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  • NEWS Materials Today Volume 18, Number 6 July/August 2015controlled. The resulting coating varied

    from a few atomic layers thick, to complex,

    interconnected nanopillars of carbon

    which increased the specific surface area

    of the material by up to 26,000 times.

    The inhomogeneity of stainless steels

    microstructure was found to produce a

    graphene coating that was not highly-

    crystalline. In addition, the wettability of

    the material was also studied, and it was

    found that when the density of carbon nano-

    pillars was highest, the coated steel was

    super-hydrophobic. The material also dis-

    played its highest corrosion resistance to

    synthetic seawater at this point, suggesting

    that it was the presence of graphene that

    increased its corrosion resistance, without

    compromising the properties or structure

    of the native stainless steel material.

    The team expect potential applications of

    their coated-steel to include thermal

    exchangers, molecular separation systems

    and bio-compatible materials.

    Laurie Winkless

    Magnesium boosts artificial bone performanceMagnesium plays an important role in the

    body, maintaining bone health and medi-

    ating cell function, so it should come as no

    surprise that this biocompatible, biodegrad-

    able, low-cost, and environmentally friend-

    ly material also boosts the performance of

    artificial bone composites. Until now there

    has been little examination of the effects of

    Mg on tissue engineered replacement bone

    scaffolds. But Thomas J. Webster and his

    team at Northeastern University have

    found that simply adding MgO nanoparti-

    cles to polymer composite scaffolds helps

    bone-forming cells stick [Hickey, et al., Acta

    Biomater. (2014), doi:10.1016/j.actbio.

    2014.12.004].

    Biomaterial scaffolds that support the re-

    generation of damaged bone tissue using

    the patients own cells and then degrade to

    leave just the new tissue are increasingly in

    demand as existing implant materials are

    invasive to install and can lead to long-term

    health problems.

    Instead of the current practice of

    implanting permanent materials to replace

    [orthopedic] tissues, we believe that biode-

    gradable scaffolds can be loaded with the

    patients own cells and implanted into

    the affected region, explains lead author

    Daniel J. Hickey. The scaffold degrades at

    the same rate that the loaded cells and

    surrounding tissues fill the void by generat-

    ing their own tissue matrix.

    The new scaffold comprises 20 nm MgO

    nanoparticles mixed with the biodegrad-

    able polymer poly(L-lactic acid) (PLLA)

    and hydroxyapatite (HA) nanoparticles.

    Adding MgO increases the stiffness and

    elasticity of HA-PLLA composites to match

    more closely the properties of native can-

    cellous bone the spongy tissue found in

    the core of vertebrae and at the end of long

    bones like the thigh (or femur). While vary-

    ing the size, shape, and concentration of

    the nanoparticles allows the mechanical

    properties of the scaffold to be finely tuned.

    But most significantly of all, the nanopar-

    ticles improve the adhesion and prolifera-

    tion of bone-forming cells (or osteoblasts).

    In fact, osteoblasts adhered twice as well to

    scaffolds containing MgO as to plain PLLA

    samples.

    At this point, we do not know the exact

    mechanisms that make this happen but we

    expect the MgO nanoparticles degrade to

    release Mg2+ ions, which are known to play

    a key role in the action of several cellular

    proteins and processes, says Hickey.

    The degradation of the MgO nanoparti-

    cles appears to release products that

    improve the adhesion of osteoblasts,

    while the PLLA preserves the scaffolds

    mechanical properties. The nanoparticles

    also appear to have an antibacterial affect

    and enhance the function of fibroblasts,

    the cells found in skin, tendons, and

    ligaments.

    This approach is extremely practical be-

    cause the materials are cheap and effec-

    tive, Hickey says. We do not see any

    direct disadvantages. . .but there is still a

    considerable amount of work to do before

    these composites will be ready for clinical

    application.

    Cordelia Sealy

    Nanoparticles spice up Alzheimers diagnosisDementia has a devastating effect on the

    40 million sufferers worldwide and costs

    billions in healthcare. Alzheimers disease

    makes up 60-80% of cases and, with no

    known cure or prevention, early diagnosis

    could be vital for new treatments seeking

    to halt or slow the disease before irrevoca-

    ble brain damage occurs.

    Magnetic nanoparticles combined with a

    derivative of the spice turmeric could help

    make earlier diagnoses of Alzheimers easi-

    er, according to researchers at the Chinese

    University of Hong Kong [K.K. Cheng, et al.

    Biomaterials 44 (2015) 155].

    Diagnosis relies on the detection of am-

    yloid b (Ab) plaques build-ups of Ab

    proteins secreted from brain cells, which

    are normally cleared from the brain but in

    the disease aggregate into deposits. Ab

    aggregates may contribute to neuronal

    damage and the debilitating symptoms

    of Alzheimers. Early on in the disease,

    plaques may be present long before the

    patient experiences symptoms. Current

    detection techniques rely on positron

    emission tomography (PET), which is ex-

    pensive and exposes patients to radiation.

    Alternatively, magnetic resonance imag-

    ing (MRI) is cheaper, widely available in

    hospitals, and does not involve radiation

    Scanning electron micrograph of the surface of a10% HA/10% MgO PLLA composite after

    incubation in cell growth media at 378C for7 days.

    310

  • Materials Today Volume 18, Number 6 July/August 2015 NEWSexposure. As MRI also offers better spatial

    resolution, it is more suitable for early

    intervention or mass screening. But the

    technique cannot detect plaques directly;

    a contrast agent is needed to bind onto

    amyloid plaques to make them visible in

    MRI. Magnetic nanoparticles are a common

    contrast agent, but Kwok Kin Cheng, Albert

    Chow, and Larry Baum have designed clev-

    er super-paramagnetic iron oxide (SPIO)

    nanoparticles treated with curcumin de-

    rived from turmeric that bind onto amy-

    loid plaques. Not only does curcumin bind

    naturally to both SPIO and amyloid plaques

    without the need for additional chemical

    linkers, it appears to have no toxic side

    effects.

    To help the curcumin magnetic nanopar-

    ticles (Cur-MNPs) sneak into the brain with-

    out detection by the immune system,

    Baums team coated the particles with the

    polymers polyethylene glycol-polylactic

    acid (PEG-PLA) and polyvinylpyrrolidone

    (PVP). The polymer coating prevents the

    nanoparticles from aggregating, prolongs

    the time they can circulate in the blood,

    and appears to facilitate crossing of the

    bloodbrain barrier (BBB).

    The researchers tested their novel Cur-

    MNPs in mice, demonstrating that the

    particles bind to plaques in the brain, which

    appear as dark spots in MRI.

    We showed that the particles can distin-

    guish transgenic mice with amyloid plaques

    from control mice without plaques, sug-

    gesting that the particles would be able to

    detect plaques in humans, Baum told

    Materials Today.

    He would now like to see the Cur-MNPs

    tested in humans to confirm their safety

    and compare their ability to detect amyloid

    plaques with PET imaging agents.

    Our approach opens up new ground for

    research and applications, he says.

    Cordelia Sealy

    Model reveals secret of natural materials successNatural materials like nacre shell, colla-

    gen, and spider silk possess an exceptional

    combination of strength and toughness

    thanks to a bricks-and-mortar-like struc-

    ture. During synthesis, whether by a

    mollusk or on a production line, defects

    occur by chance and accumulate in

    the material as it grows, which adversely

    affect its final mechanical properties.

    Natural materials show a remarkable abil-

    ity to withstand these defects or so-

    called size effects and preserve their

    characteristics from the micro- to the

    macroscale.

    Now researchers at Northwestern Uni-

    versity have come up with a mathematical

    model that describes and can predict

    the outstanding mechanical behavior of

    natural composites [Wei, et al., Acta Bio-

    mater. (2015), doi:10.1016/j.actbio.2015.

    01.040]. The staggered arrangement of

    strong, stiff filaments embedded in a softer

    matrix seen in natural materials seems,

    over multiple hierarchical levels, to cancel

    out the size effect and render their

    strength insensitive to scale. The new sta-

    tistical shear lag model reveals that there

    is a critical length scale at which the dom-

    inant failure mechanism switches from

    filament fracture to sliding at the interface

    between the two composite constituents,

    says Horacio D. Espinosa who led the

    work.

    Interestingly, the critical length scale

    found by the researchers using the new

    model coincides with a fundamental length

    observed in the statistical models of fiber-

    reinforced composites many years ago,

    explains Xiaoding Wei.

    [This] critical length was empirical and

    the understanding of its origin incom-

    plete, says Espinosa. [Our model] demon-

    strates, for the first time, how staggered

    composites can achieve size-independent

    material strength.

    Another unique finding emerging from

    the work is that the statistics describing the

    strength of hierarchical composites change

    from a type of distribution know as a Wei-

    bull distribution to Gaussian and back to

    Weibull at each level during material scale

    up. The new model provides a much deeper

    understanding of the structure-property

    relationships in natural biomaterials and

    the hierarchical composites inspired by

    them. Using the model to introduce defects

    in a controlled manner enables defect tol-

    erance to be designed into a material and

    size effects to be suppressed.

    (Top) The hierarchical structures of tendon, a biological material known for its high strength and

    toughness. (Bottom) Transition of the statistics of strength during material scaling up at each level in acomposite with nested hierarchical structures.

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