S58 Oral and Poster Presentations / Journal of Biomechanics 43S1 (2010) S23–S74

limited work on the microstructure, configuration and mechanism

of deformation of hydrogels have been reported in literature.

Therefore, the design of theoretic models and finite element

simulation analysis of auxetic hydrogel is significant to predict its

mechanical behaviours and to guide the material selection, design

and processing. A novel centresymmetric honeycomb structure with

fillings in three types of shape and packing pattern is designed,

constructed and evaluated. The assumptions of the model are

made on the basis of the preparation process, the microstructure

characteristics and experimental analysis of polyvinyl alcohol

hydrogel with three-dimension porous structure, in comparison

with the classical hexagonal honeycomb configurations.

Based on the theoretical and numerical analysis, the results show

that the effective density and the Poisson’s ratio vary as functions

of the configuration parameters of a centresymmetric honeycomb

unit cell, such as the isosceles trapezoid cell short length (L2),

the base angle (q1) and circular cell inner diameter (R2). The

honeycomb structure without fillings has the negative Poisson’s

ratio regardless the changes of the configuration parameters.

The fillings in forms of different unit shapes (circular and

hexagonal) and packing orders are proposed to represent the

dispersion medium (water) in hydrogels. In the case of the circular

filling (the unit structure thickness, t, <5mm) in square packing,

the centresymmetric honeycomb structure possess the negative

Poisson’s ratio on condition that large parameters, L2, q1 and R2,

are chosen. In another case of the hexagonal close-packing filling,

the centresymmetric honeycomb structure possess the negative

Poisson’s ratio if small values of L2, q1 and R2 are defined.

Acknowledgments: This study is financially supported by National

Natural Science Foundation of China Project (Grant No. 50773004)

M-16

A New Tool to Assess Corrosion and Metal Ion Release in

Artificial Hip Joints

Y. Yan, A. Neville, J. Hesketh, D. Dowson, S. Williams, J. Fisher.

University of Leeds, UK

Concern for the use of Metal-on-Metal hip prostheses has been

expressed about the potential problems which may arise in later

years due to the increased amount of metal ions and the number

of very fine metallic wear particles generated. The release of

metal ions is electrochemical in nature (corrosion). Hip simulators

have been extensively used to study wear. To date, all the work

on hip simulators has been focused on mechanical damage –

wear. However, hip implants operate in a corrosive and biological

environment and corrosion plays a key role in the total material

degradation processes. There has been no attempt to measure the

interactions (biotribocorrosion) between corrosion and tribology

in situ in simulated body fluids using a hip simulator. There are

no reports that identify what damage is due to pure mechanical

processes and corrosion processes in a hip simulator. This abstract

describes the first instrumentation of an integrated hip joint

simulator to provide corrosion measurements under dynamic

loading and movements. It is a pioneering study to determine

damage in real time. The contribution of corrosion-related damage

to the total material degradation is quantified. The open circuit

potential results are reported to assess the corrosion regime in

the absence and presence of movement at the bearing surfaces.

The importance of these measurements is that the real damage

mechanisms can be assessed as a function of the operating cycle.

Links of lab tests with clinical data will be discussed.

M-17

Strontium Bioactive Glass Coatings for Medical Implants

N. Lotfibakhshaiesh, E. Gentleman, M.M. Stevens, R. Hill. Imperial

College London, UK

Introduction: Metallic prosthetic implants are widely used to treat

joint and skeletal injuries. However, some implants can fail because

the metal alloys used do not bond with bone. Bioactive glass (BG)

coatings may offer a solution to this problem as they form a strong

bond with living tissue and have the added benefit that their

dissolution ions stimulate cell activity. Sr can be substituted for

calcium in BGs creating a material that may combine the bone

remodelling benefits of Sr ions with the well-established bone

stimulatory action of bioactive glass. The aim of this study is to

develop Sr containing BG coatings on metallic surfaces to bond to

an implant as an effective biomaterial choice for a range of bone

regeneration therapies.

Materials and Methods: BG in the system: SiO2-MgO-Na2O-K2O-

ZnO-P2O5-CaO with 0, 10, and 50 of the Ca being replaced by Sr

were prepared by a melt-quench route. A degradation study was

carried out in simulated body fluids. Silicon (Si), Phosphorus (P),

Calcium (Ca) and Sr were checked at different time periods.

Produced glasses were combined with RPMI media. The human

osteosarcoma cell line, Saos-2, was seeded in Sr substituted

medium. Cell metabolic activity was measured using the tetrazole

MTT. BGs were coated on to the surface of Ti6AL4V coupons and

Saos-2 cells were seeded on BG coatings and viability was assessed

with a Live/Dead stain.

Results: Glass dissolution leads to a rise of all the ion concentrations

in solution. MTT activity increased in all samples with time in

culture until 14 days. Saos-2 cultured treated with Sr-substituted

BGs had higher MTT activities than controls. Live/Dead staining

showed that cells were alive on all coating materials.

Discussion and Conclusions: Substituting strontium for calcium

increases the dissolution of the glass releasing in more ions

in solution. These ions may tend to enhance the metabolic

activity of Saos-2 cells, which may lead to an increase in

mineralization. As BGs are a well-studied biomaterial system

capable of controlled ion release and are deliverable as coatings

or scaffolds substitution of strontium for calcium in bioactive

glasses may be an effective strategy for creating materials for bone

repair/regeneration therapies.

M-18

Carbon Nanotube-Reinforced Hydroxyapatite Biocomposite

Coatings by Hydrothermal Synthesis and Electrophoretic

Deposition (EPD)

C. Kaya1, C.B. Ustundag1, F. Kaya2. 1Yildiz Technical University,

Turkey; 2Zonguldak Karaelmas University, Turkey

Carbon nanotubes (CNTs) show outstanding mechanical, physical

and electrical properties and they are interesting reinforcing

elements for a variety of materials, which includes also materials

for biomedical applications. One significant drawback of using

CNTs in liquid media is the non-wetting character of as-fabricated

CNT surfaces, therefore, CNT surfaces must be modified with

functional groups (for example with OH or COOH groups) to achieve

homogeneous CNT dispersions. Typically, CNTs are functionalized

by immersing them in an acid mixture (H2SO4+HNO3), however

it has been shown that this process may create surface defects

on CNT surfaces resulting in significant property degradation. In

the present work, an alternative functionalization process based on

hydrothermal processing is shown, which utilizes a suspension that

contains CNTs and a mixture of calcium acetate (Ca (CH3COO)3)

and phosphoric acid (H3PO4), which are the precursors to produce

stoichiometric hydroxyapatite (HA) at 200°C for 2h. The ultimate

objective is the development of CNT-hydroxyapatite (HA) nano-

composite powder mixtures exhibiting a highly homogeneous

dispersion of CNTs. The microstructure of CNT-HA mixtures

obtained under hydrothermal conditions was characterized by a

range of techniques including XRD, TEM and FEG SEM. Utilizing

the CNT-HA mixed powders, well dispersed colloidal suspensions

were prepared and used to coat various biomedical Ti alloys

using electrophoretic deposition (EPD). The characterization of the

bioactive coatings based on CNT reinforced HA will be presented