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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