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MATERIALS RESEARCH

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Mechanical properties of biocompatible protein polymer thin films Christopher J. Buchko, Margaret J. Slattery, Kenneth M. Kozloff, and David C. Martina) 2022 H.H. Dow, Department of Materials Science and Engineering, Macromolecular Science and Engineering Center, University of Michigan, Ann Arbor, Michigan 48109-2136 (Received 30 April 1999; accepted 15 October 1999

A silklike protein with fibronectin functionality (SLPF) (ProNectin F威, Protein Polymer Technologies, Inc.) is a genetically engineered protein polymer containing structural and biofunctional segments. The mechanical properties and deformation mechanisms of electrostatically deposited SLPF thin films were examined by scratch testing, tensile testing, and nanoindentation. Scanning electron microscopy and scanned probe microscopy revealed that the macroscopic properties were a sensitive function of microstructure. The SLPF films were relatively brittle in tension, with typical elongation-to-break values of 3%. Nanoindentation data were fit to a power law relationship.

I. INTRODUCTION

Research in our laboratory has identified schemes for processing genetically engineered protein polymers into porous thin films of controlled morphology as biocompatible coatings for implantable devices.1–3 The devices are micromachined silicon probes implanted in the central nervous system for neural prosthesis.4 These devices can fail during implantation as a result of migration through tissue or encapsulation by scar tissue.5 The stiffness mismatch at the tissue/device interface has the potential to create large interfacial strains during the service lifetime of the device, and minimizing these differences could prevent device failure.3 The protein polymer materials used to coat the devices were polypeptides genetically engineered to combine the structural stability of natural silk with biofunctional properties of natural proteins.6 The protein polymer used primarily in this study was a silklike polymer with fibronectin functionality (SLPF). This paper evaluates the mechanical properties of various morphologies of protein polymer coatings and compares them to the mechanical properties of soft tissue and the stiff silicon device. A value of 172 GPa for the modulus of silicon has been observed from indentation experiments parallel to the (100) face of silicon wafers using a spherical indenter,7 while a value of 0.1 MPa for the modulus of human brain (nonliving tissue at room temperature) was obtained using a dynamic viscoelastometer in a range 3–35 Hz.8 Figure 1 schematically shows this 7 orders of magnitude difference between the moduli of the device a)

Address all correspondence to this author. J. Mater. Res., Vol. 15, No. 1, Jan 2000

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and brain tissue. The presence of a continuous polymer coating provides an intermediate step along this interface. This sharp difference could be mediated further if the polymer coating was stiff and dense on the device side of the interf