Elliot Chaikof, Emory Univ
John Ranieri, Sultzer Innotec Carbomed
Jeff Schryver, Smith & Nephew Inc
Robert Valentini, Brown Univ
- Smith & Nephew Research, Ltd.
- Sulzer Innotec Carbomedics
Proceedings published as Volume 550
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
1:30 PM *II1.1
SESSION II1: BIOLOGICAL CUES AND ORGANIC/INORGANIC HYBIRDS
Chair: Robert F. Valentini
Monday Afternoon, November 30, 1998
Essex North Center (W)
PUTTING SIGNALS ON BIOMATERIALS TO ENGINEER CELL BEHAVIOR AND TISSUE REPAIR; TASTE, TOUCH AND FORCE. Adam Curtis , Chris Wilkinson, Centre for Cell Engineering, University of Glasgow, Glasgow, SCOTLAND.
Chemical and topographical signals have been applied to the surfaces of biomaterials to control cell behavior. Mechanical signals can be applied by appropriate mechanical devices to the surfaces or frameworks on and in which cells are grown, to produce the same results . The effects on the cells are similar whichever of the three approaches are used. The results are changes in cell adhesion, shape and spreading, motility, cytoskeletal organisation, tyrosine kinase activity and gene expression. These changes are important and useful in cell and tissue engineering.
The talk will be organised into three sections :
In the first the question of whether there is a common central pathway for chemical, topographic and mechanotransduction pathways will be discussed. Similarities between the three pathways will be described. The sequences of events suggest that the common pathway may involve ion channel activation. Appropriate methods of fabricating devices with such signals will be described.
In the second section the value of using either of these approaches in isolation and then in combination will be discussed. Using chemical (`taste') and topographic (`touch') together may provide the most effective set of methods.
In the third section the application of such approaches to tendon repair will be described together with a tour of the `tools' and `useful things' in the tissue builder's yard.
2:00 PM II1.2
DESIGN OF POLYMERIC COATINGS WITH TUNABLE TETHERED LIGAND DOMAINS. Pallab Banerjee , Darrell J. Irvine, Anne M. Mayes, Massachusetts Institute of Technology, Dept. of Mater. Sci. and Eng., Cambridge, MA; Linda G. Griffith, Massachusetts Insititute of Technology, Dept. of Chemical Eng., Cambridge, MA.
Control of the behavior of cells at the surface of permanent implants is a key goal for the design of successful long-term implants. Early work in biomaterials focused on rendering surfaces non-interactive or bland to the biological mileiu, however, it is now recognized that materials which interact in specific ways with the surrounding tissue may provide the kind of integration between implants and the in vivo environment necessary for long-term function of implants and the avoidance of chronic problems, i.e. inflammation. We have designed monodisperse polymer microspheres through a dispersion polymerization technique for use as cell-signaling coatings. The microspheres have a poly(methyl methacrylate) core surrounded by a comb copolymer shell. The comb copolymer consists of a PMMA backbone and hydrophilic poly(ethylene glycol) teeth, which can be selectively end-modified with small peptide ligands for receptro-mediated interactions with cells. By casting films of these microspheres on a suitable substrate and heat-treating, a transparent coating is obtained which is protein and cell adhesion resistant, which may present tethered ligands for cell interaction. By functionalizing some microspheres with ligands such as RGD-containing peptides and mixing these functionalized particles with non-functional counterparts, films with distinct ligand domains on the surface can be created which allow one to selectively design the size and spacing of ligand-modified areas of the surface. These films may be useful as coatings for intraocular lenses, where the control of epithelial cell responses to the implant are important for long-term viability of the device.
2:15 PM II1.3
EARLY OSTEOBLAST ATTACHMENT, SPREADING, AND FOCAL ADHESIONS ON RGD COATED SURFACES. Geoffrey Moodie , Diana Ferris, Benjamin Hertzog, Colin Chen, Robert Valentini, Edith Mathiowitz, Brown University, Providence, RI.
Cells recognize and interact with the extracellular matrix (ECM) through heterodimeric receptors known as integrins. The objective of our work is to immobilize integrin-stimulating peptides to orthopedic implants in order to control cellular activity and bone response. We have previously demonstrated that cysteine (C) containing peptides self-assemble onto gold-coated substrates. Investigations have focused on the RGD (Arg-Gly-Asp) peptide sequence since it is found in several bone ECM proteins. Gold was first coated onto glass coverslips by evaporation and the peptide was applied in a 0.22 mM solution. RGDC peptide attachment was verified by contact angle and surface plasmon resonance. Rat calvarial osteoblasts isolated from six day old rat pups and were used from passage one to three. Cell attachment at 20 minutes is 100% greater on RGDC than on CG (control sequence) or plain gold surfaces. Cells on RGDC also stain positively for vinculin, a protein which is present in focal adhesions (functional structures into which integrins assemble) whereas surfaces without integrin stimulating peptides do not. Scanning electron micrographs show cells to be more spread and have more processes at 20 minutes, 1, 3, and 24 hours on RGDC. Live video images of these surfaces from zero to three hours after plating confirmed earlier and greater cell spreading on RGDC. Ongoing in vitro experiments are investigating the long term response of osteoblasts to RGDC and other immobilized peptides in terms of differentiation, matrix production, and integrin expression.
2:30 PM II1.4
ANALYSIS OF PEG COVERED SILICA SURFACES BY SFA. Norma A. Alcantar , Tonya Kuhl, Eray S. Aydil, Jacob N. Israelachvili, Dept of Chemical Engineering, University of California, Santa Barbara.
A monolayer of polyethylene glycol (PEG) attached to a surface resists protein adhesion and biological attack. We have developed a method to chemically graft low MW PEG onto silica coated surfaces. The silica films can be deposited onto almost any surface using plasma enhanced chemical vapor deposition (PECVD). These thin amorphous films or coatings are then activated by water plasma. The hydroxyl end of the PEG chains react with these surface silanol groups to form an ester bond, Si-O-C. We discuss the synthesis and characterization of silica and PEG films by attenuated total reflection spectroscopy and atomic force microscopy. Surface interactions and frictional characteristics of PECVD silica films, water plasma treated silica surfaces and PEG coated surfaces were determined using the Surface Forces Apparatus. Some anomoulous behavior of PEG covered surfaces in water will also be discussed
2:45 PM II1.5
APPLICATION OF CONDUCTIVE POLYMERS IN BONE REGENERATION. Venkatram (Prasad) Shastri , Nahid Rahman, Ivan Martin and Robert Langer, Departmentís of Chemical Engineering and Materials Science and Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA.
Current clinical approaches in treating bone loss typically use either autografts (removed from a donor site such as the iliac crest) or allografts (human cadaver bone) to substitute for the bone loss. However, these clinical solutions have several drawbacks, including lack of patient compliance (autografting), disease transmission and poor integration between donor and host tissue (allografting). The emerging field of tissue engineering can provide an attractive alternative to bone grafting, by combining the principles of materials science with cell based therapies to generate functional tissue equivalents. One important challenge lies in the development of polymer structural supports (scaffolds) that can improve cell-biomaterial interactions.
Yasuda demonstrated in the early 1950's that bone tissue is piezoelectric in nature and is capable of extensive remodeling when subjected to exogenous electrical stimulation. This observation led to the evaluation of electromagnetic therapy in bone healing with some success. Encouraged by these observations, we have explored the potential of an electrically conducting polymer, polypyrrole (PPy), as an interactive substrate for bone regeneration. Past work carried out in our laboratory has shown that PPy is highly biocompatible. In this study PPy thin films containing the polyanionic dopant, poly(styrenesulfonate), were evaluated with respect to their ability to support Bone Marrow Stromal Cell (BMSC) proliferation and differentiation. We have demonstrated that the application of a constant electric field (approximately 20 V/m for 1 hour), increased the alkaline phosphatase (ALP) activity of BMSC monolayers cultured on PPy thin films by 40 % (p<0.01). This effect was not observed in BMSC monolayers cultured on conductive substrates without the PPy (controls). ALP is a well defined marker for osteogenic differentiation of BMSC's. These results suggest that polypyrrole substratum in combination with electrical stimulation can be an effective tool in the manipulation of BMSC differentiation. Studies are underway in our laboratory to understand the underlying mechanism and extend the results to a 3-dimensional system. This paradigm can prove to be very powerful in combining the principles of tissue engineering with electromagnetic therapy in providing an effective local therapy for bone regeneration and healing.
3:30 PM *II1.6
BONE HEALING AND BMP DELIVERY WITH -BSMTM
. David Knaack , D. Duke Lee ETEX Corp., University Park at M.I.T., Cambridge MA.
(BSM) is a recently developed calcium phosphate material that has been designed to promote bone healing and to be easily degraded by cellular mechanisms. BSM is commercially available for a variety of dental indications in the United States. BSM is provided as a powder and is hydrated to form an injectable paste which remains formable for hours at room temperature. At 37 degrees centigrade, BSM sets in less than twenty minutes, to produce a nano-crystalline apatitic calcium phosphate with a chemical structure comparable to normal bone. Following application in paste form, to an implant site, BSM hardens . It may also be heated and pre- hardened into a desired shape prior to implantation..BSM was used to treat a number of surgically created animal bone defects including a canine femoral slot defect and critical size defects in rabbits and sheep. In all these models complete defect bridging with coordinate BSM remodeling into bone was observed. Residual BSM was generally vascularized, well osseointegrated and found to contain presumptive osteoclasts and osteoblasts in typical cutting cone configurations. These results coupled with the relative insolubility of BSM, suggest a BSM is remodeled into bone via a cell mediated remodeling mechanism. BSM has been investigated as a carrier for therapeutic molecules. The powder may be hydrated with a variety of aqueous agents (e.g. physiologically compatible buffers, surfactants, and biological fluids) without compromise to the setting reaction or mechanical properties of the hardened material. Stability tests demonstrated antibiotics, enzymes and growth factors may be added directly to the hydration media used to prepare BSM paste. Biological activity of these molecules was detected after hardening for one hour at 37 degrees . Likewise, BSM/ rh-BMP-2 application lead to ectopic bone formation within 2 weeks in rodent soft tissue and accelerated healing and restoration of a mature bone phenotype in a rabbit critical size bone defect.
4:00 PM II1.7
INTRACRYSTALLINE ORGANIC INCLUSIONS IN BIOMIMETIC CALCIUM PHOSPHATE MATERIALS. Julien Guiu, Sandra L. Burkett , Massachusetts Inst. of Technology, Dept. of Materials Science and Engineering, Cambridge, MA.
The development of techniques for the synthesis of calcium phosphate materials containing intracrystalline organic molecules and proteins is described. Although occluded proteins are present in some biologically mineralized materials, this type of hybrid inorganic-organic material has not previously synthesized in vitro. The product materials may be applicable as implant coatings and as implantable, biocompatible, resorbable vehicles for vaccine and drug delivery. In contrast to the ceramic coatings and devices currently in use for delivery of adhesion proteins, growth factors, and anti-infective agents, in which the organic species are adsorbed to the surface of the ceramic particles and are thus not necessarily delivered at a uniform rate, release rates of intracrystalline species should be controllable since they are determined by the rate of dissolution of the calcium phosphate matrix. These unique inorganic-organic hybrid materials also provide information regarding fundamental interactions between proteins and calcium phosphate that are relevant to bone bonding and biomineralization processes.
4:15 PM II1.8
IN VITRO BIOCOMPATIBILITY TESTING OF COLLAGEN-CALCIUM PHOSPHATE COMPOSITES USING HUMAN DERIVED OSTEOBLASTS. M. Oyama, M.E. Emerton, M.J.O. Francis, A.H.R.W. Simpson, Oxford University, Nuffield Department of Orthopaedic Surgery, Nuffield Orthopaedic Centre, Oxford, UK; A.C. Lawson , J.T. Czernuszka, Oxford University, Department of Materials, Oxford, UK.
The in vitro biocompatibility of a potential bone substitute has been assessed using human-derived osteoblasts. The bone substitute is produced by the precipitation of calcium phosphate onto a collagen matrix: a method analogous to the formation of natural bone. A collagen sheet is used as a membrane separating reservoirs of calcium and phosphate ions. The ions diffuse through the membrane and precipitation of calcium phosphate occurs where the ions meet, both on and within the collagen sheet.
Trabecular bone-derived cells were seeded onto the material and the ability of the material to support cell attachment, proliferation and bone matrix synthesis was assessed using histochemical alkaline phosphatase staining and immunolocalisation of transforming growth factor b1 and type 1 collagen. The effect of calcium phosphate phase and crystal size on the cell reaction was evaluated using composites containing octacalcium phosphate and hydroxyapatite with varying crystal size.
The results suggest that calcification of the collagen reduces the cell attachment but cells attached to the calcified samples showed more intense alkaline phosphatase staining. The nature of the calcium phosphate phase did not appear to effect the cell reaction whereas the calcium phosphate crystal size had a strong influence on the cell morphology and alkaline phosphatase activity. Increasing the crystal size was found to decrease the biocompatibility of the material.
4:30 PM II1.9
PROSPECTS FOR CHEMICAL CONTROL AND ENGINEERING OF ORGANICALLY MODIFIED CERAMICS. Stephen E. Rankin , Alon V. McCormick, University of Minnesota, Dept of Chemical Engineering and Materials Science, Minneapolis, MN.
Hydrolytic polycondensation of metal alkoxides and organically modified metal alkoxides shows promise as a method of creating new materials for medical implants. Mathematical models of the processes occurring during fabrication can significantly accelerate the development of new materials and processes for their production. With a reliable model, one can rapidly explore a wide variety of options for controlling materials properties. Often, only reaction conditions such as temperature, reactant concentration, precursor type, etc. are considered as controls during chemical synthesis of organically modified ceramics. However, when other reactor configurations such as semibatch and continuous stirred tanks are considered, the possibility of control of polymer structure by spatial segregation is introduced. We describe a model for copolycondensation of organoalkoxysilanes and show that for certain operating regimes, better control of copolymer structure and homogeneity is possible by using semibatch and continuous reactors rather than batch reactors. Unfortunately, operating conditions must be optimized differently for every different structural (performance) goal. Goals considered are maximizing homogeneity, minimizing the content of cyclic (non-network-forming) species. Other possible goals (with more sophisticated structure-property relationships) are preparing an optimal UV-curable resin for dental composites and creating an organically modified ceramic with the optimal surface properties for medical implantation. Good reaction engineering may allows allow one to make high performance materials from easily obtained materials, for instance the excellent properties of some composites of polypeptides and ceramics found in nature.
SESSION II2: POSTER SESSION
Monday Evening, November 30, 1998
America Ballroom (W)
POROSITY EVOLUTION IN ELECTROSURGICAL BLADE COATINGS. Gregory Konesky , BSD Inc., Hampton Bays, NY.
During Electrosurgery, Radio Frequency (RF) electrical power is applied to a cutting blade to cauterize the incision. The desiccation of surrounding tissue results in surgery without bleeding. In the case of localized heavy bleeding, a coagulation process is employed using modulated RF power. However substantial tissue sticking to the blade may undo the clot thus formed, or even worsen the situation. To prevent this, various electrosurgical blade coatings have been developed to both permit the flow of RF power through the coating, and prevent desiccated tissue from sticking to it. The effectiveness of these coatings are highly variable. The results of studies to assess the effectiveness of several electrosurgical blade coatings are presented. General mechanisms of electrosurgery and their associated waveforms are reviewed. Several coating materials are compared in terms of their change in properties under applied electric field. The evolution of porosity of the coating is shown to be critical to the flow of RF power, maintenance of non-stick qualities, and ultimately, the durability of the coating.
SAPPHIRE MEDICAL IMPLANTS. Leonid Lytvynov , Viacheslav M. Puzikov, Academy of Sciences of Ukraine, Sciences and Technological Concern ´Institute for Single Crystals, Dept of Optical and Constructional Crystals, Kharkov, UKRAINE.
Itis noticed thatin spite of evident inertness the sapphire implants (SI) show osteogenic activity. Study of this event has led us to crystallographic approach to the problem of forming implant boarder-bone tissue, taking in consideration crystallographic correspondence between crystal lattice of implants with that of crystal fibers ( mineral part of the bone tissue), which are inside of microfibrils. Estimates of the level of crystallographic disparity showed good convergence of angular disparity of periodicity and angles between pairs of interconnected atomic chains of sapphire and hydroxilapatite. But SI biocompatibility doesn't guarantee their functional adaptation in a body. For osteogenes stimulation besides tight insertion in a bone socket it is necessary to activate the surface and create retention points. The surface activation is reached by making subsurface layer with high density of dislocations, point defects, pores, micro cracks, which decrease the energy of crystal nucleus formation and accelerate jointing. Besides of constructional hollows on the surface of SI it is desirable to have macro pores 100...300 mm, in which bone tissue can grow through. For forming such micro pores when growing adjusted profiled crystal by Stepanov method we directed flows of the melt so that they collided with the reflected from meniscus wave close to the side surface of the profile. In the places of collision of gas saturated flows the concentration of gas component of the flows increases. In these zones macro pores are formed in crystals and also increased is density of dislocations, vacancies and vacancies complexes. Such SI were designed for stomatology, Maxillo-facial surgery, orthopaedics, traumatology. Different types of sapphire scalpels were designed too for this purpous.
MOLECULARLY REINFORCED BIODEGRADABLE INTERNAL FIXATION DEVICES. Joseph D. Gresser, Debra J. Trantolo, Paul H. Fackler , Roslyn L. White, Donald L. Wise, Cambridge Scientific, Inc., Belmont, MA; Kai-Uwe Lewandrowski, Orthopedic Research Laboratories, Massachusetts General Hospital, Boston, MA.
Internal orthopedic fixation devices fabricated from resorbable polymers have several advantages over metallic devices. However, to ensure dimensional degrading and to match modulus and strength to that of bone, it is necessary to introduce a reinforcing structure for those applications to plate fixation. One approach is to disperse the major structural element, a degradable poly (lactide-co-glycolide) (``PLGA''), within a three dimensional scaffold of degradable crosslinked poly (propylene fumarate) (``PPF''). This scaffolding concept is termed ``molecular reinforcement''. PLGA, a linear polyester and PPF, an unsaturated polyester, form a compatible blend in the presence of the crosslinking agent, vinyl pyrrolidone. This plasticized blend is then compression molded. The support offered to the main structural element, PLGA, by the crosslinked PPF protects the plate against dimensional instability such as warping, arising from unequal rates of degradation on the surfaces exposed to tissue fluid and the surface adjacent to bone. In vivo
evidence of biocompatibility and osteointegration will be presented.
DOUBLE LAYERED HA COATING, ITS CHARACTERIZATION AND PREPARATION. Ping Zhou , Souichiro Asanami, Hiromasa Kawana, Keio Univ, School of Medicine, Dept of Oral Surgery, Tokyo, JAPAN; Masashi Hosonuma, Permelec Electrode Inc, Research and Development Div, Kanagawa, JAPAN; Hiroshi Mitsui, Mitsui Memorial Hospital, Dept of Orthopedic Surgery, Tokyo, JAPAN; Hui Zhou, Nippon Dental Univ, Dept of Dental Materials Science, Tokyo, JAPAN.
Hydroxyapatite(HA) has been universally used because of good biocompatibility. There are varieties of coating methods, however, they still have problems. The most significant problem is layer thickness. We introduced a new coating procedure to eliminate this difficulty. Pure CaCO3
powder was decomposed at 1200C for 3 h to yield CaO. The CaO powder was added to a 2-ethylhexanoic acid solution and then heated to 120C until all the CaO powder was dissolved. N-butanol was added to the resulting gel and stirred to form a transparent solution, and then the HA coating solution was made up. The CaTiO3
coating solution was prepared similar to a method that was used for HA, except that Ti tetraisopropoxide was added instead of the bis(2-ethylhexyl) hydrogen phosphate used for the HA solution. The characterization of the coating was studied by X-ray diffractometry, infrared spectroscopy, SEM and chemical analysis. Said examination indicated that the double layer consisted of carbonate HA and CaTiO3
and the thickness of the layer was 2-4 m. The coating was performed on the inner surfaces of 50-200 m sized pores and was also consistent in the smallest of the pores even those of 50 m. Our results indicated that the coating layer was homogeneous and thin. In addition, both layers consisted of Ca ions. The consistency of coating into even the smallest pore size suggested that our method promises to have widespread clinical applications in artificial implants because of increased surface contact area.
SYNTHESIS, SINTERING, AND MICROSTRUCTURE OF NANOCRYSTALLINE HYDROXYAPATITE. Edward S. Ahn and Jackie Y. Ying, Massachusetts Institute of Technology, Dept of Chemical Engineering, Cambridge, MA.
The synthesis, sintering, and microstructure of nanocrystalline hydroxyapatite (HAP) were examined for suitability as load-bearing implants. HAP was prepared by chemical precipitation, and the effects of pH, precursor concentration, aging time and aging temperature on particle size, particle morphology, phase purity and stoichiometry were examined. Optimal synthesis conditions resulted in powders possessing crystallites less than 50 nm, B.E.T. surface areas greater than 95 m2
/g and ultrafine particle sizes. The powder properties were subsequently correlated to sintering behavior and compact microstructure. Pressure-assisted sintering of the optimized powder resulted in transparent, fully dense HAP at a low temperature of 900C with grain sizes were less than 125 nm. The improvements in the mechanical properties of the resulting materials compared to conventional HAP demonstrate the superior characteristics of nanostructured materials.
SINTERING BEHAVIOR AND MECHANICAL PROPERTIES OF HYDROXYAPATITE-ZIRCONIA COMPOSITES PREPARED BY PRECIPITATION METHOD. Viviane V. Silva, Rosana Zacarias Domingues , Universidade Federal de Minas Gerais, ICEx, Dept of Chemistry, Minas Gerais, BRAZIL.
Hydroxyapatite is one of the materials most biocompatible with human and teeth, but its mechanical properties, especially toughness, are insufficient for hard tissue. Recent studies demonstrated that it is possible to improve ceramics mechanical properties by addition of another phase that remains dispersed in them. The aim of this study was to evaluate the sintering behavior and the mechanical properties such as, compressive strength,fracture toughness and microhardness of composites formed by dispersion of tetragonal calcia-zirconia into hydroxyapatite matrix. All composites were prepared by precipitation method. The influence of some synthesis parameters like pH, filtration time and atmosphere of medium was investigated . Powders of hydroxyapatite-zirconia composites, with zirconia content of 30 and 40 vol , and pure hydroxyapatite as well as pure calcia-zirconia were compacted at 700 MPa and sintered at 900, 1100 and 1200C for 3 hours, in air. The products obtained at 1100 and 1200C were dense, with density values above 91 of the theoretical density. It was assumed a structure of the bi modal compact for composites consisted of a sphere of zirconia phase embedded in a hydroxyapatite matrix, in which the matrix was assumed to sinter faster. The sintering model by brittle fracture mechanism was suggested to illustrate the heterogeneous sintering behavior observed in the composites investigated. Intermediary compressive strength and fracture toughness values, between those of pure hydroxyapatite and zirconia based samples, were found for composites, although these materials presented the lowest values of microhardness. These results suggest that these composites have potential applications in medicine as implant materials.
NANOPOWDERS FROM SYNTHETIC HYDROXYAPATITE FOR MEDICAL IMPLANTS. Evgenii Popov , Tetyana Shmyreva, State Metallurgical Academy, Dnepropetrovsk, UKRAINE.
The biochemical properties of synthetic hydroxyapatite that determine its suitability for surgical implants are mainly dictated by its chemical composition and structure. X-ray diffraction and electron probe microanalysis were employed to compare the structure of synthetic hydroxyapatite prepared by sol-gel method from solutions of calcium nitrate and orthophoshoric acid that of natural biomaterial (Scapula Vaccae).