molecular or physical gels.

    Chem. Asian J. 2011, 6, 3042 2011 Wi ley-VC H Verl ag G mbH & C o. KGaA, Weinheim www.chemasianj.org 31

    http://www.chemasianj.org/
  • 8/3/2019 Self Assembling

    3/13

    It is worth mentioning here that self-assembled fibrillar

    structures are also found in nature, and although they do

    not form true self-assembled gels, their functional properties

    do arise from their fibrillar nature rather than their building

    blocks per se. The most recognizable examples of this kind

    are the b-amyloids that are associated with diseases such as

    Alzheimers disease.

    [9]

    A few natural peptide and proteinhormones self-assemble into non-disease-related aggregates

    near or within their storage sites.[10]

    Finally, it is important to note that self-assembled gels are

    dynamic in nature, akin to naturally occurring self-assem-

    bled systems, such actin filaments and the above-mentioned

    b-amyloids. The dynamic nature of self-assembled gels

    allows them to adapt better to their environments and the

    changes in their surroundings, including inside living tissue.

    It is quite probable that the dynamic nature of self-assem-

    bled gels is the underlying explanation for the apparent ad-

    vantage that they seem to have over conventional polymer-

    based gels in applications such as a tissue engineering and

    drug delivery.

    What Are Gels?

    Although somewhat difficult to define in an exact manner,

    gels are typically considered as a two-component system

    comprised of a three-dimensional matrix of entangled fibers

    that encapsulate a solvent by means of capillary forces and

    surface tension (Figure 1).[11]

    Abstract in Chinese:

    Warren Ty Truong was born in Sydney,

    Australia in 1987. He received his BSc in

    Nanotechnology from the University of

    New South Wales in 2009. Currently, he is

    pursuing his PhD under the supervision

    of Pall Thordarson. His interests include

    self-assembling systems for biomedical

    applications and reading graphic novels,

    in particular anything with Batman.

    Yingying Su obtained her BSc in Bio-

    chemistry & Biotechnology in 2004 at theSun Yat-Sen University (Guangzhou,

    China). Soon after, Su started her training

    as a master in Applied Sciences and at-

    tained a major in Molecular Biotech-

    nology in 2006 at the University of

    Sydney (Australia). Currently, she works

    as a PhD student together with Filip

    Braet and Pall Thordarson on the appli-

    cation of biomodified hydrogels as a ther-

    apeutic means to treat colon cancer.

    Joris T. Meijer received his masters

    degree from Utrecht University. He ob-

    tained a PhD from Radboud University

    Nijmegen working with professor Jan

    C. M. van Hest. After postdoctoral work

    at the Radboud University he joined the

    group of Pall Thordarson at the Universi-

    ty of New South Wales. His current inter-

    ests include peptide synthesis, self-assem-

    bled systems, and biomaterials.

    Pall Thordarson obtained his BSc from

    the University of Iceland in 1996 and aPhD from The University of Sydney in

    2001, followed by a Marie Curie Fellow-

    ship at the University of Nijmegen, The

    Netherlands. He returned to Australia in

    2003 and obtained an ARC Australian

    Research Fellow at The University of

    Sydney in 2006. He was appointed a

    Senior Lecturer at the University of NSW

    in 2007 where he leads a research group

    of ten people working on light-activated

    bioconjugates for controlling enzyme ac-

    tivity and self-assembled gels for drug de-

    livery.

    Filip Braet received his postgraduatetraining in clinical chemistry and biomed-

    icine and obtained his PhD in 1999 at the

    Free University of Brussels (Belgium).

    Currently, he holds the positions of Asso-

    ciate Professor, Biomedical Scientist, and

    Deputy Director in the Australian Centre

    for Microscopy & Microanalysis at the

    University of Sydney. His research inter-

    ests include the application of correlative

    biomolecular microscopy techniques for

    exploring structurefunction relationships

    in biological systems, particularly in the

    areas of nanobiology, chemical biology,

    structural biology, and cancer biology.

    32 www.chemasianj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3042

    FOCUS REVIEWSW. T. Truong et al.

  • 8/3/2019 Self Assembling

    4/13

    Chemical and Physical Definitions

    In most gels, it is the solvent that makes up the majority (by

    weight) of the gel. Self-assembled gels typically consist of

    0.110% w/w of the gelator, whereas in polymeric gels the

    weight percentage of the covalent polymer tends to be

    higher. If the solvent is organic, then the gel is known as anorganogel (Figure 1, right), whereas if the solvent is water,

    then the gel is a hydrogel (Figure 1, left). Because of their

    high water content, hydrogels offer excellent biocompatibil-

    ity. Furthermore, owing to their ability to adjust to any

    shape required of them and their inherent mechanical stiff-

    ness, gels are different to injectable solutions. Consequently,

    the focus here will naturally be on hydrogels, although some

    interesting medical applications of organogels have been re-

    ported. For example, Leroux and co-workers have shown

    that organogels formed from N-stearoyl-l-alanine methyl

    ester 1 in refined safflower oil can be used for the controlled

    delivery of rivastigmine, which is used to treat Alzheimers

    disease.[12]

    Our recent cytotoxicty studies on the related fattyacid amino acids LMOGs 25 showed that they are only

    moderately cytotoxic at concentrations of 0.5 mm.[13]

    Based on the bonding interactions that are involved intheir assembly, all gels can also be classified into polymeric

    (covalently bonded, high molecular weight (MW)) and self-

    assembled (or supramolecular) gels from low MW building

    blocks. The nomenclature within the field is still evolving, so

    sometimes it is difficult to discern in the literature whether

    we are dealing with polymeric or self-assembled hydrogels

    (Figure 1, top vs. bottom).

    Traditionally, gels have been polymer based, with the mo-

    nomer units of the polymers linked through covalent forces;

    this leads to mechanically strong gels. However, the gel-for-

    mation is not reversible, and as a result, degradation and re-

    moval of the gel is limited by the polymer properties. [14] Fur-

    ther, properties of polymer gels can vary considerably from

    batch to batch owing to the lack of control over the length

    and shape (branching) of the polymers useda problem

    that persists in the field of polymer chemistry despite some

    recent advances in controlled polymerization techniques.

    Self-assembled gels formed from LMOGs rely on nonco-

    valent forces between these gelator molecules to self-assem-ble into larger structures. It is the combination of these

    forces (which by themselves are relatively weak) that ena-

    bles large assemblies of the gelator molecules to interact

    and form the matrix of the gel.[8] These noncovalent forces

    are notably hydrogen bonds, electrostatic interactions, hy-

    drophobic interactions, van der Waals interactions, pp

    stacking, and water-mediated hydrogen bonds.[15] Self-assem-

    bled hydrogels are attractive because of their potential ap-

    plications in areas such as drug delivery, [16] tissue engineer-

    ing,[17] three-dimensional cell cultures,[18] and as a scaffold

    for making wires.[19]

    The Use of Traditional Polymeric Gels in Biologyand Biomedicine

    For historical reasons,[20] most work has been performed so

    far on polymeric hydrogels that have been widely studied

    for drug delivery[21] and tissue engineering. Before reviewing

    how self-assembled gels have been used in biomedical appli-

    cations, it is worth looking briefly at some representative ex-

    amples of how their polymeric counterparts have been used

    for these purposes.

    In the area of tissue engineering, Quinchia Johnson

    et al.[22] were able to restore the vocal folds in rabbits six

    months after injection with a slightly modified polymeric gel

    compound, Extracel. In brief, the extracellular matrix of the

    vocal folds was treated onsite by using an injectable gel that

    was basically composed of a mixture of a thiolated deriva-

    tive of gelatin (i.e., gelatin-3,3dithiobis(propionichydrazide)

    (DTPH)) that was covalently co-cross-linked with Carby-

    lan S by using polyethylene glycol diacrylate as the thiol-re-

    active cross-linker. This was next functionalized with hyalur-

    onic acid (HA), thus forming the basis of the HA-based hy-

    drogel for tissue-regeneration purposes. The authors found

    that the treated animals had significantly less fibrosis and

    concurrently a significant improvement of the biomechanical

    properties of the vocal areas was registered. The authorsconcluded that vocal fold scars, the cause of significant dys-

    phonias, can be minimized by the prophylactic use of chemi-

    cally engineered HA gels at the time of surgery.

    In the field of controlled drug release, Lee et al.[23] dem-

    onstrated the use of an in situ hydrogeldrug complex as an

    intradiscal drug-delivery system for the treatment of lower

    back pain (Figure 2). In this study, the polymeric compound

    Pluronic F127 together with sodium hyaluronate was used

    as a local drug-delivery system to release the anesthetic bu-

    pivacaine, which itself was encapsulated within microspheres

    (MS) of polymer (polycaprolactone and poly(vinyl alcohol)

    (PVA)). This encapsulation was to compensate for the fast

    Chem. Asian J. 2011, 6, 3042 2011 Wi ley-VC H Verl ag G mbH & C o. KGaA, Weinheim www.chemasianj.org 33

    Self-Assembled Gels

  • 8/3/2019 Self Assembling

    5/13

    release of the drug from the gel itself. In this preclinical

    study, the geldrug compound was tested for its injectability

    and onsite delivery in cadaveric intervertebral discs. This all

    was monitored by means of X-ray radiographs and corre-

    sponding drug-release profile studies. In-vitro tests showed

    that 3% (w/w) of the anaesthetic loaded in the MSs were re-leased over 42 days, thereby demonstrating a good potential

    for sustained release of bupivacaine.

    For the purpose of chemotherapy, Yi et al.[24] demonstrat-

    ed that polyethylene glycol (PEG)-based hydrogels loaded

    with the anticancer drug 5-fluorouracil (5-FU) formed an ef-

    fective formulation to combat malignant cells in tumuor-

    bearing rats and nude mice. Pharmokinetic and drug release

    studies showed that this geldrug formulation resulted in a

    drug residence time that was 6- to 14-fold higher than those

    for the free administration of 5-FU. Concurrently, the

    tumuor volume was significantly reduced in the 5-FU-

    loaded hydrogel group when compared to the free 5-FU

    drug treatment group. This is to the best of our knowledge

    the first paper that unambiguously demonstrated that hydro-

    gel-based anticancer drug complexes should be considered

    by cancer biologists and drug designers as a new alternative

    delivery approach to combat cancer onsite. The advantages

    to the patient are obvious and include sustained exposure to

    the drug at much higher local concentrations onsite upon

    application of the hydrogel, thereby decreasing unwanted

    systemic side effects that are typically inherent to traditional

    chemotherapy methods. Furthermore, the chance of disease

    recurrence is also most likely to decrease because of the sus-

    tained exposure to the anticancer drug.

    Finally, by using a melanoma in vitro cancer model, ibu-profen-releasing polymeric hydrogels (Pluronic F127) have

    been shown to be an effective formulation for the onsite de-

    livery of nonsteroidal anti-inflammatory drugs that reduce

    cancer-cell migration.[25]

    Polymeric Gels Versus Self-Assembled Gels

    There are, as already outlined earlier, fundamental differen-

    ces between polymeric and self-assembled hydrogels. Data

    continue to accumulate and show that the latter is a superior

    class of hydrogels because of the chemical and physiological

    advantages they possess. From the chemical perspective,

    these include their easily controlled gel-to-sol state reversi-

    bility (e.g., pH changes) and the fact that their chemistry

    (e.g., incorporation of functional molecules) is much easier

    to manipulate than that of polymer hydrogels.

    To some degree, polymeric and self-assembled hydrogels

    are also complementary: self-assembled hydrogels will un-doubtedly excel at relatively rapid and specific release when

    complexed to proteins or drugs, whereas polymer gels might

    be more stable in biological environments (>72 h) and

    prove better for long-term delivery.[26] On the other hand, as

    useful polymeric hydrogels are, it is difficult to control their

    exact composition and their lack of biodegradabilityin

    contrast to self-assembled hydrogelshinders in part their

    use in biomedicine.

    Self-assembled hydrogels can change their pore sizes as a

    result of their dynamic nature and thus readily reassemble

    during shrinkage/swelling processes.[27] In contrast, polymer-

    ic hydrogels have less-flexible pore sizes because their ma-

    trices are linked by covalent bonds. The ability of self-as-sembled gels to self-adjust their pore sizes makes them at-

    tractive candidates for the creation of smart matrices for

    controlled drug release.

    Self-assembling hydrogels are also readily biodegradable

    due to the weak nature of the forces that hold their supra-

    molecular structure together. This is in addition to their

    high water content and the fact that most self-assembled

    gels are formed from naturally occurring components such

    as peptides and lipids, both of which are factors that assist

    with their biodegradability. For example, Banwell et al. ra-

    tionally designed and fully characterized two-component

    self-assembling hydrogels based on complementary standard

    linear peptides such as 6 and 7 with purely helical struc-

    tures.[28] These peptides formed self-supporting hydrogels of

    >99% water content that interestingly gelled only on

    mixing the two complementary peptides. They were capable

    of sustaining both the growth and differentiation of rat adre-

    nal pheochromocytoma cells for sustained periods in culture.

    Also the versatility of their synthesis opens up the possibili-

    ty of incorporating therapeutic drugs into the gelling compo-

    nent without the need to trap the compound inside the gel

    scaffold.

    The formation of self-assembled hydrogels can also be

    triggered using external biologically occurring stimuli such

    as a phosphatase enzyme.

    [29]

    The phosphatase catalyzes thedephosphorylation of a LMOG precursor 8 to trigger the

    self-assembly of the resulting b-amino acid derivate 9, there-

    by resulting in the formation of self-assembled hydrogels,

    which exhibit excellent in vivo biostability. It is noteworthy

    that this paper also demonstrated that b-amino acid deriva-

    tives afford self-assembled hydrogels with longer biostability

    than that of the related self-assembled gels generated by de-

    phosphorylation of the a-amino acid derivative 10 to the

    LMOG 11.

    Although there already exist numerous recent reviews on

    polymeric gels,[30] this review will highlight recent important

    developments in the field of self-assembled hydrogels for

    Figure 2. Schematic for the in-situ delivery of geldrug complexes for

    pain treatment of the lower back. Reproduced by permission from John

    Wiley & Sons.[23]

    34 www.chemasianj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3042

    FOCUS REVIEWSW. T. Truong et al.

  • 8/3/2019 Self Assembling

    6/13

    biomedical applications. We will also be discussing their po-

    tential possibilities in terms of approaches to specific sophis-

    ticated and practical applications.

    Applications

    Numerous potential applications for self-assembled gels

    have been outlined in the literature; however, here the main

    focus will be on successful examples of biomedical applica-

    tions, including 3D cell-culture scaffolds, drug delivery,

    tissue engineering, and regenerative medicine.

    Three-Dimensional Cell-Culture Scaffolds

    It is well known that cells behave structurally and function-

    ally different when seeded on thin surface-coated substrates

    (i.e., 2D) versus a thick layer of polymeric molecules (i.e.,

    3D), which more closely mimics their natural environ-

    ment.[31] Biomedical researchers have become increasingly

    aware of the limitations of the long-established 2D cell cul-

    ture and over the years much attention has been paid to ar-

    tificial cell-culture substrates or scaffolds that closely mimic

    the natural ECM.[32] There is an increasing awareness that

    evaluating drug efficacy in preclinical high-throughput stud-

    ies should be preferably performed in 3D in vitro cell-cul-

    ture models instead of 2D cell cultures.[33] The 3D cell cul-

    tures can offer a much better approximation of the natural

    micro- and local environment compared to the 2D cultur-

    es.[26b] Moreover, the functional properties of cells can be

    observed and manipulated in ways that are not possible inanimal models. Surprisingly, the mentioned differences in

    cell behavior between 2D and 3D cell cultures are inde-

    pendent of whether the culture substrates are derived from

    naturally purified extracellular matrix components or com-

    ponents obtained synthetically. Therefore, hydrogels (poly-

    meric or self-assembled) hold great promise as an alterna-

    tive 3D cellular microenvironment for tissue studies.[34] Not

    only because their elastic moduli closely resembles that of

    natural tissues, but also because their composition can be

    tailored to bear the appropriate chemical, physical and bio-

    logical structures that facilitate the development of tissue-

    like, and hence organoid-type cultures in vitro.[35] The impli-

    cations of 3D cell cultures would be profound.Ideally, hydrogels need to be stable and amenable to han-

    dling under physiologically relevant conditions such as a

    temperature of 37 8C, pH value of 7.27.8, and in the pres-

    ence of the required concentrations of dissolved molecules

    and enzymes required for cell cultures. Furthermore, the

    rate of gel degradation should be controlled in such a way

    that they warrant stability for the duration of the experi-

    ment and/or application, that is, commonly between 24 and

    96 h. Finally, synthetic hydrogels and their degradation

    products should not inflict any unwanted adverse cell reac-

    tion(s) such as immunogenic responses or decreased cell via-

    bility.

    It is obvious from these considerations that self-assembled

    hydrogels offer significant advantages over polymeric hydro-

    gels. The well-defined chemical nature of self-assembled

    gels (all LMOG molecules in a self-assembled gels are the

    same) is of special note here, but batch-to-batch reproduci-

    bility problems in the making of polymer-based hydrogels

    makes their application in 3D cell culture for research pur-

    poses challenging.

    Of particular interest are the findings by Liebmann and

    colleagues,[36] who demonstrated that self-assembling 9-fluo-

    renylmethoxycarbonyl (Fmoc) dipeptides such as Fmoc-

    Phe-Phe 12 (Phe=phenylalanine) form hydrogels that can

    serve as a 3D cell-culture scaffold at the microscale (Fig-ure 3ac and Table 1). This is important not only because

    3D gels better mimic the in vivo cell and tissue growth situa-

    tion than the conventional 2D cell-culture surfaces, but also

    because the composition and matrix density of the gel can

    be fully controlled by chemical tailoring approaches. This is

    a huge advantage when compared to the commercially avail-

    able cell-culture matrices, whether naturally derived or syn-

    thesised by means of large-scale biotechnological methods.

    The authors also succeeded in generating effective patterned

    3D cultures by using scaffolds when forming the gels within

    cell-culture chambers. Applications were diverse and al-

    lowed careful modelling of specific in vivo growth patterns,

    Chem. Asian J. 2011, 6, 3042 2011 Wi ley-VC H Verl ag G mbH & C o. KGaA, Weinheim www.chemasianj.org 35

    Self-Assembled Gels

  • 8/3/2019 Self Assembling

    7/13

    thereby closely mimicking the microanatomy of the tissue of

    interest. The key to success was that cells were seeded to-

    gether in the presence of the gelling agent in the cell cham-

    ber. By subsequently initiating the hydrogel self-assembly,

    the cells were immobilized within the gel and immediate

    local perfusion of the growth medium, after the gel was

    formed through the gel scaffold allowed the viability and

    growth of cells to be maintained over time.

    The flexibility of the approach of using these small self-as-

    sembling molecules is demonstrated in a paper by Zhou and

    co-workers.[37] One key to success to the practical applica-

    tion of hydrogels is the functionalization of the highly hy-

    drated gel complex with amino acids that code for the adhe-

    sion peptide sequence RGD (Arg-Gly-Asp) or RGDS (Arg-

    Gly-Asp-Ser). This has proven be a highly valuable molecu-

    lar alteration to increase cellgel interactions.[38] These short

    amino acid sequences are well known to play a key role inmany recognition systems involved in cell-to-cell and cell-to-

    matrix adhesion.[39] A gel was created that contained not

    only Fmoc-Phe-Phe 12 but also that incorporated the cell-

    adhesion peptide RGD (Arg-Gly-Asp) as Fmoc-RGD 13.

    This mixture resulted in a three-dimensional biomimetic

    nanoscaffold, which successfully allowed the adhesion,

    spreading, and proliferation of human adult dermal fibro-

    blasts. This specific mixture was shown to form a gel at

    physiologically relevant 37 8C and a pH of 7.0.

    Interestingly, when reviewing the literature, the success to

    exposing and maintaining cells under viable conditions on

    hydrogels in culture is largely dependent on both the cell

    type used and the introduction of chemical functionalitygroups. Fibroblasts and chon-

    drocytes or other cell types of

    mesenchymal origin seem to

    be a key to success when it

    comes to in vitro biocompati-

    bility studies. This is not sur-

    prising as those cells have the

    cell-membrane receptors to in-

    teract with the surrounding

    connective tissue (e.g., ECM).

    Some of these cell types se-

    crete natural ECM compo-

    nents by themselves.

    Evidently, the addition of

    smart functional groups to the

    gel, such as a multitude of

    NH2 or OH groups, creates the

    ideal microenvironment for

    tight interactions of cell-sur-

    face receptors (e.g., integrins)

    with the gel components. For

    example, Jayawarna et al.[18,40]

    elegantly demonstrated that

    mixing different LMOGs in

    self-assembled hydrogels en-hances the compatibility of the

    gel with different cell lines

    (Table 1). They showed that Fmoc-Phe-Phe 12-based gels

    proved to be good cell substrate for various mesenchymal-

    derived cell types. When self-assembled gels were formed by

    a 1:1 mixture of Fmoc-Phe-Phe 12 and Fmoc-Ser 14, a supe-

    rior hydrogel scaffold was achieved for cell culture (Fig-

    ure 3d and e).[40]

    A similar approach in adding functional groups to a self-

    assembled gel was shown recently by Hartgerinks research

    group. They successfully fabricated a series of self-assem-

    bled gels based on the multidomain peptide 15 for cell-cul-

    Figure 3. Cell culturing of different cell types on self-assembled hydrogels. a)c) The in-situ growth of cells in

    three dimensions (3D) within a self-assembled hydrogel from Fmoc-Phe-Phe 12.[37] a) Microchamber designed

    for the in-situ polymerization of the hydrogel (fibers) together with cells in suspension (spheres). b) COS-7 cells

    immobilized in a 3D self-assembled hydrogel within a microchamber, as depicted in (a). c) Illustration of the

    3D growth of MDCK cells within the microchamber. Scale bar, 50 mm. Reprinted with permission from

    BioMed Central.[37] d)f) Illustration of the long-term (6 days) cell culture of chondrocytes (d), 3T3 fibroblasts

    (e), and human dermal fibroplasts (HDF) (f) cells on self-assembled gels from a 1:1 mixture of Fmoc-Phe-Phe

    12 and Fmoc-Ser 14.[40] In these histological light microscopy sections, the dark layer represents the cells grown

    on top of the self-assembled hydrogels (fiberlike structures). Reprinted with permission from Elsevier.[40]

    36 www.chemasianj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3042

    FOCUS REVIEWSW. T. Truong et al.

  • 8/3/2019 Self Assembling

    8/13

    ture studies (Table 1). Three analogues of 15 were synthe-

    sized; 16 includes the cell adhesion motif RGD, 17 incorpo-

    rated an MMP-2 cleavage site, and 18 combines both

    (Figure 4).[41] The combination of both the RGD and

    MMP-2 motifs in 18 resulted in the largest improvements in

    cell viability as well as marked differences in cell spreading

    and morphology. In summary, the peptide functionalization

    data discussed above highlights once more the importance

    of functional tailoring of hydrogels to warrant optimal cell

    and tissue functioning.

    Drug Delivery

    Traditional methods of drug delivery rely on the bodys own

    systemic and cellular transport mechanisms to deliver drugmolecules to their target destination. Drugs are generally

    delivered into the body through oral or intravenous routes.

    The disadvantage of these methods is that drug molecules

    can come into contact with healthy tissues, thereby causing

    major side effects that prohibit treatment. This situation is

    often the cause of chemotherapy failure when bone marrow

    cell death prevents the patient from undergoing a complete

    treatment.[21]

    Localised drug delivery, on the other hand, offers numer-

    ous advantages compared to conventional dosage forms in-

    cluding improved efficacy, reduced toxicity, and improved

    patient compliance and convenience.[21] Currently, localised

    drug-delivery systems rely

    heavily on synthetic polymers

    to carry the drugs.[21] A good

    example of this is the FDA-ap-

    proved Gliadel polymer inserts

    for the treatment of glioblasto-

    ma multiforme, an aggressiveform of brain cancer. The Glia-

    del polymer comes in the form

    of wafers that are implanted in

    resected tumor sites. A chemo-

    therapeutic carmustine

    (BCNU) is then released from

    within the wafers. The aim of

    this localized form of delivery is to prevent any cancerous

    cells that were not removed during resection from metasta-

    sizing.

    There are basically two approaches for localized delivery

    with hydrogels, be they polymeric or self-assembled. The

    first is to encapsulate a therapeutic within the voids of a hy-drogel.[42] The gel can then be topically applied, and it is

    then degradation of the gel and/or diffusion of the therapeu-

    tic that allows the drug to act at the target site.[21] The

    second method is by developing drug-based gelators, that is,

    either covalent polymers that present drug molecules as side

    chains or LMOG molecules that self-assemble into a hydro-

    gel while presenting a therapeutic effect.

    Self-assembled gels are now showing significant promise

    in the field of localized drug delivery. Inherently, gels can

    easily fit into any shape that is required; this is necessary for

    easy application and efficacy, especially in the field of local-

    ized drug delivery. Also, because self-assembled gels can be

    triggered to gelate (i.e., transition from sol to gel) by means

    of various stimuli, they offer specific advantages for local-

    ized drug delivery relative to other forms of drug-delivery

    methods. For example, drugs mixed within a solution of

    LMOG, which forms a self-assembled gel on contact with

    bodily fluids such as blood (e.g., due to the resulting pH

    changes), could be delivered topically or by injection after

    tumor resection. On gelation, the gel theoretically would

    then be held in the cavity, thereby allowing the drugs to act

    locally.[43]

    For instance, when a self-assembled gel formed from a

    peptide amphiphile 19 that binds to heparin[44] was mixed

    with diazeniumdiolate nitric oxide donors, the resulting gelsystem can release nitric oxide. It was noted that the mixing

    of the nitric oxide within the gel extended the release of the

    nitric oxide significantly to four days in vitro. This mixture

    was then applied directly to the exterior of an injured blood

    vessel (rat model) after angioplasty. As an example of this

    first approach to localized drug delivery, the system showed

    clinically promising results in the limiting of neointimal hy-

    perplasia by up to 77% compared with the controls and also

    limited inflammation in the injury site.[45]

    One prominent example of the drug-gelator approach is

    in the modification of vancomycin with a pyrene group by

    Xing and co-workers, which enabled the formation of a self-

    Table 1. Cell lines used in cell-culture experiments with self-assembled g els.

    Gelators Cell type/line Reference

    Fmoc-Phe-Phe 12 kidney fibroblast cell line (COS-

    7)

    [37]

    MadinDarby canine kidney cells

    (MDCK)

    [37]

    rat cortical astrocyte cell line

    (CTX TNA2)

    [37]

    Fmoc-Phe-Phe 12/Fmoc-Ser 14 (1:1) rat-brain astrocytes cell line (DI

    TNC1)

    [40]

    mouse fibroblasts (3T3) [40]

    human dermal fibroblasts [40]

    ABA multidomain peptide (MDP) with RGD and MMP rec-

    ognition sites 1518

    human mesenchymal stem cells

    (SHED)

    [41]

    Figure 4. The structure of the multidomain peptides 1518 made by Hart-

    gerink and co-workers to form self-assembled gels for cell-culture stud-

    ies.[41] The parent ABA-block peptide 15 forms self-assembled gels in

    water. Peptides 16 and 18 incorporate the RGD cell adhesion motif

    (light gray). Peptides 17 and 18 include a MMP-2 cleavage motif (dark

    gray) with the cleavage site shown by an arrow. See also Table 1 for de-

    tails.

    Chem. Asian J. 2011, 6, 3042 2011 Wi ley-VC H Verl ag G mbH & C o. KGaA, Weinheim www.chemasianj.org 37

    Self-Assembled Gels

  • 8/3/2019 Self Assembling

    9/13

    assembled gel from the pyrene modified vancomycin 20.

    Vancomycin is an antibiotic used in the prevention and

    treatment of infections caused by Gram-positive bacteria. In

    levels of antibiotic activity, the pyrene-modified vancomycin

    20 showed an 11-fold increase, relative to plain vancomy-

    cin.[46]

    Undoubtedly, hydrogeldrug complexes can largely con-

    tribute to the treatment of various cancers as an alternative

    means, through localized and sustained chemotherapy. In

    combination with surgery, local chemotherapeutic delivery

    using hydrogels is especially well suited to deliver the treat-

    ment to the site of a recently resected tumor. This has been

    demonstrated before in polymer-based gels.

    In work by Kim and co-workers,[47] a self-assembled gel

    for possible chemotherapy was investigated. A peptide am-

    phiphile with a matrix metalloproteinase-2 (MMP-2) 21; the

    sensitive peptide sequence formed a gel when complexation

    of the carboxylic acids in the peptide sequence with cisplatin

    (CDDP) occurred, as has been shown in similar studies.[48]

    The rationale behind using an MMP-2-sensitive peptide se-

    quence was because it is known that MMP-2 is overex-

    pressed in different kinds of invasive tumors, and it plays a

    critical role in tumor progression, angiogenesis, and metasta-sis.[49] The CDDP complex is one of the extensively used

    chemotherapeutics for the treatment of various cancers such

    as testicular cancer and glioma.[50] However, severe side ef-

    fects such as acute nephrotoxicity and neural toxicity have

    limited the clinical use of CDDP. [51] To reduce these adverse

    effects and enhance its anticancer activity, the tumor-specific

    accumulation and controlled release of CDDP at the site

    was investigated (Figure 5). As expected, CDDP release

    from the peptide amphiphile gel was triggered by the cleav-

    age of the MMP-2-sensitive sequence in the peptide amphi-

    phile and was found to be dependent on the concentration

    of the enzyme. Also, the amounts of CDDP loaded in the

    gel were found to be approximately 2.53-fold greater than

    its aqueous solubility. Although this study was preclinical, it

    demonstrates the potential of self-assembled gels for drug

    delivery.

    Another chemotherapeutic delivery system was investigat-

    ed by Gao and co-workers, who modified paclitaxel (Taxol)

    through the 2-position with a linker, self-assembling motif,

    and enzyme-cleavable group to yield 22.[52] Upon the addi-

    tion of alkanine phosphatase, dephosphorylation of 22 gives

    23, which readily forms self-assembled gels in water. Pacli-

    taxel is a notoriously highly insoluble hydrophobic anticanc-

    er drug, and the group established a new, facile method to

    convert this drug into a gel without compromising its biolog-

    ical activity. This work demonstrates the versatility of self-

    assembled gels in being the first enzyme-instructed, self-as-

    sembly, and hydrogelation of a complex, bioactive smallmolecule. It further proves that a therapeutic can act as

    both the drug-delivery vehicle and the drug itself.

    In contrast to enzyme-triggered hydrogels, there are also

    pH-triggered gels such as 24 and 25, which can form self-as-

    sembled gels that encapsulate the chemotherapeutic doxoru-

    bicin.[53] Self-assembled gels from Fmoc-Phe-Phe 12 and re-

    lated dipeptides conjugated to other bulky aromatic groups

    (such as naphthalene in the case of 26 and 27) that are trig-

    gered by pH changes have been investigated in detail by the

    groups of Xu,[54] Ulijn,[37,40] and Gazit.[55] They have been

    shown to self-assemble in the right conditions, based mainly

    on a change in pH.[56]

    Figure 5. Release profiles of CDDP from CDDP mixed with self-assem-

    bled gels from 21 at different concentrations of type IV collagenase

    (MMP-2) solution. The release study was performed with Franz diffusion

    cells at 37 8C, and the enzyme solution was prepared in phosphate buffer

    solution (PBS) containing 0.5 mm CaCl2. *All points are significantly dif-

    ferent from those of 2 mgmL1 collagenase (p

  • 8/3/2019 Self Assembling

    10/13

    Topical application of hydrogeldrug formulations at the

    site of injury/disease undoubtedly offers additional advan-

    tages of delivering the active drug compound to the specfic

    site. For instance, in a study from the Xu group, it was

    shown that the topical application of a self-assembled hydro-

    gel based on the mixture of Fmoc-Leu 28, the uranyl nitrate

    binding ligand pamidromate 29, and e-Fmoc-Lys 30 could be

    used to treat wounds on the skin of mice that had been con-

    taminated with uranyl nitrate. The treated mice recovered,

    whereas untreated mice weighed 35% less or died, presuma-

    bly from radiation damage caused by the uranyl-nitrate-con-

    taminated skin wounds.[57]

    In localised drug delivery, the rate of drug release is an

    important aspect. Self-assembled hydrogels have shown to

    be capable of having controlled release kinetics. Liang and

    co-workers did the first in vivo imaging of a self-assembledhydrogel formed from a naphthalane d-Phe dipeptide 31

    and showed that it had controlled release kinetics by using125I isotopes.[58]

    For example, Ellis-Behnke et al. demonstrated that the

    known self-assembling peptide Ac-(RADA)4-NH2 32[59]

    (RADA=Arg-Ala-Asp-Ala) forms a self-assembled gel on

    contact with bodily fluids such as blood. They reported that

    the self-assembling peptide 32 establishes a nanofiber barri-

    er (

  • 8/3/2019 Self Assembling

    11/13

    The peptide amphiphile 33 was designed to form cylindrical

    nanofibers that display to cells in the spinal cord the laminin

    epitope IKVAV (Ile-Lys-Val-Ala-Val). As a control in these

    experiments, a peptide amphiphile 34 with the nonphysio-

    logical epitope EQS (Glu-Gln-Ser) was also synthesized.

    The three-dimensional network of nanofibers constructed

    from the peptide amphiphiles incorporated the pentapeptideepitope IKVAV, which is also found in laminin, a protein

    found in the extracellular matrix. The IKVAV epitope was

    incorporated within the gel because it is known to promote

    neurite sprouting and to direct neurite growth. The ability

    to have a dense population of biologically active factors

    (IKVAV) incorporated in the self-assembled nanofibers of

    the gel from 33 presenting themselves to the NPCs was de-

    termined to be the critical factor in the observed rapid and

    selective differentiation of cells into neurons compared to

    the peptide amphiphile control 34 (Figure 6).[61]

    Following on from the work in 2004, Stupp, Tysseling-

    Mattiace, and co-workers in 2008[63] used the same peptide

    amphiphile 33 without exogenous proteins or cells as a ther-

    apy in a mouse model of spinal cord injury (SCI). When a

    liquid solution of the peptide amphiphile was injected,

    changes in ionic strength of the in vivo environment trig-

    gered self-assembly within the extracellular spaces of the

    spinal cord, thereby resulting in nanoscale gel-like struc-

    tures. In this work, in vivo treatment with the gel after SCI

    reduced astrogliosis, reduced cell death, and increased the

    number of oligodendroglia at the site of injury. Furthermore,the nanofibers promoted regeneration of both descending

    motor fibers and ascending sensory fibers through the lesion

    site. Treatment with the peptide amphiphile 33 also resulted

    in significant behavioral improvement, that is, at nine weeks,

    the control groups demonstrated no hindlimb movement,

    whereas the peptide amphiphile IKVAV epitope group had

    hindlimb movement.[63] Another example from the field of

    tissue engineering comes from the group of Kisiday et al.,[64]

    which designed a peptide 35 (KLD12) related to the above-

    mentioned RADA peptide 32. The KLD12 peptide 35

    formed a self-assembled hydrogel that was used as a scaffold

    to support chondrocyte growth and development for carti-

    lage repair. During one month of culture in vitro, chondro-cytes seeded within the hydrogel retained their morphology

    and developed a cartilage-like extracellular matrix rich in

    proteoglycans and type II collagen, which is indicative of a

    stable chondrocyte phenotype. As time progressed, the stiff-

    ness of the material increased, thereby indicating that new,

    mechanically functional cartilage was formed. The outcome

    of this experiment established the potential of a self-assem-

    bling peptide hydrogel as a tool for the synthesis and accu-

    mulation of a cartilage-like extracellular matrix for tissue re-

    generation.

    The versatility of self-assembled gels in medical applica-

    tion is neatly demonstrated in their combination with a tra-

    ditional prosthetic material for regenerative medicine. The

    regeneration or replacement of hard tissue in the body has

    proven to be a challenge due to its mechanical properties.

    One approach is the use of a metal implant to replace the

    tissue. However, these materials do not incorporate a bioac-

    tive component. A peptide amphiphile 36 formed a self-as-

    sembled gel within a porous Ti-6Al-4V bone implant. This

    hybrid material was shown to be able to mineralize with cal-

    cium phosphate and cells could be encapsulated in these hy-

    brids in a controlled manner. In vivo experiments showed

    that de novo bone is formed adjacent to and inside the PA

    Ti hybrid by 4 weeks, thus offering strong evidence of osteo-

    conduction.

    [65]

    Figure 6. Percentage of total cells that differentiated into neurons after

    1 d in nanofiber networks containing different amounts of IKVAV-PA 33

    and EQS-PA 34 (solid line) and in EQS-PA nanofiber networks to which

    different amounts of soluble IKVAV peptide were added (dashed line).

    Reprinted with permission from AAAS.[61]

    40 www.chemasianj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3042

    FOCUS REVIEWSW. T. Truong et al.

  • 8/3/2019 Self Assembling

    12/13

    In the area of tissue engineering, the previously men-

    tioned Ac-(RADA)4-NH2 32 peptide was originally reported

    by Zhang and co-workers[59] to form a self-assembled hydro-

    gel (0.51.0% w/v), which was later patented under the

    tradename PuraMatrix. It self-assembles to form macroscop-

    ic gels in aqueous solution by hydrophobic and ionic bond-

    ing due to the beta sheet structure that is encoded in itsamino acid sequence. Its mechanical properties are depen-

    dent on its initial concentration. The peptide 32 is soluble at

    low pH and osmolarity; when the conditions are changed to

    physiological pH and osmolarity, it quickly forms fibers on

    the order of 510 nm and assembles into interwoven 3D

    scaffolds. The high volume fraction (%99%) of water within

    these hydrogels and the structural resemblance of these pep-

    tides to natural collagen along with the ability to customize

    the peptide backbone have yielded favorable cell-culture re-

    sults such as the culture of osteocytes,[66] neural cells,[67] and

    chondrocytes.[64] For the application of liver-tissue engineer-

    ing, in comparison with primary hepatocytes cultured on

    collagen, primary hepatocytes cultured on the self-assem-bled hydrogel, PuraMatrix (made from 32) yielded better

    liver-specific function. This is promising in the field of liver-

    tissue-engineering applications.[68]

    Conclusion and Perspectives

    Since the start of the new millennium, a variety of self-as-

    sembled hydrogel complexes have been shown to be suc-

    cessful candidates for biomimetic materials. They have been

    shown to be a viable material for in-vitro 3D cell studies be-

    cause they closely mimic the in vivo environment for the

    cells. Furthermore, self-assembled hydrogel-based drug-de-

    livery complexes have been proven to be useful as an attrac-

    tive therapeutic alternative to the existing arsenal of drug-

    carrier systems, such as liposomes and metal-based nanopar-

    ticles, and as such can be used to cure life-threatening dis-

    eases. Finally, the materials themselves have been shown to

    be viable candidates to treat various pathological conditions.

    However, despite these promising findings, key challenges in

    the synthesis and functionalization remain before self-as-

    sembled gels can be completely implemented for therapeu-

    tic purposes in humans. This challenge goes to the heart of

    one the key problems in the areawe simply do not under-

    stand self-assembled gels.

    [69]

    A better understanding of themechanism and structure of self-assembled gels will also

    allow us to tackle some of key questions with regards to

    their applications in medicine, including how we can control

    their stability.

    There is no doubt that self-assembled gels have a bright

    future ahead as novel biomaterials; however, careful assess-

    ment on their biocompability and the possible immunogenic

    response they may inflict remains a topic of great interest.

    Furthermore, the development of innovative ways in admin-

    istrating or applying these gels in situ is a major challenge.

    Much can be expected in fabricating gel complexes that

    gelate onsite under carefully controlled conditions such as

    changes to local pH environment or temperature; further-

    more, enzyme-controlled gelation reactions or multicompo-

    nent systems are possible ways forward. Besides the produc-

    tion of these responsive hydrogels, the material should be in

    such a form that it can be easily handled within cavities and/

    or tissues. The use of an injection device that subsequently

    allows local gelation and incorporation of compounds of in-terest would be the preferred modus operandi.

    The major advancement in the near future should be

    sought for in the fabrication of smart self-assembled gel

    complexes that specifically recognize the target of interest

    (i.e., stealth-based delivery). Even more, the ideal gel com-

    plex should only release its cargo and/or stimulate the cellu-

    lar target when in the vicinity of the targeted cell population

    or tissue. Finally, in this context, the addition of a fluores-

    cent reporter molecule would be a welcomed addition. This

    would not only allow researchers to monitor the effective-

    ness of the gels in vivo but also assist the operator in apply-

    ing the gel more effectively at the site of interest by using

    fluorescence-assisted imaging technology. The inherent mod-ularity of self-assembled gels will be an important asset in

    the challenge to incorporate all these functionalities in the

    functional material.

    Acknowledgements

    The authors acknowledge the facilities, and technical and administrative

    assistance from staff of the AMMRF at the Australian Centre for Micros-

    copy and Microanalysis (ACMM), The University of Sydney. We would

    also like to thank the Australian Research Council (ARC) for a Discov-

    ery Project Grant (DP0985059) to P.T. and F.B. as well as the NSW

    Cancer Institute (08/RFG/1-29) for supporting our work and the Univer-sity of New South Wales for a Scholarship to W.T.T.

    [1] N. Huebsch, D. J. Mooney, Nature 2009, 462, 426432.

    [2] a) M. S. Lesniak, H. Brem, Nat. Rev. Drug Discovery 2004, 3, 499

    508; b) E. P. Sipos, B. Tyler, S. Piantadosi, P. C. Burger, H. Brem,

    Cancer Chemother. Pharmacol. 1997, 39, 383389.

    [3] W. D. Comper, Extracellular Matrix, Harwood Academic, Amster-

    dam, 1996.

    [4] P. Roughley, C. Hoemann, E. D esRosiers, F. Mwale, J. Antoniou, M.

    Alini, Biomaterials 2006, 27, 388396.

    [5] D. G. Wallace, J. Rosenblatt, Adv. Drug Delivery Rev. 2003, 55,

    16311649.

    [6] a) J. P. Jung, J. Z. Gasiorowski, J. H. Collier, Biopolymers 2010, 94,

    4959; b) O. D. Krishna, K. L. Kiick, Biopolymers 2010, 94, 3248.

    [7] A. R. Hirst, B. Escuder, J. F. Miravet, D. K. Smith, Angew. Chem.2008, 120, 81228139 ; Angew. Chem. Int. Ed. 2008, 47, 80028018.

    [8] L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 12011218.

    [9] I. W. Hamley, Angew. Chem. 2007, 119, 8274 8295; Angew. Chem.

    Int. Ed. 2007, 46, 81288147.

    [10] C. Keeler, M. E. Hodsdon, P. S. Dannies, J. Mol. Neurosci. 2004, 22,

    4349.

    [11] M. George, R. G. Weiss, Acc. Chem. Res. 2006, 39, 489 497.

    [12] A. Vintiloiu, M. Lafleur, G. Bastiat, J.-C. Leroux, Pharm. Res. 2008,

    25, 845 852.

    [13] L. Y. G. Lim, Y. Su, F. Braet, P. Thordarson, Aust. J. Chem. 2009, 62,

    653656.

    [14] a) J. M. Lehn, Science 2002, 295, 2400 2403 ; b) N. A. Peppas, J. Z.

    Hilt, A. Khademhosseini, R. Langer, Adv. Mater. 2006, 18, 1345

    1360.

    Chem. Asian J. 2011, 6, 3042 2011 Wi ley-VC H Verl ag G mbH & C o. KGaA, Weinheim www.chemasianj.org 41

    Self-Assembled Gels

    http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1002/adma.200501612http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1126/science.1071063http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1071/CH09211http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1007/s11095-007-9384-3http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1021/ar0500923http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1385/JMN:22:1-2:43http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/anie.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1002/ange.200700861http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1021/cr0302049http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/anie.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/ange.200800022http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21333http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1002/bip.21326http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.addr.2003.08.004http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1016/j.biomaterials.2005.06.037http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1007/s002800050588http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1038/nature08601http://dx.doi.org/10.1038/nature08601
  • 8/3/2019 Self Assembling

    13/13

    [15] S. G. Zhang, Nat. Biotechnol. 2003, 21, 11711178.

    [16] T. P. Richardson, M. C. Peters, A. B. Ennett, D. J. Mooney, Nat. Bio-

    technol. 2001, 19, 10291034.

    [17] K. Y. Lee, D. J. Mooney, Chem. Rev. 2001, 101, 18691879.

    [18] V. Jayawarna, M. Ali, T. A. Jowitt, A. E. Miller, A. Saiani, J. E.

    Gough, R. V. Ulijn, Adv. Mater. 2006, 18, 611614.

    [19] M. Reches, E. Gazit, Science 2003, 300, 625627.

    [20] a) S. D. Bruck, J. Biomed. Mater. Res. 1973, 7, 387404 ; b) M. S.

    Jhon, J. D. Andrade, J. Biomed. Mater. Res. 1973, 7, 509522.[21] K. E. Uhrich, S. M. Cannizzaro, R. S. Langer, K. M. Shakesheff,

    Chem. Rev. 1999, 99, 31813198.

    [22] S. L. Thibeault, S. A. Klemuk, X. Chen, B. H. Quinchia Johnson, J.

    Voice 2010, in press, PMID 20456912.

    [23] J. W. Lee, T. H. Lim, J. B. Park, J. Biomed. Mater. Res. Part A 2010,

    92, 378 385.

    [24] H. Yi, H. J. Cho, S. M. Cho, D. G. Lee, A. M. Abd El-Aty, S. J.

    Yoon, G. W. Bae, K. Nho, B. Kim, C. H. Lee, J. S. Kim, M. G. Bar-

    tlett, H. C. Shin, BMC Cancer 2010, 10, 211.

    [25] M. Redpath, C. M. Marques, C. Dibden, A. Waddon, R. Lalla, S.

    Macneil, Br. J. Dermatol. 2009, 161, 2533.

    [26] a) M. Hamidi, A. Azadi, P. Rafiei, Adv. Drug Delivery Rev. 2008,

    60, 16381649; b) F. Zhao, M. L. Lung, B. Xu, Chem. Soc. Rev.

    2009, 38, 883891.

    [27] S.-L. Zhou, S. Matsumoto, H.-D. Tian, H. Yamane, A. Ojida, S.Kiyonaka, I. Hamachi, Chem. Eur. J. 2005, 11, 11301136.

    [28] E. F. Banwell, E. S. Abelardo, D. J. Adams, M. A. Birchall, A. Corri-

    gan, A. M. Donald, M. Kirkland, L. C. Serpell, M. F. Butler, D. N.

    Woolfson, Nat. Mater. 2009, 8, 596600.

    [29] Z. M. Yang, G. L. Liang, M. L. Ma, Y. Gao, B. Xu, Small 2007, 3,

    558562.

    [30] J. Kopecek, J. Polym. Sci. Part A 2009, 47, 59295946.

    [31] a) W. Mueller-Klieser, Am. J. Physiol. 1997, 273, C1109C1123;

    b) R. Z. Lin, H. Y. Chang, Biotechnol. J. 2008, 3, 11721184.

    [32] a) J. L. Ifkovits, H. G. Sundararaghavan, J. A. Burdick, J. Vis. Exp.

    2009, 32, 1589; b) M. H. Kim, M. Kino-oka, M. Taya, Biotechnol.

    Adv. 2010, 28, 716.

    [33] G. D. Prestwich, Acc. Chem. Res. 2008, 41, 139148.

    [34] M. W. Tibbitt, K. S. Anseth, Biotechnol. Bioeng. 2009, 103, 655663.

    [35] M. C. Cushing, K. S. Anseth, Science 2007, 316, 11331134.

    [36] T. Liebmann, S. Rydholm, V. Akpe, H. Brismar, BMC Biotechnol.2007, 7, 88.

    [37] M. Zhou, A. M. Smith, A. K. Das, N. W. Hodson, R. F. Collins, R. V.

    Ulijn, J. E. Gough, Biomaterials 2009, 30, 25232530.

    [38] M. Colombo, A. Bianchi, Molecules 2010, 15, 178197.

    [39] C. A. Buck, A. F. Horwitz, Annu. Rev. Cell Biol. 1987, 3, 179205.

    [40] V. Jayawarna, S. M. Richardson, A. R. Hirst, N. W. Hodson, A.

    Saiani, J. E. Gough, R. V. Ulijn, Acta Biomater. 2009, 5, 934 943.

    [41] K. M. Galler, L. Aulisa, K. R. Regan, R. N. DSouza, J. D. Hartger-

    ink, J. Am. Chem. Soc. 2010, 132, 32173223.

    [42] Z. M. Yang, H. W. Gu, Y. Zhang, L. Wang, B. Xu, Chem. Commun.

    2004, 208 209.

    [43] R. G. Ellis-Behnke, Y.-X. Liang, D. K. C. Tay, P. W. F. Kau, G. E.

    Schneider, S. Zhang, W. Wu, K.-F. So, Nanomedicine 2006, 2, 207

    215.

    [44] K. Rajangam, H. A. Behanna, M. J. Hui, X. Han, J. F. Hulvat, J. W.Lonasney, S. I. Stupp, Nano Lett. 2006, 6, 20862090.

    [45] M. R. Kapadi a, L. W. Chow, N. D. Tsihlis, S. S. Ahanchi, J. W. Eng, J.

    Murar, J. Martinez, D. A. Popowich, Q. Jiang, J. A. Hrabie, J. E.

    Saavedra, L. K. Keefer, J. F. Hulvat, S. I. Stupp, M. R. Kibbe, J.

    Vasc. Surg. 2008, 47, 173 182.

    [46] B. Xing, C.-W. Yu, K.-H. Chow, P.-L. Ho, D. Fu, B. Xu, J. Am.

    Chem. Soc. 2002, 124, 14846 14847.

    [47] J.-K. Kim, J. Anderson, H.-W. Jun, M. A. Repka, S. Jo, Mol. Pharm.

    2009, 6, 978 985.

    [48] a) X. Yan, R. A. Gemeinhart, J. Controlled Release 2005, 106, 198208 ; b) J. R. Tauro, R. A. Gemeinhart, Bioconjugate Chem. 2005, 16,

    11331139 ; c) H. Ye, L. Jin, R. Hu, Z. Yi, J. Li, Y. Wu, X. Xi, Z.

    Wu, Biomaterials 2006, 27, 59585965.

    [49] a) J. D. Mott, Z. Werb, Curr. Opin. Cell Biol. 2004, 16, 558564;

    b) G. Klein, E. Vellenga, M. W. Fraiije, W. A. Kamps, E. S. de Bont,

    Crit. Rev. Oncol. Hematol. 2004, 50, 87100.

    [50] J. W. Ho, Recent Pat. Anti-Cancer Drug Discovery 2006, 1, 129134.

    [51] a) V. Pinzani, F. Bressolle, I. J. Haug, M. Galtier, J. P. Blayac, P.

    Balmes, Cancer Chemother. Pharmacol. 1994, 35, 1 9; b) D. Screnci,

    M. J. McKeage, J. Inorg. Biochem. 1999, 77, 105110.

    [52] Y. Gao, Y. Kuang, Z.-F. Guo, Z. Guo, I. J. Krauss, B. Xu, J. Am.

    Chem. Soc. 2009, 131, 13576 13577.

    [53] J. Naskar, G. Palui, A. Banerjee, J. Phys. Chem. B 2009, 113, 11787

    11792.

    [54] Z. Yang, G. Liang, M. Ma, Y. Gao, B. Xu, J. Mater. Chem. 2007, 17,850854.

    [55] A. Mahler, M. Reches, M. Rechter, S. Cohen, E. Gazit, Adv. Mater.

    2006, 18, 13651370.

    [56] C. Tang, A. M. Smith, R. F. Collins, R. V. Ulijn, A. Saiani, Langmuir

    2009, 25, 94479453.

    [57] Z. M. Yang, K. M. Xu, L. Wang, H. W. Gu, H. Wei, M. J. Zhang, B.

    Xu, Chem. Commun. 2005, 44144416.

    [58] G. L. Liang, Z. M. Yang, R. J. Zhang, L. H. Li, Y. J. Fan, Y. Kuang,

    Y. Gao, T. Wang, W. W. Lu, B. Xu, Langmuir 2009, 25, 84198422.

    [59] H. Yokoi, T. Kinoshita, S. Zhang, Proc. Natl. Acad. Sci. USA 2005,

    102, 84148419.

    [60] T. C. Holmes, Trends Biotechnol. 2002, 20, 1621.

    [61] G. A. Silva, C. Czeisler, K. L. Niece, E. Beniash, D. A. Harrington,

    J. A. Kessler, S. I. Stupp, Science 2004, 303, 13521355.

    [62] H. Okano, J. Neurosci. Res. 2002, 69, 698707.

    [63] V. M. Tysseling-Mattiace, V. Sahni, K. L. Niece, D. Birch, C. Czeis-ler, M. G. Fehlings, S. I. Stupp, J. A. Kessler, J. Neurosci. 2008, 28,

    38143823.

    [64] J. Kisiday, M. Jin, B. Kurz, H. Hung, C. Semino, S. Zhang, A. J.

    Grodzinsky, Proc. Natl. Acad. Sci. USA 2002, 99, 9996 10001.

    [65] T. D. Sargeant, M. O. Guler, S. M. Oppernheimer, A. Mata, R. L.

    Satcher, D. C. Dunand, S. I. Stupp, Biomaterials 2008, 29, 161171.

    [66] A. Horii, X. Wang, F. Gelain, S. Zhang, PloS ONE 2007, 2, e190.

    [67] F. Gelain, D. Bottai, A. Vescovi, PloS ONE 2006, 1, e119.

    [68] a) S. Wang, D. Nagrath, P. C. Chen, Tissue Eng. Part A 2008, 14,

    227236; b) N. Navarro-Alvarez, A. Soto-Gutierrez, J. D. Rivas-Ca-

    rillo, Cell Transplant 2006, 15, 921 927.

    [69] J. H. van Esch, Langmuir 2009, 25, 83928394.

    Received: August 20, 2010Published online: November 12, 2010

    42 www.chemasianj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3042

    FOCUS REVIEWSW. T. Truong et al.

    http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1186/1471-2407-10-211http://dx.doi.org/10.1186/1471-2407-10-211http://dx.doi.org/10.1186/1471-2407-10-211http://dx.doi.org/10.1186/1471-2407-10-211http://dx.doi.org/10.1186/1471-2407-10-211http://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1039/b507314fhttp://dx.doi.org/10.1039/b507314fhttp://dx.doi.org/10.1039/b507314fhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1371/journal.pone.0000190http://dx.doi.org/10.1371/journal.pone.0000190http://dx.doi.org/10.1371/journal.pone.0000190http://dx.doi.org/10.1371/journal.pone.0000190http://dx.doi.org/10.1371/journal.pone.0000190http://dx.doi.org/10.1371/journal.pone.0000119http://dx.doi.org/10.1371/journal.pone.0000119http://dx.doi.org/10.1371/journal.pone.0000119http://dx.doi.org/10.1371/journal.pone.0000119http://dx.doi.org/10.1371/journal.pone.0000119http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.1021/la901720ahttp://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.3727/000000006783981387http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1089/tea.2007.0143http://dx.doi.org/10.1371/journal.pone.0000119http://dx.doi.org/10.1371/journal.pone.0000190http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1016/j.biomaterials.2007.09.012http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1073/pnas.142309999http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1523/JNEUROSCI.0143-08.2008http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1002/jnr.10343http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1126/science.1093783http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1016/S0167-7799(01)01840-6http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1073/pnas.0407843102http://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1021/la804271dhttp://dx.doi.org/10.1039/b507314fhttp://dx.doi.org/10.1039/b507314fhttp://dx.doi.org/10.1039/b507314fhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1021/la900653qhttp://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1002/adma.200501765http://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1039/b611255bhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/jp904251jhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1021/ja904411zhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1016/S0162-0134(99)00135-Xhttp://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.1007/BF00686277http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.2174/157489206775246485http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.critrevonc.2003.09.001http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.ceb.2004.07.010http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1016/j.biomaterials.2006.08.016http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1021/bc0501303http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1016/j.jconrel.2005.05.005http://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/mp900009nhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1021/ja028539fhttp://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1016/j.jvs.2007.09.005http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1021/nl0613555http://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1039/b310574ahttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1021/ja910481thttp://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1016/j.actbio.2009.01.006http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.1146/annurev.cb.03.110187.001143http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.3390/molecules15010178http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1016/j.biomaterials.2009.01.010http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1186/1472-6750-7-88http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1126/science.1140171http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1002/bit.22361http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1021/ar7000827http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1016/j.biotechadv.2009.08.002http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/biot.200700228http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1002/smll.200700015http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1038/nmat2479http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1002/chem.200400677http://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1039/b806410phttp://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1016/j.addr.2008.08.002http://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1111/j.1365-2133.2009.09220.xhttp://dx.doi.org/10.1186/1471-2407-10-211http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1002/jbm.a.32377http://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1021/cr940351uhttp://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070604http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1002/jbm.820070503http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1126/science.1082387http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1002/adma.200501522http://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1021/cr000108xhttp://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt1101-1029http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt874http://dx.doi.org/10.1038/nbt874