The obtained resalts show us that these Kinetics are controlled by a Fickian diffusion step, into the over all heterogeneous media studied.

However, in the homogeneous media obtained by solubilizing the carriers in hydroethanolic solution ( with several pHs), a unitary order characterise these kinetics.

True resalts show us that:

Key words: monomer carrier- copolymer carrier- drug release- diffusion- reactional order- kinetic constant.

Referances:
(1): O. Schiavori, G. Pasut, S. Moro, P. Orsolini, A. Guiotto, F. M. Veronese ; European Journal of Medicinal Chemistry, 39 (2004) 123-13.
(2): Li. SM, X. Chen, R.A. Gross, S. Mc Carty, J. of Materials Science: Materials In Medicine, 11 (2000) 227-233.
(3): N. CHafi, A. Benghalem, A. Mesli, European Polymer Journal, 39 (2003) 1063-1070.
(4): S. Li, L. Liu, H. Garreau & M. Vert, Biomacromolecules, 4 372-377 (2003).

GEN053.6

Software Application for Assessing Real-Time Patient Responsiveness to Functional Electrical Stimulation Devices
R. D. Trumbower, P. D. Faghri, University of Connecticut, Storrs, CT

Recording real-time physiological and biomechanical reactions to the application of functional electrical stimulation (FES) is an important concern. To our knowledge such a system presently does not exist.

Objective: To develop a software application that acquires real-time physiological and biomechanical data from subjects during FES use.

Methodology: The software was developed using LabVIEW® (National Instruments, USA) and is executable on PC platforms running Windows OS 98/NT/2000/XP. Physiological and biomechanical data were collected from six persons with spinal cord injury during FES-induced leg cycling (ERGYS®, TAI Inc., USA). Crank kinematics, power output, stimulation intensity, flywheel resistance, and heart rate, data were collected in real-time using the developed software.

Conclusions: The application provides a user-friendly graphical interface for real-time analysis of a patient’s response to FES-induced leg cycling. It offers offline data-logging management and report generating capabilities. The application may also be used for clinical assessment of other developed FES devices.

GEN053.7

Osteophone as a device to detect bone fractures
P. D. Leeuw, J. Brouwers, G. Streekstra, L. Blankevoort, University of Amsterdam, Utrecht, Netherlands

INTRODUCTION
Diagnosing a hip fracture without the availability of a X-ray is sometimes hard. Osteophony is a non-invasive technique that could eventually assist general practitioners in correctly diagnosing hip fractures. Detection of bone fractures with osteophony started in 1816 with the introduction of the stethoscope by Laennec [1]. Osteophony is the assessment of bone integrity by analyzing its vibrations. In case of a hip fracture, percussing the patella and auscultating the symphysis pubica results in a different sound, as compared to the healthy contra lateral side [2]. The pitch of the sound represents the resonant frequency. Nevertheless listening with a stethoscope provides subjective results. By recording and analyzing the signals, the results become objective. Bone can be easily excited on prominences without much soft tissue coverage. The skin covering this prominence could influence the resonant frequency [3]. The goal of this study is to assess the influence of skin covering the point of excitation and to assess the influence of a standardised osteotomy on the resonant frequency of bone, with the eventuel idea to develop a screeningstool for general practioners in correctly diagnosing a fracture.

METHODS
For the experiments, 16 goat tibiae (8 fresh cadavers) are used. The medial malleolus is percussed with a hammer, instrumented with a force transducer. The response is measured with an acceleration sensor screwed into the tuberositas tibiae. After measuring with the skin covering the malleolus, the skin is removed and the experiment is repeated. Subsequently, halfway between the malleolus and the tuberositas a stepwise osteotomy is made (1/3, 2/3 and completely through). An osteophonogram is made by analyzing the transfer function between both signals, using a 2 channel data acquisition and analysis system (Pulse, Bruel & Kjaer, Denmark) coupled to a notebook PC. In the osteophonogram the resonant frequency is detected by the software.

RESULTS
There is no statistically significant difference between the resonant frequency of a goat tibia with or without skin covering the malleolus (paired T-test). The Pearson correlation coefficient is 0.992 (p< 0.05). There is a significant decrease in the mean resonant frequency between an intact and a fully osteotomised goat tibia (521 vs. 428 Hz; p< 0.05, Anova and Bonferonni post hoc analysis). For a typical example see figure I. No significant difference is found between intact and 1/3 (497 Hz) or between intact and 2/3 (497 Hz) (Figure II).

CONCLUSION
Soft tissue covering bone prominences at the point of excitation does not influence the resonant frequency. The resonant frequency of a fully osteotomised tibia is significantly decreased as compared to the intact goat tibia. Osteophony seems to be a promising method to detect bone fractures.

REFERENCES
1. Lipmann, RK. The use of auscultatory percussion for the examination of fractures. J. Bone Joint Surg. Am. 1932;14:118-126.
2. Tiru M., et al. Use of percussion as a screenings tool in the diagnosis of occult hip fractures. Singapore Med J 2002 September;43(9):467-9.
3. Nokes LD. The use of low-frequency vibration measurement in orthopaedics. Proc Inst Mech Eng [H] 1999;213(3):271-90.

Keynote Session I

Session 6C: Boston Chapter Student Competition

Session 1A: Fabrication Processes-Stents

GEN056.1

Novel Micro-Joining Techniques to Improve Stent Radiopacity. A Comparison of Welding and Riveting Processes
G. Siekmeyer¹, R. Steegmüller¹, B. Schrader¹, A. Hegel¹, M. Strobel², A. Schuessler², (1)Admedes Schuessler GmbH, Pforzheim, Germany, (2)ADMEDES SCHUESSLER GmbH, Pforzheim, Germany

Nitinol material for implants offer superior stent characteristics due to their super-elasticity and biocompatible behaviour. A drawback of this material, however, is the limited radiopacity. To correct this disadvantage enhanced radiopaque materials such as Gold or Tantalum are added to the Nitinol structure. By selective attachment of these markers to the stent implant superior material properties are maintained with added radiopacity.

The gold standards to join these different metal materials is micro-riveting. Each manufacturer has developed their own proprietary, novel micro-joining technology to attach, anchor, press and process a variety of rivet or inlay designs.

Available solutions from stents being marketed today have been investigated, analysed and will be reviewed. We will compare results from innovative concepts to achieve optimum 3D inlay contours and retention as well as new ideas to ensure minimal potential for crevice formation. Visual criteria, SEM images, mechanical, stress and corrosion data from potentiodynamic polarization curves will be presented. A benchmark for an optimum inlay and process will be given and discussed. Final process parameters such as material, inlay surface, radiopacity, alignment and joining quality of the anchored micro inlay are characterized.

The technological ranking, advantages and disadvantages of these process technologies are discussed. Finally, a decision matrix will be presented to guide in choosing the appropriate joining process for stent designs.

GEN056.2

Microstructural Characterization of NiTi Vascular Stents
W. Van Geertruyden¹, A. Toro², W. Z. Misiolek³, X. Han⁴, M. H. Wu⁵, (1)EMV Technologies, LLC, Bethlehem, PA, (2)National University of Colombia, Medellín, Colombia, (3)Lehigh University, Bethlehem, PA, (4)Institute of Microstructure and Property of Advanced Materials, Beijing, China, (5)Edwards Lifesciences, Irvine, CA

NiTi alloys are currently being used for biomedical applications due to their biocompatibility, high corrosion resistance and unique mechanical properties, especially those related to Shape Memory Effect (SME) and Superelasticity (SE).

A detailed study of the microstructural features in NiTi stents was performed, which included crystallographic texture analysis by EBSD, as well as TEM examination to determine the grain structure and precipitation kinetics after heat treatment of the stent tube. The samples were extracted from vascular stents, as well as from the parent tubes used in early stages of the production process. Care was taken to avoid changes in the microstructure derived from mechanical polishing during the specimen’s preparation for EBSD observation, while the Focused Ion Beam (FIB) equipment was used for preparation of TEM discs.

The specimens were analyzed at different stages of the stent production process, such as laser cutting, shape setting and electropolishing, and correlations between the microstructure and the manufacturing process were established. Characterization of the main morphologic features of martensitic phase was based on TEM imaging, and the effect of the different processing steps on the crystallographic texture was determined by adequate selection of samples location.

Key Words: Shape memory alloy, martensite, precipitation kinetics, stent manufacturing, laser cutting

GEN056.3

Optimization of NiTi Shape Setting Processes Through Modification of the Pre-Shape Setting Surface Finish
J. MacWilliams, Norman Noble, INC, Cleveland, OH

Nitinol is often used in medical devices for its unique mechanical properties. It is also used for the ability to produce complex geometry via shape setting, eliminating costly and difficult machining operations. The process of shape setting requires deformation of a finished or partially finished device. If this deformation is too severe, cracking can result leaving the device unusable. While multiple shape setting operations may be used to minimize the possibility of cracking, limiting the number of these operations is also important. Each shape setting cycle will affect the Austenite to Martensite transformation temperature and mechanical properties, potentially increasing variation in the end product performance or moving the properties outside the allowable limits.

This paper will study if the level of achievable shape setting deformation can be affected by modifying the surface finish of the pre-shape set component. The study will evaluate different laser cutting processes, and post laser cutting processes, and their effect on the level of deformation achievable before cracking occurs. Increasing the deformation achievable with a single shape setting cycle may minimize the number of cycles required, decreasing variation of end product performance as well as decreasing manufacturing cost and complexity. The goal will be to present an analysis of pre-shape setting surface finish criteria shown to maximize the deformation achievable during shape-setting heat treatment.

GEN056.4

L605 Precipitates and Their Effects on Stent Applications
P. Poncin¹, B. Gruez¹, P. Missillier¹, P. Comte-Gaz¹, J. L. Proft², (1)Minitubes, Grenoble Cedex 2, France, (2)Metallurgical Solutions, Foster City, CA

Among the various cobalt-chromium alloys, L605 (ASTM F90), with its high elastic modulus and density, has gained wide acceptance as a suitable alloy for stent applications. The presence of precipitates in this alloy, due to the physical metallurgy of the alloy system, leads to difficulties in predicting material behaviour and therefore in optimizing its processing. The initial aim of this work is thus to investigate the nature, morphology and distribution of these precipitates. Secondly, their nucleation, dissolution and conditions of stability are studied. Based on these results, predictable effects of precipitates on stents and their manufacturing steps are discussed. This analysis is used to facilitate the tube fabrication process in order to optimize properties appropriate for stent applications.

Session 1B: Fabrication Processes-Novel Laser Processes

GEN057.1

Damage-Free Cutting of Medical Devices Using the Water Jet Guided Laser
T. Levesque¹, D. Perrottet², R. Housh², B. Richerzhagen², (1)Synova-USA, Inc., Lewisville, TX, (2)Synova SA, Ecublens, Switzerland

Material processing in manufacturing of medical instruments and devices proves most of the time to be challenging for standard cutting technologies. In the medical field, indeed, the requirements related to manufacturing processes, in terms of surface cleanness, edge smoothness, thermal damages and cutting accuracy, are particularly high. The water jet guided laser, a hybrid technology coupling a laser beam and a water jet, is a new tool whose characteristics are advantageous for such delicate operations. One of its main advantages is the cleanness of the process, as particle contamination and burrs are avoided because of the use of water. Moreover, the processed materials suffer no thermal or mechanical constraints. If lately, regarding medical applications, this technology has been mostly used for stent cutting, it is also employed with several other devices such as blades and needles, or electronic devices.

GEN057.3

Homogenous Tube(HT) for Medical Applications
J. M. Carlson¹, L. Tysdal², G. May², T. Trozera³, (1)Cook Incorporated, Bloomington, IN, (2)K - Tube Corporation, Poway, CA, (3)Tom Trozera, DelMar, CA

Laser welding a cold formed metal or alloy strip and subsequent cold drawing and annealing processes is used to produce a weld free or “homogenous” tube. No evidence of a preexisting weld is present after the cold drawing and annealing has been completed. The grain structure across the cross section of the tubing consists of fine in size and recrystallized grains. Examples of the microstructure of 3XX SS, 4XX SS, and 17 – 7 PH SS alloys will be presented after laser welding, cold drawing and subsequent annealing. Typical mechanical properties of these alloys after the various processing operations will be presented and correlated to the microstructure

GEN057.4

Matrix Assisted Pulsed Laser Evaporation of biodegradable Poly-(bis-carboxyphenoxy-propane-sebacic acid) p(CPP:SA) thin films
E. Chyau¹, T. M. Patz¹, A. Doraiswamy², C. Jin², R. J. Narayan², R. Modi³, D. Chrisey³, (1)Georgia Institute of Technology, Atlanta, GA, (2)University of North Carolina, Chapel Hill, NC, (3)Naval Research Laboratory, Washington, DC

Session 1C: Fabrication Processes-MIM and Powder Processes

GEN058.1

Metal-Injection-Molded Suture Needles
F. R. Cichocki, Ethicon Inc., Somerville, NJ

Metal-injection molding (MIM) techniques have been used to produce three different types of suture needle. Martensitic and martensitic-aged stainless steel alloys were chosen to attain good strength performance and a hot isostatic pressing process was employed to eliminate residual porosity and enhance ductility. The tissue penetrating performance of cutting point needles produced via metal-injection molding was found to be markedly superior to the tissue penetrating performance exhibited by conventional suture needles of identical design. Examination of the penetration sites revealed that the MIM needles tended to incise through tissue, producing dissection marks, but no visible puncture holes. In contrast, the conventional suture needles left a pronounced puncture mark at the penetration site. Mold design played an important role in achieving sharp cutting edges along the cutting tip of the MIM needle and was largely responsible for the exceptional penetration performance observed. Other desirable features such as suture receiving holes and I-beam needle bodies were easily incorporated into the suture needles produced via MIM.

GEN058.2

Metal Injection Molding of Co-28Cr-6Mo
J. L. Johnson¹, D. F. Heaney², (1)Penn State University, University Park, PA, (2)The Pennsylvania State University, University Park, PA

Cobalt-chromium alloys are commonly used for biomedical applications because of their high strength, superior corrosion resistance, non-magnetic behavior, and biocompatibility. In this work, metal injection molding of gas- and water-atomized Co- 28Cr-6Mo powders is evaluated. Debinding and sintering are conducted in different atmospheres to evaluate their effects on sintering response and carbon, nitrogen, and oxygen contents. Hardness, tensile strength, and ductility are measured after hot isostatic pressing. The results are correlated to the interstitial content and microstructure. Nitrogen in the sintering atmosphere results in higher yield strength and hardness. The mechanical properties exceed ASTM F-75 requirements for cast material.

GEN058.3

Fabrication Techniques for the Production of Porous Structures
D. F. Heaney, The Pennsylvania State University, University Park, PA

Porous structure fabrication techniques such as loose powder sintering, binder assisted shape forming, the use of sacrificial binder particles, foam precursors, electrophoretic deposition, and electroforming are discussed. A design guide for process technique versus final properties is presented to provide a tool to determine the appropriate processing route for the final application.

GEN0523.1

A Comparison of the Properties of Powder Metallurgy Processed versus Cast-Wrought Processed ASTM F1537 Alloy 1 Barstock
M. J. Walter, Carpenter Technology Corporation, Reading, PA

ASTM Standard F1537 covers the requirements for three wrought cobalt - 28chromium - 6molybdenum alloys used for surgical implants. This paper compares the physical, mechanical and metallurgical properties of low carbon Alloy 1 barstock produced using powder metallurgy processed (Carpenter Micro-Melt®) and conventional cast wrought processed billets. Additional comparisons are also made with ASTM F75 Castings. The metallurgical advantages of cast wrought Co-Cr-Mo components over those manufactured exclusively via casting are well known. It was demonstrated that similar advantages could be found when comparing powder processed (Micro-Melt®) low carbon Alloy 1 Co-Cr-Mo barstock to cast-wrought processed barstock. Micro-Melt® was found to have a finer and more uniform grain size, significantly less micro-constituent segregation and improved mechanical properties in both the Un-annealed warm-worked and solution annealed conditions. Additionally, it was found that the Micro-Melt® powder process enhanced the fabricability of typical alloy 1 material thus allowing for more versatile processing options. For example, Micro-Melt® low Carbon Alloy 1 can now be manufactured into smaller cross sectional sizes without the need for cold drawing which has known deleterious effects on micro-structure. Micro-Melt® powder processing also allowed for the production of shaped products, which were found to exhibit superior mechanical properties and more preferred microstructural features versus typical F75 castings.

Session 2A: Fatigue Life I

GEN059.1

Implant Device Design based on Failure not Survival: A Damage-Tolerant Analysis of a Cardiovascular Stent
R. V. Marrey¹, R. Burgermeister¹, R. B. Grishaber², R. O. Ritchie³, (1)Cordis Corporation, a Johnson & Johnson company, Warren, NJ, (2)Cordis Johnson and Johnson, Warren, NJ, (3)University of California, Berkeley, CA

To design against premature mechanical failure, most implant devices are assessed on the basis of survival, i.e., if a fatigue life of 10⁸ cycles is required, testing is performed to ascertain whether the device will survive 10⁸ cycles under simulated physiological loading. This is a highly unsatisfactory approach as the safety factors, which essentially tell you how close you are to failure, remain unknown. Indeed, this approach to fatigue design is quite unlike that used in most others fields of engineering, e.g., in automobile or aerospace applications, where the probability of fatigue failure is instead assessed on the basis of failure. In this work, we present a new damage-tolerant analysis of a cardiovascular stent, where the design life is conservatively evaluated using a fracture mechanics methodology. In addition to enabling estimates of safe in vivo lifetimes to be made, this approach serves to quantify the effect of flaws in terms of their potential effect on device failure, and as such provides a rational basis for quality control.

GEN059.2

Fatigue Evaluation of a Prosthetic Heart Valve
J. Crompton, S. Yushanov, S. Canchi, J. Dydo, K. Koppenhoefer, Advanced Computational & Engineering Services, Gahanna, OH

Implanted Björk-Shiley convexo-concave (BSCC) heart valves have experienced approximately 1% failure due to fatigue. An engineering assessment has identified the influence of valve design, manufacturing processes and physiological conditions on the factors influencing valve failure. Computational analyses identified the effect of valve operation on the expected lifetime of the valve.

GEN059.3

Comparison of Corrosion Fatigue of Implant Quality Stainless Steel and Titanium Alloys
L. D. Zardiackas, M. Roach, S. Williamson, University of Mississippi Medical Center, Jackson, MS

Some evaluations of failed devices have suggested CF as a possible contributory mechanism for the fracture of stainless steel and certain titanium alloys, while others have been in disagreement. Fatigue was performed in tension-tension in distilled water and in Ringers solution at 37EC using an MTS servohydraulic testing system following the guidelines of ASTM F1801. Samples were prepared using low stress grinding. A minimum of three samples at each of five stress levels were cycled to failure or to a maximum of 106 cycles. S/N curves for each material under each condition were plotted. Analysis of fracture surfaces was performed by SEM and EDS. Comparison between the fatigue response of each material in distilled water and Ringer=s solution as well as between alloys was evaluated. Results showed typical S/N curve response comparison in distilled water vs. salt solutions and the affect of the two test media was clearly evident. SEM evaluation showed variations in striation spacing as a function of loading as well as a transition in fracture surface morphology as a function of crack propagation rates and stress intensity.

GEN059.4

Fatigue Failure Analysis of Enhanced 35N LT®
L. E. Kay, R. Bouthot, Fort Wayne Metals Research Products Corporation, Fort Wayne, IN

MP35N® (ASTM F 562, UNS R30035) is the most common alloy used for leads in pacing devices. The unique requirements of this application require the balance of biocompatibility, corrosion resistance, and high fatigue strength offered by this alloy system. Drawn Filled Tube (DFT®) has been produced from this allow system to offer a low resistance material. This composite wire uses an outer layer of ASTM F562 (cobalt-nickel-chromium-molybdenum alloy) material for strength and corrosion resistance combined with a core material of silver to provide high conductivity properties. Previous studies of MP35N and Drawn Filled Tube (DFT®) have been conducted to analyze the effect of titanium content of the melt. The new enhanced chemistry of 35N LT® provides measurable improvements in surface finish and dramatic improvements in fatigue life. These improvements have been achieved while maintaining conformance to the requirements of the applicable material standards. This detailed study of the fatigue fractures uses Scanning Electron Microscopy (SEM) analysis to determine the cause of the failures in both material systems. It will be shown that the optimized process of 35N LT accomplishes the desired reduction in Titanium Nitride based inclusions.

MP35N® is a registered trademark of SPS Technologies 35N LT® is a registered trademark of Fort Wayne Metals Research Products Corp.

Session 2B: Fatigue Life II

GEN0510.1

Review of Fatigue and Fracture Behavior in Nitinol
R. A. Sire, B. James, L. E. Eiselstein, Exponent Failure Analysis Associates, Menlo Park, CA

Over the past four decades, a considerable amount of fracture and fatigue data has been collected for nitinol alloys. This review will evaluate current and past stress/strain-life, fatigue crack growth, and fracture toughness studies. Effects of environment, frequency, processing and texture will also be examined. This paper will outline the current state of knowledge with respect to nitinol fatigue and fracture behavior, point out conflicting data, identify areas where information is lacking, and suggest areas for further study

GEN0510.2

The Effects of Varying Active A_f Temperatures on the Fatigue Properties of Nitinol Wire
M. M. Patel¹, R. Bouthot¹, D. Plumley¹, J. L. Proft², (1)Fort Wayne Metals Research Products Corporation, Fort Wayne, IN, (2)Metallurgical Solutions, Foster City, CA

The influence of deformation temperature on mechanical properties of Shape Memory and Superelastic Nickel-Titanium (Nitinol) alloys has been studied and is well documented. In determining the effectiveness of a device when it is deployed and maintained under strain at ambient or body temperature, both the material properties and the environments encountered by the final device must be taken into account. In device design, it is important to understand the thermomechanical history of the wire supplied. However, data is lacking in terms of deriving a relationship between transition temperatures to a breadth of mechanical properties. Engineers of medical devices tailor the final material properties of wires used in a medical device application typically through a shape-setting process. Heat treating Nitinol wires in this type or a similar process aids in obtaining a desired shape as well as in reaching a target Active Af prior to final surface preparation. These processing steps can affect fatigue life. By applying alternating tension and compression states through rotary beam fatigue testing, one may generate data to predict the life expectancy of Nitinol wires. Specimens with varying Active Austenitic Finish temperatures have been subjected to multiple strain levels. The relevancy of this type of study, which involves the generation of fatigue data, supplements thermal and mechanical data to provide design engineers additional information for the development of Nitinol wire implants.

GEN052.3

Maximize Durability and Performance of Medical Devices: Optimize Design, Materials and Processes
J. J. Scutti¹, R. A. Mayville², (1)NMT Medical, Inc., Boston, MA, (2)R. A. Mayville & Associates, Inc., Newton, MA

An optimized product development process is critical to the development of medical devices that meet customer durability and performance criteria. Such a process involves identifying needs, developing requirements, defining concepts, creating a detailed design (including modeling, materials selection, manufacturing approach, process selection), verification, pilot production, validation, clinical trials, followed by production. Often, the durability and performance requirements are met during the design phase, through trade-off analyses that balance durability and performance criteria with such factors as ease of implanting/ease of use, variabilities in patients, manufacturability, production costs, and volume. This paper discusses these seemingly competitive factors, and provides insight into detailed design features, materials and processes that maximize or degrade durability and performance in medical devices. A stent, intramedullary nail, and a surgical instrument will be included.

Mechanical and Fatigue Properties of Nitinol Wire Under Uniaxial Tension
N. B. Morgan, AdvaNiTi Consulting, Wilts, United Kingdom

N/A

Session 2C: Materials Modeling

GEN0511.1

Finite Element Modeling of the Interaction of a Self-expanding Stent with an Artery
S. Prabhu, Guidant Corporation, Santa Clara, CA

Self-expanding stents have been used extensively to treat occlusions in endovascular arterial lumens, especially in carotid and peripheral vascular applications. These stents have proven to be very successful in maintaining the patency of the lumen and provide a less invasive alternative in the treatment of endovascular disease. However, poor performance of the intravascular device can result in undesirable clinical outcomes such as thrombosis and restensosis. In order to minimize these adverse events, it is important to characterize the stent-artery interaction and obtain a thorough understanding of the factors that contribute to these events during the stent deployment and its subsequent performance under the cyclic systolic and diastolic blood pressure loading. These factors could include the stent geometry (i.e. design, length, diameter, strut dimensions), the stent material and stent deployment procedure. In this paper, a finite element methodology, that can be used to study the nature of the stent-artery interaction, is presented. This technique can be used to determine the impact of various stent design attributes on the stresses generated on the arterial wall, as well as the impact that the artery has on the performance of the stent. The knowledge gained from such analyses can be used to generate stent designs that minimize the injury caused to the arterial wall and thereby greatly reduce the occurrence of adverse clinical events.

GEN0511.3

Modeling of Brazed and Welded Assemblies
S. Khurana, G. Jung, Edison Welding Institute, Columbus, OH

Many medical device designs include brazed subassemblies that are subsequently welded to other components. Examples include ceramic feedthroughs for implantable devices and lenses for endoscopic surgical devices. The difference in coefficient of thermal expansion of the materials and the proximity of the braze joint to the subsequent weld joint create failure concerns due to the thermal gradients, residual stresses, and distortion induced by the joining and welding processes.

During new product design, computational modeling can be used to predict thermal gradients, residual stresses, and distortion of brazed and welded assemblies. A case study will be presented involving a ceramic disk (alumina) brazed to a metal ring or disk (stainless steel) then laser welded to a metal cylinder (stainless steel). The finite-element analysis (FEA) procedure developed was verified with laboratory experiments. This type of analysis has been coupled with optimizing routines to predict the optimum configuration and joining process parameters.

GEN052.5

Non-Linear Finite Element Analysis of a Polyurethane Diaphragm Used in an Artificial Heart
S. Doshi, R. Bell, Ottawa-Carleton Institute of Mechanical and Aerospace Engineering, Ottawa, ON, Canada

Artificial heart polyurethane diaphragms are subjected to cyclic loading which makes devices scheduled for long term use prone to fatigue failure. Stress concentrations that develop during the deformation of the diaphragm are sources of micro cracks that could lead to untimely fatigue failure. Using finite element analysis (FEA) it is possible to analyze the stress and strain in a proposed design and adjust dimensions such as thickness to alleviate these concentrations. In operation, the diaphragm will invert under the load of the driving fluid. The bending, buckling and dynamic snap-through event is modelled using non-linear large deformation analysis in ABAQUS/Standard, an FEA software package. To represent the non-linear tensile response of polyurethane elastomer, the constitutive Mooney-Rivlin model is fitted to uniaxial and biaxial tensile data. A 3D model of the diaphragm was analyzed using a structured hexagonal mesh of second-order and hybrid elements. A reduced integration scheme was also incorporated. Strains over 50% and stress over 5MPa appear near the periphery of the model when the diaphragm is inverted and pressurized to 30Kpa (255mmHg).

Session 3A: Materials R&D I

GEN0512.2

Titanium Alloy with Bone-Matching Elastic Modulus and Super-Elasticity
Y. Hao¹, S. Li², S. Sun¹, C. Zheng¹, Q. Hu¹, R. Yang¹, (1) Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, (2)Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Due to their relatively low elastic modulus, beta-type titanium alloys are expected to improve the stress distribution to adjacent bone tissues and minimize stress shielding both of which may lead to bone resorption and eventual failure of implanted devices. This study focuses on investigation of elastic deformation behaviors of a recently developed beta-type titanium alloy composed of non-toxic elements. The results show that the as-forged alloy exhibits non-linear elastic deformation with incipient Young’s modulus matching that of human bone and high recoverable elastic strains up to 3.3%. We suggest sluggish, partially reversible processes of stress-induced phase transformation and/or incipient kink bands as the origin of these properties desirable for biomedical applications.

GEN0512.3

Strengthening of Low Young's Modulus Beta Ti-Nb-Sn Alloys by Thermomechanical Processing
S. Hanada, H. Matsumoto, S. Watanabe, Tohoku University, Sendai, Japan

Young’s modulus of quenched beta Ti-Nb-Sn alloys was investigated as a function of alloy composition. It is found that a minimum of Young’s modulus appears at a composition in an unstable beta phase where athermal omega phase transformation as well as martensitic transformation is almost completely suppressed. Based on the obtained result, quenched beta Ti-Nb-Sn alloys with low Young’s modulus were cold rolled and heat treated to evolve various microstructures. Orthorhombic martensite is stress-induced by cold rolling in Ti-Nb-Sn alloy having Ms just below room temperature. With increasing rolling reduction the volume fraction of martensite increases and most martensite plates become parallel to the rolling direction. With increasing temperature reverse martensitic transformation starts above room temperature, proceeds gradually and finishes at about 250°C, thereby yielding very fine beta phase grains elongated to the rolling direction. A high density of dislocations and fine alpha precipitates are included in the beta grains. With further increasing temperature the alpha phase precipitation occurs significantly, leading to increases in strength and Young’s modulus. Low Young’s modulus and high strength are simultaneously attained by controlling heat treatment condition

GEN0512.4

Fatigue Properties of Beta Type Titanium Alloy for Biomedical Applications Under Various Fatigue Conditions
T. Akahori¹, M. Niinomi¹, H. Toda¹, H. Fukui², M. Ogawa³, (1)Toyohashi University of Technology, Toyohashi, Japan, (2)Aichi-Gakuin University, Nagoya, Japan, (3)Daido Steel Co., Ltd., Nagoya, Japan

Implant instrumentations like bone plates, screws and nails, artificial spines, and artificial femoral and hip joints are used under fatigue conditions and sometimes failed due to monotonic loading, fatigue and corrosion fatigue. Mechanical performance, in particular, plain, fretting and notch fatigue performances are very important factors for titanium alloys for biomedical applications. A new beta type titanium alloy composed of non-toxic and non-allergic elements like Nb, Ta, and Zr, Ti-29Nb-13Ta-4.6Zr(TNTZ), has been recently developed in order to achieve lower Young’s modulus and excellent mechanical performance.

Fatigue performances are important mechanical properties to confirm the reliability of TNTZ as metallic biomaterials. It is well known that for titanium alloys these properties are changed according to the microstructures obtained by heat treatments or thermo-mechanical treatments in order. Therefore, fatigue properties of TNTZ conducted with various thermo-mechanical treatments were investigated under various fatigue conditions in this study.

Session 3B: Materials R&D II

GEN0513.2

Preparation of carbonated apatite and its evaluation
F. A. Zakaria¹, J. T. Kalitheertha Thevar¹, S. H. Abu Bakar¹, Z. H. Hussin¹, S. Muhamad², J. F. Mikan Venegas³, (1)SIRIM Berhad, Kulim Kedah, Malaysia, (2)Institute for Medical Research, Kuala Lumpur, Malaysia, (3)Universidad Militar, Bogota, Colombia

Introduction

The mineral in bones and teeth is an impure form of hydroxyapatite (ideal formula: Cav₁₀(PO₄)₆(OH)₂), with carbonate as the major impurity from 2 to 8 wt%[1]. It is well known that hydroxyapatite can host carbonate at two different sites: site A, where they subsitute for OH- ions, and site B, where they replace PO^3-₄ ions. Although hydroxyapatite has been widely used as osteoconductive biomaterial, carbonated apatite are more similar to the inorganic component of bone and can be used as bioresorbable materials for filling bone defects or as coating of metal alloys [2].

Several techniques can be used to prepare carbonated apatite where hydrolysis technique is the most popular option [2,4,5]. In this research, a simpler approach in preparing carbonated apatite, a solid state reaction, was used and characterized.

Materials and Method

Calcium carbonate (CaCO₃) powder (Fluka brand) and b-tricalcium phosphate (Ca₃(PO₄)₂) powder (Fluka brand) were used as chemical precursors for the synthesis of carbonated apatite (CAp). The precursor chemicals were mixed at Ca/P molar ratio of 1.8 and ball-milled for between 24 hours to get a homogenous mixture. The homogenous mixture was then pressed using uniaxial press machine (Carver, Australia) at 2 US tons in a 20mm diameter die. The disc was fired in a controlled atmosphere furnace (Modutemp, Australia) in air or carbon dioxide between 1000 and 1400oC in air, held for 1 hour and followed by cooling carbon dioxide to obtain carbonated apatite [3].

The fired discs were soaked in 40ml simulated body fluid solution as suggested by Miyazaki et al [7] for between 1 and 14 days at 36.5oC. The SBF solution has ion concentration and pH concentration almost similar to human blood plasma. After the given duration, discs were taken out from the solution, washed with double distilled water and dried in an oven.

Sample was characterized using X-ray diffraction analyzer (XRD, Bruker, Germany), Field Emission Scanning Electron Microscope (FESEM, Leo 1525, German with Energy dispersive X-ray spectroscope option (EDX, Oxford, UK) and Fourier Transform Infrared (FTIR, Nicolet, USA).

Results and Discussion

The result from sintering atmosphere showed a prominent result in producing carbonated apatite. When sintered and cooled in carbon dioxide, the results showed the conversion of the precursor into carbonated apatite was not complete. Instead, due to incomplete reaction of the precursor, tri-calcium phosphate, and calcium carbonate is present together with calcium oxide. However, when sintering was done in air and cooling in carbon dioxide, pure carbonated apatite was obtained. The precursor powder after sintering showed a highly crystalline pattern of pure carbonated apatite (CAp) from XRD analysis (Figure 1). FTIR result (Figure 2) shows carboxyl functional group presence at 1541 cm^-1, 1463 cm^-1 and 819 cm^-1 which confirms that the apatite obtained has the presence of carbonate and it is present at A-site [6]. EDX result (Figure 4) also confirms the presence of carbon which represents the carbonate. Figure 3 shows the microstructure of the sintered powder.

Immersion in SBF solution showed an increased FTIR absorption peak at 1630-1635 cm^-1 CO^2-3 band which is formed on the original carbonate apatite scaffold. However this peak was not observed at 1 day immersion. The peak intensities of XRD pattern was also observed to change with the duration of immersion. The peak reduced in height after immersion until day 7 but at day 14 the peak height increased. This result was due to apatite precipitation on the surface of the CAp as observed from the SEM images.

Conclusion

Carbonated apatite powder was synthesized with tricalcium phosphate and calcium carbonate through solid state reaction followed by heat treatment and carbon dioxide gas introduction during cooling. The single phase of carbonated apatite obtained using this technique is very simple and recommendable to the mass production of highly crystalline A-type carbonated apatite.

GEN0513.4

Biomimetic Polymer Composites for Orthopedic Implants
M. Campbell¹, H. A. Bougherara¹, M. N. Bureau², L. '. Yahia¹, J. G. Legoux³, J. Denault³, (1)École Polytechnique de Montréal, Montreal, QC, Canada, (2)National Research Council of Canada, Boucherville, QC, Canada, (3)Industrial Materials Institute, Boucherville, QC, Canada

Although fairly successful, total hip arthroplasty is subjected to long-term bone remodeling because the inert synthetic materials involved cannot mimic the biological and biomechanical functions of bones. The first cause of this incapacity, leading to aseptic loosening, is the production of wear debris at the hip joint. The second is stress shielding, leading to bone resorption, and is caused by the stiffness difference between cortical bone and the metallic stem.

The development of biomimetic femoral stem based on composite materials is presented. It is composed of a carbon fiber-reinforced polymer composite coated with a bioactive bone-like coating. The polymer composite structure consisted of a hollow conical-shaped stem formed by inflatable bladder molding and the coating was produced by plasma spraying.

Results concerning the physico-chemical and mechanical characteristics of the biomimetic THP stem will be presented, including strength, bone-matching rigidity and fatigue. Preliminary results regarding biocompatibility will also be discussed.

Session 3C: Materials R&D III

GEN0514.1

Strain, Texture and Phase-Fraction Measurements during Loading in Superelastic NiTi Wires
C. Rathod¹, D. C. Dunand², R. Vaidyanathan¹, (1)University of Central Florida, Orlando, FL, (2)Northwestern University, Evanston, IL

The large recoverable strains associated with NiTi (commonly called NiTiNOL) make it a valuable material for use in medical devices. Upon mechanically loading superelastic NiTi, a stress-induced phase transformation occurs that can result in macroscopic strains of up to 8%. These strains are fully recovered upon unloading due to the reverse transformation. By recording diffraction spectra during mechanical loading, such reversible stress-induced phase transformations can be investigated as they occur. Synchrotron X-rays at Argonne National Laboratory were used to investigate superelastic NiTi wires in situ during loading. A data analysis routine was implemented to analyze diffraction patterns and follow the strain, texture and phase fraction evolution during loading and unloading. Such measurements, while benefiting from the added penetration and decreased data acquisition times of synchrotron X-rays, linked macroscopic and microscopic phenomena in superelastic NiTi with immediate relevance to stent, orthodontic and guide wire geometries. This work is supported by NSF (CAREER DMR- 0239512).

GEN0514.2

Characterization of Torsional Properties of Implant Quality Stainless Steel and Titanium Alloys
M. Roach, S. Williamson, L. D. Zardiackas, University of Mississippi Medical Center, Jackson, MS

Stainless steel and titanium alloys have a long history of successful use for orthopedic screw applications. The purpose of this research was to compare the torsional characteristics of three stainless steel and four titanium alloys used for implants. Samples were tested on a multi-turn torsional assembly at 1080º/min with the axial load held constant. Maximum torque (Mmax), rotation at maximum torque (Θmax), 2º offset yield torque and rotation (My and Θy), and the breaking torque and angle (Mbreak and Θbreak) were calculated and compared using ANOVA and Duncan’s multiple range analysis. Comparison of torque and rotation values showed that the high strength stainless steel alloys had significantly higher Mmax than the other alloys tested, the α and α/β titanium alloys reached Θmax just before over-load fracture, the β-titanium alloy reached Θmax on average 180 degrees before fracture. The low-Ni high strength stainless steel alloy demonstrated the highest strength value, and the largest elastic range and rotation to failure values suggesting it is a candidate for torsional implant applications such as orthopedic screws.

GEN0514.3

Austenitic Stainless Steels with Improved Performance for Medical Devices
A. H. Heuer¹, F. Ernst¹, H. Kahn¹, G. M. Michal¹, S. R. Collins², P. C. Williams², S. V. Marx², (1)Case Western Reserve University, Cleveland, OH, (2)Swagelok Company, Solon, OH

A low-temperature carburization process that improves the mechanical and electrochemical properties of austenitic stainless steels will be reviewed. The process yields homogeneous, carbide-free solid solutions with >10at% interstitially dissolved carbon which is more than 600 times the equilibrium solubility during carburization. Such low-temperature "colossal" supersaturation (LTCSS) heat treatments introduce enormous biaxial compressive surface stresses (~2 GPa), provide exceptional hardness (~1200 HV25) and greatly improve the corrosion resistance with minimal loss of ductility. Greatly increased fatigue strength has been observed in a series of fully reversed high cycle tests. These properties have been achieved with 316 austenitic stainless steel having carburized layers ~25 mm thick. The potential to attain similar property improvements by extending the application of low temperature carburization processing to titanium and cobalt-based alloys will be discussed.

GEN0523.2

Evaluation of Titanium Implant Components Directly Fabricated through Electron Beam Melting technology
O. L. A. Harrysson, B. Deaton, J. Bardin, H. West, O. Cansizoglu, D. R. Cormier, D. J. Marcellin-Little, North Carolina State University, Raleigh, NC

Custom designed orthopedic implant components based on computed tomography (CT) or magnetic resonance imaging (MRI) data has gained in popularity over the past years. The literature shows many instances where custom designed implants are the only option. A common problem has been to efficiently fabricate the custom designed components and rapid prototyping technologies have been used to some extent. Direct fabrication of fully dense biocompatible metals like Ti6Al4V might offer a solution. This paper will cover direct fabrication of custom designed components using electron beam melting (EBM) technology. Material properties and microstructures of thin-walled sections will be discussed and compared to traditionally machined components. Finish machining of near-net shaped parts produced with the EBM system will be discussed as well.

Session 4A: Surface Engineering-Orthopedic

GEN0515.1

Surface Engineering of Orthopedic Bearing Materials
L. A. Pruitt, University of California Berkeley, Berkeley, CA

Total joint arthroplasty (TJA), is a widespread and highly successful surgical treatment for several types of degenerative joint disorders such as osteoarthritis and rheumatoid arthritis. Development of optimized surfaces and biomaterials for the bearing surface is a leading research area in orthopedics. This work will discuss cuurent surface modification schemes for ultra high molecular weight polyethylene. Topics will include surface crosslinking, plasma modification, and functionalization of this polymer. This work involves understanding the functionality of surface modifications and the implications for improved lubricity and wear resistance.

GEN0515.2

Improved Performance of Orthopedic Devices by Ion Implantation Surface Treatment Processes
M. A. Sambito, E. J. Tobin, M. Drory, Spire Biomedical, Inc., Bedford, MA

Surface treatments are used to improve materials without adversely affecting bulk properties or changing component dimensions. A wide range of mechanical, biological and other properties can be affected. Ion implantation is a robust process that can impart improvements such as reduced friction, increased hardness (for metallic materials), and increased wettability to medical devices. Nitrogen implantation is particularly effective for Co-Cr and Ti-6Al-4V alloys used for artificial orthopedic components in articulating environments, leading to reduced wear on mating polyethylene surfaces. Other potential applications for ion implantation include metal-on-metal systems and artificial spinal implants. This paper will provide an overview of ion implantation surface modification techniques and discuss data generated on ion implanted surfaces.

GEN0515.3

Matrix Assisted Pulsed Laser Evaporation of biodegradable Poly(lactide-co-glycolide) (PLGA) thin films
N. Johansen¹, J. Horowitz¹, A. Doraiswamy², T. M. Patz¹, R. J. Narayan², R. Modi³, D. Chrisey³, (1)Georgia Institute of Technology, Atlanta, GA, (2)University of North Carolina, Chapel Hill, NC, (3)Naval Research Laboratory, Washington, DC

Investigation of CoCrN Films Deposited Using a Dual Evaporation Ion Beam Assisted Process
E. J. Tobin¹, J. E. Burns¹, F. Namavar², (1)Spire Corporation, Bedford, MA, (2)University of Nebraska Medical Center, Omaha, NE

Session 4B: Surface Engineering-Cardiovascular

Hemocompatibility Evaluation of Ti Alloy and Diamond-like Carbon Coatings for Cardiovascular Applications
J. Arps, K. A. Dannemann, Southwest Research Institute, San Antonio, TX

GEN0516.2

Electrochemical Properties and Stability of PVD Coatings for the Application in Cardiac and Neurological Stimulation
H. Specht¹, F. Krüger², H. J. Wachter², O. Keitel², M. Frericks², C. Leitold², (1)W. C. Heraeus GmbH, Hanau, Germany, (2)W. C. Heraeus GmbH & Co. KG, Hanau, Germany

An overview about the opportunities to apply PVD coatings with different morphologies on implants will be given. Using magnetron sputtering technology single and multiple layers of metals, alloys, composites and ceramics can be deposited. Stimulation electrodes for example are coated to optimise the impedance properties of the electrode tissue interface for cardiac and neurological stimulation. Various metallic and ceramic coatings have been sputtered and the influence of material, thickness and surface morphology on their impedance characteristics and stability has been investigated by electrochemical impedance spectroscopy and cyclic voltametry. An increase in electrochemical capacitance that yields an impedance enhancement is best achieved by morphological changes resulting from process parameters or thickness. The stability of coatings can be influenced by the coating composition. Knowing the physical and chemical coating properties permits coatings to be applied with tailored characteristics for use in cardiac and neurological stimulation applications.

GEN0516.3

Hydroxyapatite Coatings for Coronary Stents
A. Rajtar¹, T. Troczynski², (1)MIVI Technologies Inc., Vancouver, BC, Canada, (2)University of British Columbia, Vancouver, BC, Canada

In this presentation we will report on our progress of development of HAP coatings for coronary stents, within the collaborative R&D program between UBC and MIV Therapeutics, Vancouver, BC. Our research focused on wet chemical technologies (sol-gel; electro-chemical deposition; electrophoretic deposition) for ultra-thin (<1um) and micro-thin (1-10um) calcium-phosphate based bio-ceramic films. The ultra-thin films are designated as a surface modification of metallic implants, whereas the micro-thin films are evaluated also as a potential vehicle for drug delivery purposes for implantable medical devices. In the extremely demanding application on stents the coating not only has to withstand deformation during manufacturing (i.e. stent crimping) and at the implantation stage and remain un-damaged in such operation. If this was not enough, the coating has to maintain its integrity and resist fatigue stresses in concert with the heart beat over the years after deployment in human heart. The variety of deposition technologies developed to address these difficulties, and preliminary evaluation results (in-vitro and in-vivo) will be reviewed.

GEN0516.4

Is Electropolishing Equal Electropolishing? A Comparison Study of Nitinol Stents
R. Steegmueller¹, T. Fleckenstein², A. Schuessler², (1)Admedes Schuessler GmbH, Pforzheim, Germany, (2)ADMEDES SCHUESSLER GmbH, Pforzheim, Germany

Electropolishing is the state-of-the-art finishing process of Nitinol implants, such as for stents, filters and connectors. While several studies have shown that electropolished Nitinol surfaces exhibit a better corrosion resistance, biocompatibility and overall surface quality of electropolished surfaces compared to other finishing methods, little is known about differences in the surface quality of Nitinol achieved among the different electropolishing procedures. Polishing solutions, parameters and basically all process details are proprietary to each manufacturer. However, the aim of the study was to determine to what extent Nitinol stent products currently being marketed differ in their surface quality.

The quality of the stent surface is evaluated by using Scanning Electron Microscopy. The chemical composition and structure of protective surface films is investigated by Auger Electron Spectroscopy depths profiles. Atomic force microscopy is used to measure the surface roughness of the different products. Finally, a first comparison of the corrosion behaviour of the different is performed by potentiodynamic polarization curves at a small amount of parts.

Session 4C: Surface Engineering-Novel Coating Processes

Microfused Coatings: Applications from Radiopacity to Electrophysiology
R. Sahagian, Implant Science Corp., Wakefield, MA

GEN0517.3

Characteristics and Properties of Advanced Surface-Engineered Device Components
S. Sastri, R. Cooke, M. Smith, N. Gunda, R. Raman, S. Jha, Surmet Corporation, Burlington, MA

Emerging medical device applications are demanding a combination of wear, corrosion and biocompatibility surface characteristics of the base material. Surface modification of the base substrate material is gaining several critical applications because if its demonstrated capability as an enabling route to meet these critical material specifications. Surmet’s Plasma Enhanced Chemical Vapor Deposition (PECVD) and Ionized Physical Vapor Deposition (iPVD) Processes coupled with our Interface Engineering Technology (IET), have gained and are poised to gain a number of important non-invasive and in-vivo device applications. In this presentation, we will give an overview of uniqueness of the deposition processes and products resulting from these deposition technologies including coating conformality, low temperature of deposition and stress-free high-thickness coatings. High purity coating materials developed including Diamond-Like Carbon (DLC), gold, and other advanced alloys and their characteristics will be discussed. Corrosion, wear, and performance-specific properties of coated components in selected applications will be illustrated.

GEN0517.4

Electrolytic Deposition of Calcium Phosphate Coatings for Biomedical Applications
R. O. Davis¹, G. M. Janowski¹, R. Venugopalan², (1)University of Alabama at Birmingham, Birmingham, AL, (2)Codman, a J&J Company, Raynham, MA

Calcium phosphate (CaP) coatings have been used to enhance the osteoconductivity of dental and orthopedic implants. Most are applied using plasma spraying, which utilizes high temperature and possibly requires a post treatment to improve the coating crystallinity. These can cause problems such as phase changes and thermal spalling. Electrolytic deposition (ELD) of CaP was investigated in this study to overcome these limitations. ELD is a process capable of depositing uniform coatings at temperatures well below those used in plasma spaying (less than 100 °C). This study investigated the effects of substrate modification and deposition parameters on the CaP coatings using chemical, morphological, and surface characterization. Coating bond strength and solubility were also measured. Adjusting the electrolytic bath starting pH, Ca and P composition, and the applied current density allowed controllable deposition of a continuous CaP coating with desired solubility from a simple unbuffered CaP containing electrolyte.

Session 4D: Surface Engineering-Biological Interactions

GEN0518.1

Effect of Surface Treatment on the Surface Characteristics of AISI 316L Stainless Steel
S. Trigwell¹, G. Selvaduray², (1)Kennedy Space Center, KSC, CA, (2)San Jose State University, San Jose, CA

The ability of 316L stainless steel to maintain biocompatibility is critical to its effectiveness as an implant material. The critical surface tension has been shown to correlate with the thrombogenecity, where a range of critical surface tension (γc) of 20 – 30 mJ/m² is considered to be the zone of biocompatibility; however, the biocompatibility is dependent upon the surface characteristics of the material. In this study, the surface of mechanically polished (MP) and electropolished (EP) 316L stainless steel coupons were studied by contact angle measurements at 37 °C, X-ray Photoelectron Spectroscopy (XPS), and Atomic Force Microscopy. The critical surface tension of the MP surface was 40 mJ/m² and the EP surface was 47 mJ/m², considerably higher than the zone of biocompatibility, even though the EP surface was considerably smoother than the MP surface (Ra = 607 Å vs. 1379 Å, respectively). The XPS data showed the EP surface had enhanced Cr:Fe and O:C ratios, typical of electropolished stainless steel, but the Cr was present more as Cr(OH)₃ on the EP surface, compared to Cr₂O₃ on the MP surface. The EP surface showed significant amounts of P, S, and Ca which were probably residues from the electropolishing process. It is well known that trace contaminants on a surface can greatly affect contact angles, and may have caused the high critical surface tension measurements for the EP. Work is in progress to further treat the 316L surfaces with Atmospheric Plasma Glow Discharge (APGD) to remove the contaminants and enhance the oxidation. This research is aimed at correlating surface characteristics, particularly the roughness and chemistry, to the critical surface tension and thereby to biocompatibility. The critical surface tension data obtained at 37oC will also be compared with data obtained at room temperature so that the usability of published surface tension data, for purposes of determining biocompatibility, can be evaluated.

GEN0518.2

Focused Ion Beam and Electron Microscopy of Bone/Dental Implant Interfaces
L. A. Giannuzzi¹, N. J. Giannuzzi², M. J. Capuano³, (1)FEI Company, Hillsboro, OR, (2)Nicholas J. Giannuzzi, DDS, Miller Place, NY, (3)Li Oral and Maxillofacial Surgery, Selden, NY

A focused ion beam (FIB) column and a scanning electron microscope (SEM) on the same platform (i.e., a DualBeam) were used to characterize the osseointegration of bone onto a Ti-plasma sprayed Ti metal dental implant. FIB methods were used to cross-section the bone-dental implant interface revealing varying degrees of osseointegration. Subsequent FIB slicing and SEM imaging of the bone/implant interface provided 2D images which were stacked and rendered into a 3D tomographic volume. The 3D analyses show regions of intimate bone growth into the pores of the implant ceramic coating. The FIB in-situ lift-out technique was used to prepare a specimen for transmission electron microscopy (TEM) of the bone/implant interface. TEM micrographs also show regions where the bone/implant surface is both discontinuous and continuous. TEM and x-ray energy dispersive spectrometry (XEDS) reveal interdiffusion between Ti, Ca, and P across continuous regions between the bone and coating, indicating that chemical bonding as well as mechanical bonding occurs place during osseointegration.

GEN0518.4

Effect of Grain Refinement of Titanium Alloy on Cell Proliferation
S. Li¹, Y. Zhao², S. Sun³, C. Zheng³, Y. Hao³, R. Yang³, (1)Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, (2)China Medical University, Shenyang, China, (3) Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

An investigatiion of the effects of grain size on cell adherence and proliferation of a recently developed biomedical titanium alloy is reported. Three group samples with average grain size of about 50 nm, 0.5 µm and 100 µm were produced by hot working or heat treatment of cold-rolled sheet. The results show that the adherence and proliferation of fibroblasts are enhanced by grain refinement. Such improvement is most significant at the early stage of cell culture. After culture time of 3h, 24h and 72h, the number of fibroblasts on the 50 nm fine-grain alloy was found to be about 3~5, 3, and 1.5 times that on the 100 µm coarse-grain alloy. Two surface conditions of the 0.5 µm alloy, smooth and ultrafine-porous, were prepared and subjected to the same experiment. No significant difference in cell proliferation was noted within the range of experimental error, suggesting that surface condition is not a critical factor at sub-micron grain size level.

Session 5A: Corrosion I

GEN0519.1

Galvanic Corrosion Evaluation of Zr-2.5Nb Coupled with Orthopaedic Alloys
M. Marek¹, V. Pawar², S. Tsai³, R. Thomas³, G. Hunter², A. Salehi³, (1)Georgia Institute of Technology, Atlanta, GA, (2)Smith & Nephew, Inc., Memphis, TN, (3)Smith & Nephew, Memphis, TN

Due to their superior wear characteristics, oxidized Zr-2.5Nb heads are coupled to hip stems made of conventional orthopaedic alloys. Galvanic interactions between specimens of Zr-2.5Nb (Zr) and Ti-6Al-4V (Ti), cobalt-chromium (CoCr), and 316L stainless steel (SS) alloys were evaluated.

Galvanic current density was measured for Zr/Ti, Zr/CoCr, Zr/SS, CoCr/Ti, and CoCr/SS couples under static conditions in a neutral Ringer's solution and in an acidic (1.7 pH) solution. To simulate fretting, one or both coupled alloys in the neutral solution subsequently were abraded by a bone cement pin. An extended (7-day) static test in the acidic solution was performed for Zr/SS and CoCr/Ti to simulate crevice conditions. The dissolved metal ion concentration was determined using direct-coupled plasma emission spectrometry.

In the galvanic couple, Zr behaved as an anode under static and fretting conditions. The Zr/SS couple produced the maximum current density in both neutral (3 µA/cm²) and acidic (0.6 µA/cm²) solutions. These results were greater than that the standard of CoCr/Ti (0.05µA/cm²). The greater current density and anodic behavior of Zr was attributed to its strong tendency to passivate, which protected the other alloy in the galvanic couple. This was verified in an extended static test where metal ion release for coupled Zr/SS was 1.8 times less than when they were not coupled (p=0.007).

Based on this investigation it is expected that when Zr-2.5Nb alloy is galvanically coupled to other conventional orthopaedic alloys, the couples will perform as well as other presently used couples.

GEN0519.2

Comparison of Stress Corrosion Cracking of a Series of Implant Quality Stainless Steel and Titanium Alloys
S. Williamson, M. Roach, L. D. Zardiackas, University of Mississippi Medical Center, Jackson, MS

The purpose of this research was to evaluate and compare the stress corrosion cracking (SCC) of three implant quality stainless steels and four implant quality titanium alloys. Smooth tensile samples and notched samples were prepared using low stress grind procedures. All samples were tested in distilled H₂O and Ringers solution at 37°C using the slow extension rate method at a stroke rate of 10^-5 mm/sec according to the guidelines established in ASTM G129. The ratio of the % elongation (PER) and reduction of area (ROAR) of smooth and notched samples tested in Ringers solution and distilled water was evaluated. The fracture surfaces of representative samples were also examined for fracture mode using SEM. Evaluations of the % elongation and reduction of area ratio’s (PER and ROAR) showed no indication of SCC failure mechanisms. SEM examination of the fracture surfaces showed no differences in the fracture morphology regardless of the testing solution. These results were consistent with the mechanical testing data. It is therefore concluded that SCC mechanisms were not operating or contributing to the fracture of these alloys under the conditions evaluated.

Corrosion Testing of Implantable Medical Devices
R. Venugopalan, Codman, a J&J Company, Raynham, MA

Session 5B: Corrosion II

GEN0520.2

Corrosion and Metallurgical Analysis of a Cast 316L TPLO Plate
R. D. Sisson¹, R. Boudrieau², (1)Worcester Polytechnic Institute, Worcester, MA, (2)Tufts University School of Veterinary Medicine, North Grafton, MA

Four cases of osseous neoplasia have been observed at TUSVM over a 5½ year time frame out of 594 cases of Tibial Plateau Leveling Osteotomy (TPLO) performed for repair of the cranial cruciate ligament in the dog (7/25/97 to 12/31/02). In 3 of 4 of these cases, a focal area of osteolysis was observed immediately adjacent to the plate. The latest case of an undifferentiated sarcoma 5½ years after a TPLO revealed this osteolysis; furthermore, an examination of the underside of this plate where it contacted the bone revealed a dull and apparently pitted appearance. Metallurgical examination of the TPLO identified the plate as cast 316L SS with some ferrite. The pale was also observed to be magnetic. The pitting was identified to be pitting corrosion in the creviced region between the plate and the bone. The possible role of corrosion of 316L SS in the osteolysis is discussed.

GEN0520.4

Electrochemical Characterization of Nitinol in Phosphate-Buffered Saline
B. G. Pound, Exponent, Menlo Park, CA

The effect of potential and surface condition on the corrosion behavior of nitinol in phosphatebuffered saline was examined using electrochemical impedance spectroscopy. Tests were performed on mechanically-polished and electropolished nitinol wire at the corrosion potential and various anodic potentials. The impedance data were analyzed using equivalent circuit models to evaluate the capacitive and resistive components of the surface oxide. Considerable differences were observed in the impedance components between the two surface conditions. In addition, the components exhibited a marked dependence on potential. These effects were interpreted in terms of surface analytical results from previous studies on the oxide composition.

Session 5C: Advanced Materials I

GEN0521.1

Novel Titanium Alloys and Titanium-Ceramic Composites by Advanced Powder Metal Technology for Biomedical Applications
S. M. Abkowitz¹, S. Abkowitz¹, H. Fisher¹, P. J. Schwartz¹, D. C. Dunand², (1)Dynamet Technology, Inc., Burlington, MA, (2)Northwestern University, Evanston, IL

Novel titanium materials produced by advanced powder metal techniques will be discussed. Unique compositions with properties tailored to biomedical applications are produced by solid-state powder metal processing. These compositions can offer improved wear resistance, superior properties and/or enhanced biocompatibility while retaining the excellent imaging capability and in vivo corrosion resistance characteristic of commercial biomedical titanium materials. This paper presents examples of innovative titanium compositions, their properties, and potential applications for orthopaedic implants and other biomedical devices.

Nanotechnology for Medical Devices: Issues for Commericialization
M. Helmus, Advance Nanotech Incorporated, New York, NY

GEN0521.2

Titania Thermal Spray Coatings Made from a Nanostructured Feedstock: An Alternative as a Biomedical Coating
R. S. Lima¹, B. R. Marple¹, K. A. Khor², H. Li³, (1)National Research Council of Canada, Boucherville, QC, Canada, (2)NANYANG TECHNOLOGICAL UNIVERSITY, Singapore, Singapore, (3)Beijing University of Technology, China, Beijing, China

Plasma sprayed hydroxyapatite (HA) coatings have been applied with success for the last 20 years on hip-joint implants to promote the fixation of the implant to the bone. Despite the success of the plasma sprayed HA coatings there are still concerns regarding the long term performance of these materials due to the (i) dissolution and weakening of the HA in the human body and (ii) low mechanical performance of plasma sprayed HA coatings. Owing to these concerns a high velocity oxy-fuel (HVOF) sprayed nanostructured titania coating is proposed to replace HA plasma sprayed coatings for long term performance implants. Titania is a non-toxic and non-absorbable material. Nanostructured thermal sprayed titania coatings have demonstrated superior mechanical performance when compared to conventional titania coatings [1]. Refereed references indicate that nanostructured titania exhibits enhanced osteoblast cell proliferation and adhesion when compared to conventional titania [2]. The typical bond strength values of plasma sprayed HA coatings on Ti-6Al-4V substrates are generally below 30 MPa [3], whereas, the bond strength of the HVOF-sprayed nanostructured titania coating reported in this work is higher than 77 MPa. Concerning the biocompatibility characteristics, the cell culture (in vitro) and the MTT cell test show that the osteoblast cells attach, grow and proliferate well on the HVOF-sprayed nanostructured titania. In addition to these characteristics, the HVOF-sprayed nanostructured titania coatings exhibit regions with nanotopography (nanoroughness) on the their surfaces. These nanostructured zones may enhance the adsorption of the adhesion proteins (e.g., vitronectin and fibronectin), which may lead to an enhanced cell adhesion to the coating [4].

1) R. S. Lima and B. R. Marple, “Enhanced Ductility in Thermal Sprayed Titania Coating Synthesized Using a Nanostructured Feedstock”, Materials Science and Engineering A, 395 (2005) 269-280.
2) T. J. Webster, C. Ergun, R. H. Doremus, R. W. Siegel, R. Bizios, “Specific Proteins Mediate Enhanced Osteoblast Adhesion on Nanophase Ceramics”, Journal of Biomedical Materials Research, 51(3) (2000) 475-483.
3) Y. W. Gu, K. A. Khor, D. Pan, P. Cheang, “Activity of plasma sprayed yttria stabilized zirconia reinforced hydroxyapatite/Ti–6Al–4V composite coatings in simulated body fluid”, Biomaterials, 25 (2004) 3177-3185.
4) R. S. Lima, B. R. Marple, H. Li, K. A. Khor, “Biocompatible Titania Thermal Spray Coating Made from a Nanostructured Feedstock”, Patent application, March 2005.

Session 5D: Advanced Materials II

GEN0522.1

Materials Characterization of Bulk Metallic Glass for Potential Use in Low Wear Articular Surface
R. Overholser, B. Aboud, S. Aust, DePuy Orthopaedics, Inc., Warsaw, IN

Of continuing interest to orthopedic implant manufacturers is the development of implant alloys designed to minimize in-vivo articular wear against a UHMWPE bearing counterface. In this investigation, three Zr-based bulk metallic glasses: LM-001 (Vitreloy 1), LM-002 (Vitreloy 1T) and LM-010, are evaluated with the objective of developing a material that has a favorable combination of wear resistance, fracture toughness, fatigue strength, corrosion resistance and biocompatibility. Pin-on-disc wear data suggest that certain Zr- based single phase bulk metallic glasses achieve UHMWPE counterface wear rates and PMMA cement abrasion resistance superior to cast Co-28Cr-6Mo. Rotating beam fatigue results for LM-001 show a fatigue limit at 20 million cycles that is higher than that of cast Co-28Cr-6Mo. Linear polarization data show LM-010 to exhibit a passivation breakdown potential that approaches that observed in cast Co-28Cr-6Mo.

GEN0522.2

Low Temperature IPD AgO Bacterial Static / Bactericidal Coatings for Various Medical Applications
D. Storey, Ionic Fusion Corp., Longmont, CO

Low temperature Ionic Plasma Deposition (IPD) has been successfully used to deposit silver and silver oxide large area (> 18,000 in²/hour) thin films on several types of wound care products, catheters, implantables, and surgical tools. The IPD process has been proven to be able to provide predictable infection control for short term bacterial-stasis implantable devices, to long term (> 60 day) bacterial toxicity. Silver and silver oxide has been deposited on substrates including PTFE, polypropylene, PVC, 440 stainless steel, Al₂O₃, and Ti6Al4V to name a few.

GEN0522.3

Nanostructured Titanium Alloy for Biomedical Application
Y. Hao¹, S. Li², R. Yang¹, (1) Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, (2)Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

Nanostructuring of structural materials for biomedical use promises to improve surface biochemical compatibility. For metallic alloys, however, bulk nanostructures are usually difficult to achieve. In general strengthening accompanies grain refinement through Hall-Petch relation due to dislocation-mediated deformation mechanisms. We report nanostructuring in bulk form of a biomedical titanium alloy achieved by highly localized plastic deformation. Coarse starting grains were easily refined to <50nm grain size during conventional cold rolling, but curiously only ~5% increase in strength was observed. We classify this type of materials as soft nanostructured metallic materials (NMMs) which contrast with several times increase in strength for previously-reported NMMs. This alloy can be used to investigate the instinct effect of nanosized grains on properties related to biomedical applications.

Session 6A: Materials R&D IV

Session 6B: Medical Device Applications

GEN0524.2

Biomechanical Design of a Shape Memory Alloy Spring for the Activation of a Hand Rehabilitation Device
M. Torri¹, S. Viscuso¹, S. Pittaccio², A. Nespoli¹, S. Besseghini¹, (1)Institute for Energetics and Interphases - Italian National Research Council, Lecco, Italy, (2)CNR IENI Institute for Energetics and Interphases - Italian National Research Council, Lecco, Italy

The rehabilitation of patients whose hand was affected by the outcomes of cerebrovascular injuries (e.g. stroke) relies on hand exercise. Home-synthetised Ni₄₀Ti₅₀Cu₁₀ was used for designing a spring-actuated work-out glove. Mechanical tests were carried by a commercial thermo-mechanical machine. Differential scanning calorimetry provided the transformation temperatures (As=43°C and Af=54°C). A simple hinged model of finger Kinematics was developed, based on movement analysis measurements. An Inverse Dynamic model comprising finger inertia and viscoelastic resistance was then used to quantify the force-time waveform required from the spring to replicate the natural movement of finger extension. From the measured alloy properties and the calculated forces the actuation temperature curve was found. The required input current curve to heat the device was then evaluated through power transfer equation including the transformation enthalpy term. Finger flexion was achieved by a bias spring whose properties depend on the SMA spring curves. A functioning prototype was built.

GEN0524.3

Feasibility Study of a New Sternal Closure Device Using Tubular Braided Superelastic Nitinol Structure
Y. Baril¹, V. Brailovski¹, R. Cartier², P. Terriault¹, (1)Ecole de technologie superieure, Montreal, QC, Canada, (2)Montreal Heart Institute, Montreal, QC, Canada

To reduce a risk of sternal dehiscence of the patients subjected to median sternotomy, a new sternal closure device is developed. This device prevents the cut in and through the sternum bones during aggressive post-surgery physiological activities as coughing, deep breathing and brusque movement. Furthermore, the new device is capable of maintaining a merely constant pressure between sternum halves during rehabilitation of such patients, thus improving the bone healing conditions. The principal element of such a device consists in a hollow tubular braided structure made of superelastic Nitinol, which tends towards a flat form when it is in contact with the sternum. Finite element modeling and in-vitro laboratory testing of the tubular superelastic device confirm a 30% reduction in contact pressure exerted by this device on the sternum bones, and a 25% increase in compressive pressure between the sternum halves, as compared to the conventional sternal steel wire closure device.