1999 MRS Spring Meeting & Exhibit

Chairs

Jim Brauker
W. L. Gore and Associates, Inc
PO Box 300
Flagstaff, AZ 86002
800-437-8181

David Clapper
SurModics Inc
Eden Prairie, MN 55344-3523
612-829-2749

Stuart Williams
Biomedical Engineering Program
Univ of Arizona
PO Box 248054
Tucson, AZ 85724
520-626-4707

Joan Zeltinger
Technology Development
Advanced Tissue Sciences Inc
La Jolla, CA 92037-1005
619-713-7959

* Invited paper

SESSION FF1: BIODEGRADABLE POLYMERS
Chairs: David L. Clapper and Joan Zeltinger
Wednesday Morning, April 7, 1999
Salon 15 (M)

8:30 AM *FF1.1
CORRELATIONS BETWEEN POLYMER STRUCTURE, IN-VITRO CELLULAR RESPONSE, AND IN-VIVO TISSUE BIOCOMPATIBILITY IN A LIBRARY OF DEGRADABLE POLYMERS. Joachim Kohn , S. Brocchini, K. James, V. Tangpasuthadol, Dept of Chemistry, E. Tziampazis, P. Moghe, Dept of Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ.

Our research effort is focused on the development of degradable polymers based on the adaptation of combinatorial approaches to polymer design. Our earlier work has resulted in the identification of non-toxic, tyrosine-derived diphenols that provide ideal building blocks for the synthesis of polyiminocarbonates, polycarbonates, polyarylates, polyurethanes, and polyethers. Tyrosine-derived polycarbonates and polyarylates are particularly useful in the fabrication of tissue-engineering scaffolds and show a wide range of physicomechanical properties and interactions with cells. Using a newly developed library of tyrosine-derived polyarylates, clear correlations could be identified between the chemical structure of individual polymers, the adsorption of fibronectin on the polymer surface, and the attachment (and growth) of various cell lines in vitro. For example, in a series of polyarylates, the adsorption of fibronectin on the polymer surface could be manipulated by minor changes in polymer structure. For these polymers, the amount of fibronectin adsorbed to the polymer surface was predictive of the ability of rat lung fibroblasts (RLF) and L929 mouse fibroblasts to attach and grow on these surfaces. In another set of experiments, the length of an alkyl ester pendent chain could be directly correlated with the degree of bone bonding in-vivo in a clinically relevant, long-term fracture fixation model. In summary, within a combinatorially designed library of polymers, new correlations between polymer structure, ECM protein adsorption, and the biological response in-vitro and in-vivo can assist in the identification of optimum material properties for specific tissue engineering/implant applications.

9:00 AM FF1.2
MICROPOROUS MEMBRANES FROM POLYLACTIDES. S. Gogolewski , P. Michel, Polymer Research, AO/ASIF Research Institute, Davos, SWITZERLAND.

Bioresorbable polymers are finding an increasing applications for internal fixation of bone fractures, drug delivery devices or scaffolds for seeding of cells in tissue engineering. More recently, porous resorbable membranes and 3-D scaffolds from various polylactides have been used experimentally for the treatment of segmental long bone defects and defects in the cranio- and maxilofacial skeleton. Depending on the intended application the membranes should be of various pore sizes and pore structures. Techniques used for the preparation of porous polymeric membranes with closed-cell or open-cell structures primarily involve the salt leaching, the so-called eutectic solidification, freeze-drying or the phase-inversion process. This study aimed at the preparation of membranes from polylactides with varying molecular weight and chemical composition with well-defined porous structures using the liquid system consisting of polymer solution in good or poor solvents and various nonsolvents. The membranes produced had the pore size and pore structure which were dependent on the polymer molecular weight, the molecular weight distribution, the chemical structure, concentration of polymer in solution, miscibility of solvent with nonsolvent, differences in the liquids solubility parameters, temperature, pressure and humidity of the environment, and the surface free energy of the substrate onto which the membrane was cast. Increasing the polymer concentration in solution and the rate of the liquids evaporation decreased the pore size and led to membranes with the closed-cell structure. Increasing the amount of nonsolvent in the polymer solution increased the membrane pore size but reduced the membranes mechanical properties. The use of nonsolvents miscible with the solvents led to porous membranes with round-shaped, interconnected pores. The use of nonsolvents which do not mix with the solvents resulted in canal like pores running perpendicularly to the membrane cross-section.

9:15 AM FF1.3
PHOTOCROSSLINKED POLYANHYDRIDES AS AN IN VIVO POLYMERIZABLE BIOMATERIAL. Amy K. Burkoth , Kristi S. Anseth, University of Colorado, Department of Chemical Engineering, Boulder, CO.

A new class of photoreactive biopolymers has been developed from multifunctional anhydride monomers which can be polymerized in situ. The resulting crosslinked networks are dimensionally stable, with minimal solvent uptake from the environment, and exhibit strengths intermediate to cortical and trabecular bone, making them favorable candidates for orthopedic applications. While the strengths of these polymers are controlled by the degree of crosslinking, the degradation rate of these networks can be controlled independently by the chemical composition. Specifically, the degree of hydrophobicity in the backbone chemistry in combination with the hydrolytically degradable anhydride crosslinks result in a controllable surface erosion mechanism, with degradation timescales ranging from 48 hours to 1 year. Furthermore, photopolymerizations provide additional advantages, including fast curing at ambient temperatures, and spatial and temporal control of the photoinitiation process, which has opened up the possibility of in vivo polymerizations. The proposed anhydride monomers react on clinically acceptable timeframes (seconds to minutes) with the time and heat of polymerization controlled by the initiation scheme (e.g., light intensity and inititor concentration). Furthermore, these polymers have shown good biocompatiblity, with high vascularization and cell in-growth subcutaneously in rats.

10:00 AM FF1.4
IN VITRO DEGRADATION OF POLY[1,6-BIS(ORTHO- CARBOXYPHENOXY)HEXANE ANHYDRIDE]. Meng Deng, Christi Bedell, Kathryn Uhrich , Rutgers University, Department of Chemistry, Piscataway, NJ.

Desirable characteristics for implantable polymeric systems include biocompatibility, suitable physical properties for device fabrication, and flexibility before and during degradation so that the device does not fragment during use. Previously, we described polymers that maintain the desirable characteristics (biodegradability, biocompatibility, mechanical properties) and overcome the solubility (or processing) problems associated with aromatic polyanhydrides. This work describes an in vitro degradation study performed to determine the degradation characteristics of poly[1,6-bis(ortho-carboxyphenoxy)hexane anhydride] at body temperatures. Polymer was compression-molded into discs and place into phosphate buffer solution at pH 7.4. Molecular weight, glass transition temperatures and decomposition temperatures were determined for each time point up to 35 days. The polymers are fast-degrading: the molecular weight is only 55% of the original value after three days. The degradation curve as a function of molecular weight decreased logrithmically whereas the glass transition temperatures decreased linearly. There was no significant decrease in polymer decomposition temperatures throughout the five week study.

10:15 AM FF1.5
BIOMEDICAL SCAFFOLDS FOR TRANSPLANTABLE TISSUE ENGINEERED SUBSTITUTES. Joan Zeltinger , Lee Landeen, Noushin Dunkelman, Jill Sherwood¹, Linda Griffith², Jonathan Mansbridge and Ronnda Bartel, Advanced Tissue Sciences, Inc., La Jolla, CA; ¹Therics, Inc., Princeton, NJ; ²MIT, Department of Chemical Engineering, Cambridge, MA.

In vitro tissue formation by numerous cell types was tested on biodegradable or biostable synthetic scaffolds to engineer dermis, cartilage or smooth muscle for human transplantation. Scaffolds differed by their chemical formulation, structure (e.g., dimensions, architecture, pore size, or void fraction [VF]) and fabrication (e.g., woven, knitted, felted, braided, solvent cast as sponges, or TheriForm processed [i.e., 3-D printed]). Materials included nylon, poly(glycolic acid), poly(ethylene terephthalate), poly(-caprolactone), poly-L-lactic acid or poly(D,L-lactide co-glycolide) / poly(L-lactic acid). Human- or animal-derived cells (dermal and arterial fibroblasts, keratinocytes, articular chondrocytes, arterial smooth muscle cells and arterial endothelial cells) were cultured on scaffolds under static or dynamic conditions for up to eight weeks. Analyses were customized per engineered tissue (quantitative MTT and DNA tests for metabolic activity and cell number, respectively; DMMB assay for glycosaminoglycans, Sirius Red assay for collagen, image analyses for pre- and post-culture dimensions, scaffold and tissue mechanics, and qualitative immunostaining and histology). The data showed that human and animal cell types adhered to, proliferated and readily produced tissue within scaffolds of various chemical formulations; however, the ingrowth, distribution, orientation, and viability of cells and the gross morphology of constructs were influenced by both cell type and scaffold features (pore size, VF, fiber density, degradation). The depth and uniformity of colonization and amount of extracellular matrix formed by chondrocytes, fibroblasts, smooth muscle cells and endothelial cells corresponded to the pore size in TheriForm scaffolds. Fibroblast orientation in felt and braided scaffolds followed the random or linear polymer fiber arrangement, respectively. Fibroblasts on nylon meshes formed monolayers or 3-D tissue depending on the particle sieve size. An overview of cell-polymer interactions will be presented. By prescribing scaffold features, one can potentially regulate the cellular destination, orientation and extracellular matrix production on scaffolds in vitro to consistently form viable, confluent tissues for transplantation.

10:30 AM *FF1.6
APPLICATIONS OF AUTOLOGOUS AND ALLOGENEIC CELL THERAPY. Frank T. Gentile , Kermit M. Borland, Helen M. Nugent and Daniel R. Omstead Reprogenesis, Inc., Cambridge, MA.

We have developed an autologous cell therapy to treat two urological disorders: vesicoureteral reflux in pediatric patients and urinary stress incontinence in adults. This product contains autologous cells (chondrocytes) suspended in a crosslinked alginate hydrogel which is injected submucosally at critical sites surrounding the ureteral orifice (in reflux) or the urethra and bladder neck (in incontinence). In reflux, the cell-alginate matrix serves as a bulking agent to prevent retrograde flow of urine from the bladder to the ureter and kidney. In incontinence, the bulking agent allows proper closure of the urethral sphincter. The key manufacturing components of this product are: 1) isolation of chondrocytes from an auricular cartilage biopsy, 2) expansion, and possible cryopreservation, of those cells in vitro, and 3) formulation of the cells into a crosslinked alginate hydrogel matrix. Similar approaches (i.e. autologous cells injected in a hydrogel matrix) are under active investigation in a number of other therapeutic areas. We are also actively developing an allogeneic cell therapy using endothelial cells to re-establish vascular homeostasis following vascular injury.
 

SESSION FF2: IN VITRO AND IN VIVO EVALUATIONS OF BIOMATERIALS
Chairs: Jim Brauker and Stuart K. Williams
Wednesday Afternoon, April 7, 1999
Salon 15 (M)

1:30 PM *FF2.1
CLINICAL AND HISTOLOGICAL OBSERVATIONS OF SEAREMATRIX - POROUS SURFACED SILICONE RUBBER IN TWO TO EIGHT WEEK SHEEP RECIPIENTS OF DIRECT CARDIAC COMPRESSION DEVICES DCC. William J. Seare, Jr. , Seare Biomatrix Systems, Inc., Salt Lake City, UT; Yifei Huang, Takeshi Yuasa, Stephen N. Hunyor, Cooperative Research Centre for Cardiac Technology, Sydney, NSW, AUSTRALIA.

Introduction - Body implant and device placements continue to be disappointing for both short and long term use. The body responds by forming scar tissue and collagen capsules. The stimulation of healthy, non-scarred, vascularized tissue at the tissue-implant interface has been elusive. A new materials fabricating technology (SeareMatrix SM) has been shown to prevent fibrous capsule formation and promote vascularity. SM creates advantages for implants and devices both during the initial biointegration and during long periods of implantation, in term of mechanical or bacterial challenge. We have extended the application of this new technology in achieving tissue adhesion and promoting vascularity by studying its biointegration on the surface of a device implanted onto the heart.
Aim - The purpose of this study was to investigate the time-related extent and quality of the biointegration of SeareMatrix onto the surface of a beating heart in adult sheep.
Methods - Four Merino cross adult sheep were implanted for 8 weeks {n=2}, 3 weeks {n=1}, and 2 weeks {n=1} with DCC devices integrally fabricated from SM silicone rubber. Cardiac parameters were examined with and without direct cardiac compression, including pressure-volume loops and cardiac output, x-ray fluoroscopy with image intensification (with and without contrast injection), echocardiography for cardiac wall movement. In one eight week sheep, effective cardiac assist over four hours was undertaken.
Results - At explant, all animals showed complete attachment of vascularized non-scarred biointegrated tissues on both epicardially opposed and pericardially opposed device surfaces. There were no areas of seroma, hematoma or detachment present. In an eight week animal, an area of smooth (non-treated) silicone surface 2 cm x 8 cm opposed against the epicardial surface (used to allow sizing for the device) showed significant thickening and scarring. No such scarring was seen where SM biointegration occurred in adjacent areas. Dissection of the pericardial and epicardial surfaces of the device was readily accomplished with finger dissection, with the release similar to Velcro strips being separated. The SeareMatrix released without clinical or histologic damage to the coronary vessels. Histology confirmed features in previous studies. There were arterioles, venules, and capillaries in a loose collagen background with a mild histiocytic response. In the area where a smooth part of the device contacted the epicardial surface, a fibrous capsule ten times the thickness of the thin overlying loose collagen membrane adjacent to the SeareMatrix was found. Untreated Dacron mesh at the periphery of the implanted device showed typical incomplete tissue penetration with microcyst formation and dense interfiber fibrosis, accompanied by signs of acute and chronic inflammation.
Discussions - DCC devices have found limited clinical acceptance for reasons of infection, materials failure and catastrophic ejection of the device from the heart if suction is removed. The use of SeareMatrix as tissue interface on DCC device shows promise in overcoming many of the drawbacks of previous DCC devices.

2:00 PM *FF2.2
MICE THAT LACK THE ANGIOGENESIS INHIBITOR, THROMBOSPONDIN 2, MOUNT AN ALTERED FOREIGN BODY REACTION CHARACTERIZED BY INCREASED VASCULARITY. Themis R. Kyriakides, Dept of Biochemistry; Kathleen J. Leach, Allan S. Hoffman, Buddy D. Ratner, Dept of Bioengineering; Paul Bornstein , Dept of Biochemistry, University of Washington, Seattle, WA.

Disruption of the thrombospondin 2 gene in mice results in a complex phenotype characterized chiefly by abnormalities in fibroblasts, connective tissues, and blood vessels. Consideration of this phenotype suggested to us that the foreign body reaction (FBR) might be altered in thrombospondin 2 (TSP2)-null mice. To investigate the participation of TSP2 in the FBR, silicone rubber (polydimethylsiloxane, PDMS) and oxidized PDMS (ox-PDMS) disks were implanted in TSP2-null and control mice. Adherence of TSP2-null and control skin fibroblasts in vitro was also evaluated on both types of disks. Normal fibroblasts grew as a monolayer on both surfaces, but attachment of the cells to the ox-PDMS was weak and sensitive to movement. TSP2-null fibroblasts grew as aggregates on both surfaces, and their adherence was also compromised on ox-PDMS. After a four week implantation period, both types of PDMS elicited a FBR with a collagenous capsule in both the TSP2-null and control mice. However, strikingly, the collagenous capsule that formed in TSP2-null mice was highly vascularized and thicker than that formed in normal mice. In addition, abnormally shaped collagen fibers were observed in capsules from mutant mice. These observations indicate that the presence or absence of an extracellular matrix component, TSP2, can influence the nature of the FBR, in particular its vascularity. The expression of TSP2 could therefore provide a molecular target for local inhibitory measures when vascularization of the tissue surrounding an implanted device is desired.

2:30 PM FF2.3
FTIR-FEW SPECTROSCOPY: NEW DIAGNOSTIC TOOL FOR MATERIALS RESEARCH. Natalia Afanasyeva , Angelique Brooks, Reinhard Bruch, Department of Physics, University of Nevada Reno, Reno, NV.

The investigations of complicated, soft condensed material such as human and animal skin and polymer surfaces on a molecular level have been developed using Fourier Transform Infrared Fiberoptic Evanescent Wave (FTIR-FEW) spectroscopic method. These types of surfaces are very difficult to investigate by traditional FTIR methods even with the use of accessories. The FTIR-FEW technique is sensitive enough to detect changes in the vibrational spectra of polymer and skin surfaces without heating or damaging it. In addition, this method is fast (15 seconds), remote (fiber length up to 3 meters), nondestructive in real time and on-line with computer for rapid (10 seconds) treatment of large database of spectral statistical results. For the first time we have recorded surface of the human skin tissue non-invasively in vivo. Surface and bulk (within the epidermis), spectra of normal, acupoints, pre-cancerous and cancerous skin tissue have been analyzed in detail. The depth of penetration of the evanescent wave into the tissue is about 10-15 m utilizing this technique. In addition to skin, we have measured the surfaces of polyethylene crumpled bags and rumpled films, as well as polytetrafluoroethylene. Weak but distinct spectra were also obtained for carbon fibers (black thin fibers with a diameter of about 10 m). This method could be developed and applied to: polysaccharides, proteins, gels, and granular materials of small sizes, etc. In this paper, we present this new fiberoptical FTIR method for many applications including biomedicine, industry, and geology.

3:15 PM *FF2.4
TISSUE REPLACEMENT AND REPAIR USING ELASTIN BIOMATERIALS. Kenton W. Gregory , Michio Kajitani, Yasmin Wadia, Monica Hinds, Rui-Quing Qian, Kristy Hanna, Shalaby Shalaby¹, Andrew Barofsky, Oregon Medical Laser Center, Providence St. Vincent Medical Center, Portland, OR; ¹ Poly-Med, Inc., Anderson, SC.

Elastin is an extracellular matrix protein that has excellent properties for tissue replacement and repair-excellent durability, thermal and chemical stability with active cell signaling, and little reported cross species antigenicity. Elastin was investigated as a biomaterial for arterial and gastrointestinal tissue replacement using biodegradable cyanoacrylates or dye targeted tissue fusion for deployment. Elastin biomaterials (EB) of porcine origin were isolated, prepared in sheets or tubes, autoclaved and stored in 80% ethanol. Conventional sutures,methoxy-propyl cyanoacrylates (PolyMed) or laser fusion using indocyanine green dye applied at tissue interfaces to selectively absorb 800 nm pulsed diode laser energy (5 ms 5 J) pulses joined the EB to native arterial or duodenal tissues in vitro and in vivo studies using anesthetized 40 kg domestic swine. EB, application integrity and host response were assessed acutely and up to 2 months. Excellent tissue bonding could be achieved by all means and minimal thrombogenicity and chronic inflammatory response was observed. No infections of the biomaterial was observed. We conclude that elastin may be an excellent native protein biomaterial for vascular and gastrointestinal tissue replacement and repair.

3:45 PM FF2.5
OSTEOGENIC CELLS IN NANO-HA/COLLAGEN MATRIX. C. Du, F.Z. Cui , Q.L. Feng, Tsinghua Univ, Dept of Materials Science and Engineering, Beijing, P.R. CHINA; X.D. Zhu, Beijing Medical Univ., China-Japan Friendship Hospital, Beijing, P.R.CHINA; K. deGroot, Leiden Univ., Biomaterials Research Group, NETHERLANDS.

Nano-HA/collagen (nHAC) composite that mimics the natural bone in both composition and microstructure to some extent was expected to be a good matrix for tissue engineering of bone. Using organ culture techniques and a convolving method, we developed a three dimensional osteogenic cells/nHAC construct in vitro . The interaction between cells and materials was investigated by histology, scanning electron microscopy and transmission electron microscopy. Spindle-shaped cells migrating out of bone fragments continuously proliferated and migrated throughout the network of the coil. The porous nHAC scaffold provided a microenvironment resembling that seen in vitro and cells within the composite eventually acquired a tridimensional polygonal shape. Collagenous matrix was synthesized at the interface of cells and nHAC. The collagen fibers were observed obliquely oriented to the surface of the material, suggesting the bioactivity and bone-bonding property of the composite. The mineralization of this matrix was observed to involve matrix-vesicle mediated process.

4:00 PM FF2.6
PARTICLE SIZE EFFECTS ON OSTEOBLAST ATTACHMENT TO AND MINERALIZATION OF POLYMETHYLMETHACRYLATE- HYDROXYAPATITE COMPOSITES. S. Ruan¹, A. Winnard², J. Lannutti ¹, A. Moursi², R. Seghi², ¹Department of Materials Science & Engineering, ²College of Dentistry, The Ohio State University, Columbus, OH.

The mechanical properties of ceramic-polymer composites are controlled in part by the surface area of the reinforcing phase. Hydroxyapatite (HA) was explored as a reinforcement for polymethylmethacrylate (PMMA), long a component of bone cement. HA particle size was varied to determine the influence of this parameter on the attachment, proliferation, viability, morphology and mineralization of primary rat calvarial osteoblasts obtained from fetal (day 21) rat calvariae by collagenase digestion. Attachment was assessed after a 4 hr incubation followed by washing. Proliferation was examined by culturing osteoblasts followed by fluorimetry of dye-containing media on days 2, 4, 6 and 8. In contrast to the attachment results, osteoblast proliferation on day 8 was found to be significantly higher on PMMA/HA than on Ti/HA. Primary rat osteoblasts formed a multilayered cell culture and produced an extensive, well-organized extracellular matrix (ECM) within 3 days of plating. Mineralization of the ECM began within 14 days. Morphology, ECM production and mineralization were examined by electron microscopy and immunohistochemistry at both early and late time points. Cell viability was examined on day 14 by acradine-orange staining. No clear correlations between cell proliferation and HA particle size were obvious although ECM production showed dependence on loading.

4:15 PM FF2.7
BIOMIMETIC INTERPENETRATING POLYMER NETWORK (IPN) AS ORTHOPEDIC BIOMATERIAL. Guy P. Beauregard, Susan P. James , Rocky Mountain Materials Research, Department of Mechanical Engineering, Colorado State University, Fort Collins, CO.

A novel, biomimetic, interpenetrating polymer network (IPN) between poly-L-lysine (PLL) and ultra high molecular weight polyethylene (UHMWPE) has been synthesized in an attempt to decrease wear in joint prostheses. A gradient IPN of cationic PLL and UHMWPE has been synthesized (in the surface of bulk UHMWPE) to recruit the poly-anion, hyaluronic acid, from the synovial fluid. It is hypothesized that the hyaluronic acid molecules and their associated hydration layer will improve lubrication between the articulating surfaces, thus lowering both friction and wear. The synthesis involves four steps. Siylation of the PLL-HBr to PLL-SiMe₃ utilizing bis(trimethylsiyl)acetamide (BSA). Swelling of the UHMWPE in a solution of PLL-SiMe₃/xylenes at 60C with ultrasonics. Crosslinking of the PLL-SiMe₃ within the UHMWPE with 1,8-diisocyanatooctane (a.k.a. OMDI). Finally, de-swelling and drying of the IPN under vacuum at 50C. Visual observations show an adhered film on the IPN surface. Reflective FTIR spectra contain the characteristic peaks associated with UHMWPE. Two additional peaks, at 3410 and 1690 cm^-1, are associated PLL. SEM shows a morphology dominated by PLL spheres with diameters ranging from <1 micron up to 3 micron. This shows that the PLL-SiMe₃ has been crosslinked by the OMDI and was not rinsed away by either xylenes or sonicated water rinses. High contact angle of the PLL in contact with the UHMWPE demonstrate that the PLL has been de-silylated and returned to its hydrophilic nature. The spheres attached to the surface of the UHMWPE indicate that PLL has infiltrated the UHMWPE physical network and is entangled there.

4:30 PM FF2.8
IN VITRO FIBROBLAST GROWTH ON TRIETHYLENE GLYCOL DIMETHACRYLATE POLYMER REINFORCED WITH NANOPOROUS SILICA: EFFECT OF CERAMIC LOADING. Janice Taylor¹, John J. Lannutti ² and Robert R. Seghi³, ¹Center for Biomedical Engineering, ²Department of Materials Science & Engineering, ³College of Dentistry, The Ohio State University, Columbus, OH.

Ceramic-reinforced polymers have obvious similarities to normal hard tissues. In direct dental restorations, such composites are commonly used in contact with gingival tissue. Their ability to bond with such tissue plays a critical role in the long-term success of such implants. We conducted a series of experiments aimed at resolving the effect of ceramic loading on cell proliferation when said ceramic is of limited biological compatibility. We used silica, SiO₂, suspended in a standard dental matrix, triethylene glycol dimethacrylate (TEGDMA). Gingival fibroblasts derived from healthy human adults were used for in vitro cell viability studies. Passage lines 2-5 were incubated at 37C in a humidified environment containing 5% CO₂. At days 1, 3 , 5, and 7 the disks were incubated in Alamar Blue and fluorescence readings taken at excitation wavelengths of 560 nm. Population results indicated that silica had negative consequences for cell growth, thereby improving little on the cytotoxicity of pure TEGDMA. Not surprisingly, rougher surfaces resulted in increased cell proliferation. Microscopy was used to examine the correlation between composite substructure and cell proliferation.

4:45 PM FF2.9
HOLLOW POWDER BIOMATERIALS. David J. Sypeck and Haydn N.G. Wadley, University of Virginia, School of Engineering and Applied Science, Department of Materials Science and Engineering, Charlottesville, VA.

Porous structures based upon hollow metal spheres with thin walls and strong interparticle bonds are a new class of cellular materials that posess excellent weight specific mechanical properties and an array of multi-function capabilities. A by-product of inert gas atomization, hollow spherical powders are available from a variety of metal and alloy systems including those of biomedical interest such as stainless steel and titanium. One significant advantage of structures fabricated from hollow powders is the ability to engineer the structure with a desired set of properties through control of the sphere type, size and mix, wall thickness and bond strength. Functionally graded biomaterials with controlled amounts open and closed porosity can be made stiff, strong and very light in weight. Such structures allow fluids to flow through them, facilitate fixation owing to their surface porosity and can be designed to have properties which mimic that of the structure they set out to replace. In this work, hollow metal powders are separated by size, shape and density. They are then consolidated to a porous structure and their compressive mechanical properties are assessed. Future applications as biomaterials are discussed.
 

SESSION FF3: POSTER SESSION:
BIOMEDICAL MATERIALS
Chairs: David L. Clapper and Joan Zeltinger
Wednesday Evening, April 7, 1999
8:00 P.M.
Salon 7 (M)

FF3.1
ON THE PIEZOELECTRIC OF ANIONIC COLLAGEN FILMS. Julio C. Goes , Sonia D. Figueiro, Jose Airton C. de Paiva and A.S.B. Sombra, Departamento de Fisica, LONLCM, Universidade Federal do Ceara (UFC), Fortaleza-Ceara, BRAZIL.

Polymeric composite materials, both of natural and synthetic origin, constitute by far the broadest and most diverse class of biomaterials. Collagen have great potencial, in the field of bioactive biomaterials. Collagen, the most abundant protein of the animal kingdom, has a long history as biomaterial. We can find it in prostheses of heart valves, in artificial skins, in contact lenses and in injectable gels for soft tissue augmentation. In this work we did a study of the physicochemical, dielectric and piezoelectric properties of anionic collagen films, considering the development of new biomaterials which have potential applications in coating of cardiovascular prostheses, support for cellular growth and in systems for controlled drug delivery. Results obtained from shrinkage temperature of collagen membranes, casted in pH 7.4, showed that the native collagen membranes had highest thermal stability than anionic collagen films. The piezoelectric strain tensor element d14, the elastic constant s55, and the dielectric permittivity e11 were measured for the native collagen and anionic collagen films. Resonance measurement of the piezoelectric strain constant d14 of native collagen film gives 0.066 pC/N, while samples of anionic collagen obtained with alkacaline treatment give 0.072 pC/N. We believe that alcaline tratment lead to an increase of the organization of the microscopic structure of the sample, which could result in an increase of the piezoelectricity.

FF3.2
EFFECT OF POLARITY OF PIEZOELECTRIC CERAMIC PZT ON CRYSTALLIZATION OF CALCIUM PHOSPHATE. Chunlai Ma , Xiaodan Sun and Hengde Li, Tsinghua University, Department of Material Science and Engineering, Beijing, P.R.CHINA.

This work is studied on effects of polarity of polarized ferroelectric ceramic of lead zirconato titanate (PZT) on crystallization of calcium phosphate from saturated solution. The PZT was choose as a substrate for his high polarizability. Started PH level of the saturated solution is about 7.0, polarized samples are with d₃₃ equal to 300x10^-12 and 450x10¹² C/N, respectively. Precipitates on PZT substrate were analyzed and specified by Scanning Electric Microscopy(SEM), Energy Dispersive X-ray Linear Scanning(EDXLS) and X-ray diffraction(XRD). PH level of the solutions was measured in certain interval time. The results showed that Ca-P layers are deposited on the positive and negative pole surface of either non-polarized or post-polarized PZT immersed in same condition; Although the size and regularity of deposits were very different on different substrate faces, yet the mineral films morphologically were all made up two similar layers: microcrystalline cluster-shape layer adjacent to the substrate and long sheet-like crystal layer which is closely grown on the former layer; Depending on the different substrate faces, the growing rate of the sheet-like layer is different and the order of that from large to small scope is: on N-pole surface > on non-polarized surface > on P-pole surface meanwhile the cluster layers on different faces were nearly the same in thickness of about 4-6 m. The deposit minerals are consist of OCP and DCPD. There are significantly influence of polarity on orientation of sheet-like crystalline(OCP) and which growing on the N-pole face of PZT were developed with (002) preferred orientation. The possible mechanism of effect of polarity on Ca-P crystallization from solution is discussed.

FF3.3
PROPERTIES OF POLY (- HYDROXYALKANOATES) COMPOSITE AS IMPLANT BIOMATERIAL. Rubén Sánchez , Norma Galego, Chavati Rozsa, Polymer Section, Advanced Material Laboratory, North Fluminense State University, Campos, R.J., BRAZIL.

Various synthetic materials have been routinely utilized as reinforcement for bone. These selected primarily on the bases of their compatibility, adequate fracture toughness and fatigue strength. In considering the replacement of bone by mechanical equivalent composite it is pertinent to start from the microestructure of bone itself. Hydroxyapatites (HA) composite were used as reforcing phase with similar chemical and structural characteristic of the human bond. By the other hand the bacterial PHB and copolymers P(HB-co-xHV), used as second composite components, beside being biocompatible, exhibit a appropiate mechanical properties (Young's Modulus 3.5 to 1.2 GPa). The rate of chemical hydrolysis of PHAs is very slow in vitro, may be in off to biomedical implants where the bioactivity of HA and the biodegradability of PHA can be combined to avoid particle migration from the implant region before growth of a new tissue and a favorable answer to induce a bone formation taking advantage of the slow polymer degradation. The PHB and copolymers P(HB-co-xHV) (5, 8, 14 and 22HV) previously characterizated were used to formulation hydroxyapatite composites. The polymer composites were studied between 20 to 40 of PHA composition. The mechanical testing were develped using hydroxyapatite (HA) of synthetic and natural origin with particle diameter of 0.1 mm and PHA polyesters. The mixture were prepared and homogenization at different composition and pressing by hot press at temperature 5C above melting point of respective polyesters. The composite properties like variation of mechanical properties under wet condition (by function of polymer structures and loaded of inorganic phase) were studied. The fracture region was explored by Scanning Electron Microscopic (SEM). The composite P(HB-co-8HV) (30) showed a mechanical strength to compression between dense bond (137.8 MPa) and spongy bond (41.4 MPa). The mechanical properties of HA/PHA composite constitute a promise results to use for orthopedic purposes.

FF3.4
ION BEAM BOMBARDMENT FOR TREATMENT OF GPC BIOMATERIAL'S SURFACE. Marcello G. Rodrigues , Robert L. Zimmerman, FFCLRP-USP, Dept of Physics and Mathematics, Ribeirão Preto/SP, BRAZIL; Eric K. Williams, Daryush Ila, Alabama A&M University, Center For Irradiation of Materials, Normal, AL.

Glassy Polymeric Carbon (GPC) is an amorphous organic polymeric carbon material widely used as bio-material mainly in the manufacturing of prosthetic heart due to high inertness and chemical bio-compatibility. But, in spite of these characteristics, GPC mechanical valves still may present thromboembolic problems related with the interface between its surface and the biological tissue mainly due to the natural cicatrization process over the replaced artifact. In fact, thromboembolism is today the most important problem to be solved in cardiac prosthesis research and it can yield dysfunction of the cardiac valve. Thromboresistence can be improved by treatment of the surface of the GPC devices. We have used energetic ion bombardment (10 MeV ion gold, 6 MeV ion carbon, 5 MeV ion silicon and 8 MeV ion oxygen), in many fluences (10¹³ to 10¹⁶ atoms/cm²), for increasing the roughness of GPC artifacts. The samples were prepared at 700C (where the material presents high density of pores available) and 1500C (final stages of carbonization, where the GPC is a very hard and smooth material and non-permeable). The improved roughness can make the GPC artifacts more biocompatible. Our results with this method showed that: a) Independent on HTT, the roughness of GPC artifacts can be increased and controlled with ion bombardment and such improvement can be of the order of 10 times. b) It was verified that the roughness increase is less for samples prepared at higher temperatures. c) In general, the roughness increasing seems to be function of the fluence with peaks around 10¹⁴ atoms/cm². d) By comparisons with early results, the absorption of lithium by the GPC pieces is related to its roughness. Although the roughness obtained is less than the minimum allowed for effective increasing in the biocompatibility, the methodology is pointed in the right direction.

FF3.5
EFFECT OF STERILIZATION PROCESSES ON THE IONÍS RELEASE OF NiTi ALLOYS. B. Thierry¹, M. Tabrizian ¹, O. Savadogo², C. Trepanier³, L'H. Yahia¹, ¹Biomedical Engineering Institute, Ecole Polytechnique of Montreal, ²Metallurgical Department, Ecole Polytechnique of Montreal, ³Nitinol Devices & Components (NDC).

As a potential biomaterial for many medical applications, NiTi alloys derive their good corrosion resistance from a homogeneous oxide layer, mainly composed of TiO₂. This passive surface layer, similar to the one observed on titanium-based alloys, provides them their good stability in most corrosive environment. However, due to the aggressiveness of the human environment, the release of metallic ions (nickel and titanium) in the surrounding tissue can occur which may affect the biocompatibility of such alloys. Indeed, it has been demonstrated that Ni ions (from pure nickel) induce DNA strand breaks, DNA-protein cross-link and chromosomal aberrations. Since sterilization processes are able to modify the biomaterial surface, they may modify the amount of metallic ions released during implantation, and thus modify the implant toxicity. The aim of this work is to study the effect of sterilization techniques on the corrosion resistance and the ionís release of NiTi alloys.
To achieve these goals, surface characterizations and cyclic polarization assays were undertaken on electropolished NiTi discs processed by commonly used sterilization techniques such as Steam Autoclave, EtO, Dry Heat, Plasma Sterrad-100S and Peracetic Acid. The passive diffusion of Ni and Ti from processed NiTi was measured by atomic absorption spectrophotometry in Hankís physiological solution.
From our results, calculation of corrosion rates (ASTM G102) based on Faraday's law indicated a mean Mass loss Rates of 4.3 10^-7 mg/cm²s for NiTi. Some slight differences in processed NiTi may be related to the surface modifications observed by AFM and AES. We are now evaluating in vitro toxicity of released ions in physiological solution.

FF3.6
SEM ANALYSIS OF Nd-YAG LASER INDUCED MODIFICATION ON MATERIALS FOR APICAL MICROSURGERY. Marco Melis, Norberto Berna , Alessandro Benvenuti, Dpt of Experimetal Medicine, Univ. La Sapienza, Rome, ITALY; Gabriele Pecora, Dpt of Endodontics, Univ. of Pennsylvania, Philadelphia, PA; Fabrizio Pierdominici, Sebastiano Tosto, Dpt of Innovation, ENEA Casaccia, Rome, ITALY.

The recent introduction of processing and diagnostic techniques like laser and surgical microscope, has significantly modified procedures and materials of apical dental microsurgery. The Nd-YAG laser is particularly suitable for the microsurgery because the beam can be veiculated by an optical fiber and focused to realize an istantaneus local sterilizzation. However the thermal shock due to the laser pulses can induced dimensional instability and microleaks of the filling materials on the apex whit possibility of persistent apical inflammation. This result is in contrast with the main purpose of apicectomy i.e. to eradicate irritant substances coming from the root canal system and to eliminate the potential cause of inflammation in the apical zone. The present paper aim to investigate the dimentional and morphological changes of three different material used for retrograde filling of new apex after Nd-YAG laser irradiation.The Endo-Tecnic Nd-YAG pulsed laser ha a pulse length of 800 nanoseconds with a repetition rate of 50Hz. The emission wawelength is 1064 microns and the diameter of the optical fibre is 300 microns. The ouput power was no greater than 10W. In this study, we have irradiated 12 sample for each material embebbed in plexiglas supports having a depth of 3mm and different shapes in order to simulate the real condition expected in vivo apical microsurgery. After hardening, the sample were irradiated with different power levels of the laser beam and then cutted perpendicularly to the vertical axis by a microtome. These cross section were investigated by SEM.

FF3.7
ADDITION OF SYNTHETIC POLYMERS ALTERS STRUCTURE AND MECHANICAL PROPERTIES OF MUCUS GELS. Rebecca Kuntz Willits , W. Mark Saltzman, Cornell University, School of Chemical Engineering, Ithaca, NY.

Mucosal surfaces of the body are exposed continuously to pathogens that are able to penetrate the semi-permeable mucus layer. The prevalence of serious, often life threatening, infectious sexually transmitted diseases is increasing; therefore the need for better methods of disease prevention is escalating. We propose that altering the physical structure of the mucus gel might inhibit the migration of infectious agents. Mucus gels are composed of large glycoproteins which entangle to form a complex network of fibers. We examined several non-toxic synthetic polymers, such as polyethylene glycol (PEG), polyacrylic acid and polyvinyl pyridine, to determine their effect on the structure of the mucus gel. Using scanning electron microscopy, we visualized the three-dimensional fiber network and found that the addition of either low (3400) or high (1  10⁶) molecular weight PEG caused the fiber network to collapse into a film. Because mucus is composed of greater than 90 water, the gels were also examined using rheological methods that determine the dynamic properties of the gel in a more natural state. As a model for one mode of disease transmission, which may be particularly relevant for HIV exchange between partners, these results were correlated to changes in white blood cell migration rates within the gel constructs.