RESEARCH NEWS

OCTOBER 2007 | VOLUME 10 | NUMBER 1014

A nanowire biocoating for Ti

providing an effective surface

for long-lasting implants has

been developed by researchers

from the University of

Arkansas, Arkansas Cancer

Research Center, and the

University of New Mexico

[Dong et al., Chem. Mater. (2007) 19, 4454].

The researchers believe this

nanowire bioscaffold will

improve success in numerous medical procedures

including bone replacements, vascular stenting,

and drug release, as well as allowing photocatalytic

sterilization of bacteria-rich environments.

A challenging area of material science is the fabrication

of bioscaffolds for human implantation that are

physically robust but sufficiently macroporous to allow

tissue growth. Biocompatible Ti alone is tough, but

often does not contain the macropores essential for

implant longevity. Nanoscale coatings have previously

been reported but none have all the properties required

for successful and lasting

implantation.

The new coating, however,

can be structurally tailored

for its specific intended use

by varying the conditions

and time of exposure during

fabrication to control the shape

and pore size of the nanowires,

the researchers show. The

nanowire bioscaffold is not

degradable and has potential

to improve drug delivery. The nanowires form self-

assembling macroporous structures that begin by

rooting down into the Ti and grow on its surface.

The researchers also report implanting a Ti nanowire-

coated joint into mice. “We saw beautiful tissue

growth… we’ve added one more function to the

currently-in-use Ti implant,” says Z. Ryan Tian.

The Ti nanowire bioscaffold also has photocatalytic

sterilization properties that could be useful in both the

hospital and food-processing environments.

Rebecca Williams

Researchers at the University of

California, Santa Barbara (UCSB)

suggest that layered clays could be

an alternative to current hemostatic

agents [Baker et al., Chem. Mater.

(2007) 19, 4390]. These clot-

promoting wound dressings are used

routinely by the military.

QuikClot (QC), a zeolite 5A composite,

is among the most effective wound

dressings currently available. QC is

thought to prevent hemorrhaging via

local dehydration and heat generation

at the wound. Using infrared imaging

on a water model in place of blood,

the group show the heat produced on

contact of QC with the water is up to

100°C. The heat released risks thermal

injury to the patient.

Instead, the group proposes the

aluminosilicate family of layered

clays. These are functional, fully

hydrated, and with negligible heat

generation. An array of clays was

synthesized with varying chemical

and physical properties and those

with the best hemostatic performance

selected. The most effective was

kaolin and, to confirm its viability as

an alternative dressing, its clotting

ability was compared to QC using

thrombelastrography (TEG) in porcine

whole blood. They have very similar

clotting characteristics. Kaolin is

composed of sheets of Si atoms

tetrahedrally bonded to oxygen and

sheets of Al atoms octahedrally

bonded to oxygen and hydroxyl

groups. This structure gives kaolin a

negatively charged surface at plasma

pH, resulting in preferential binding

of blood coagulation factor XII to its

surface, which activates the innate

blood coagulation pathway and

therefore speeds up clotting.

“We also wish to see if we can... target

and selectively terminate internal

bleeding,” says Galen D. Stucky.

Rebecca Williams

Clays for cool clottingBIOMATERIALS

Hydrogen peroxide lights the way to diagnosis

Researchers have created a nanoparticle capable of imaging

physiological concentrations of hydrogen peroxide in mice

[Lee et al., Nat. Mater. (2007) doi: 10.1038/nmat1983]. The

group from Georgia Institute of Technology, Emery University

School of Medicine, and Atlanta VA Medical Center believe

that these nanoparticles could be used as a simple diagnostic

tool, capable of detecting any chronic inflammatory disease at

its earliest stage.

Hydrogen peroxide is a good early indicator of disease

because it is synthesized by macrophages and neutrophils in

response to infection. The low physiological concentrations

of hydrogen peroxide mean contrast agents for it are a

challenging area of research. To date, boronate-based

fluorescent probes are the only agents able to detect

hydrogen peroxide at physiological concentrations but the

tissue penetration they provide is too low to be useful for in

vivo study.

Instead researchers devised peroxalate nanoparticles that

have no such limitation and can image hydrogen peroxide

at unprecedented levels of specificity and sensitivity.

“These nanoparticles are incredibly sensitive so you can

detect nanomolar concentrations of hydrogen peroxide.

That’s important because researchers aren’t yet certain

what amounts of hydrogen peroxide are present in various

diseases,” explains Niren Murthy from Georgia Tech. The

nanoparticles consist of a fluorescent dye surrounded

by peroxalate esters and detect hydrogen peroxide via a

two-step chemiluminescent reaction. First, the hydrogen

peroxide diffuses into the nanoparticle reacting with the ester

groups generating a high-energy dioxetanedione inside the

nanoparticle. Then the dioxetanedione chemically excites the

fluorescent dye resulting in photon emission detectable with a

simple photon-counting scan.

High wavelength emissions, above 600 nm, are suitable for

deep tissue imaging. By varying the type of encapsulated

fluorescent dye, the emission wavelength can be tuned for

different depths of imaging. The chemiluminescent emission

intensity has a half life of 25 mins at 10 µM hydrogen

peroxide concentration so, according to the group, is suitable

for in vivo imaging. The researchers injected the peroxalate

nanoparticles, thought to be completely nontoxic, to image

both exogeneous- and endogeneous-derived hydrogen

peroxide in mice with encouraging results.

The overproduction of hydrogen peroxide is implicated in the

onset of many inflammatory diseases including liver hepatitis,

atherosclerosis and, chronic obstructive pulmonary disease.

The ability to image hydrogen peroxide in vivo using these

novel nanoparticles could some day lead to early diagnosis of

chronic inflammatory disease.

Rebecca Williams

BIOMATERIALS

Scanning electron micrograph of nanowire

scaffolds grown on Ti mesh from 1.0 M NaOH

at 240°C. (© 2007 ACS.)

Longer implants with nanowire coatsBIOMATERIALS