Laser Fabrication and Characterization of Adhesive-Free Joints For Encapsulation of Biomedical Implant Devices
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Laser Fabrication and Characterization of Adhesive-Free Joints For Encapsulation of Biomedical Implant Devices G. Newaz, D.G. Georgiev, A. Mian, G. Auner, H. Herfurth1, R. Witte1 Center for Smart Sensors and Integrated Microsystems, College of Engineering, Wayne State University, Detroit, MI 48202, U.S.A. 1 Fraunhofer Center for Laser Technology, 46025 Port St., Plymouth, MI 48170, U.S.A. ABSTRACT Laser-fabricated joints of sub-millimeter widths between biocompatible, dissimilar materials have the potential for applications as encapsulation of miniature implant biomedical devices. In this work, we briefly describe the laser joining method of a very promising system, polyimide/titanium-coated borosilicate glass, and present and discuss results from characterization of such laser joints by means of mechanical failure (tensile) tests, optical microscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Our results suggest the formation of strong chemical bonds between Ti-containing species and certain polymeric functional groups. Mechanical tensile strength failure test showed that such joint experience only limited, disappearing with time degradation as a result of soaking in physiological solutions. INTRODUCTION The design of the encapsulation of advanced medical implants, such as devices that electrically stimulate and / or record neural activity, poses a number of serious requirements to the materials that are used. Among these requirements, biocompatibility and long-term stability are crucial. Often, the functionality of such devices requires the employment of dissimilar materials for which there is no reliable ways of joining that would also satisfy the need for biocompatibility and stability. Poor biocompatibility and lack of long-term stability, high heat input during soldering, and adhesive shrinkage during cure are indeed among the major drawbacks of the existing joining techniques. Such limitations can be overcome by laser joining techniques that intrinsically provide excellent focusing to spot sizes in the micrometer range, combined with precise control of the laser power in the focal spot to enable highly localized processing with minimum heat effect outside the joint region. Recently we studied the nature of polyimide/bulk-Ti laser-fabricated joints, which were made in ambient conditions and our results gave evidence for a very sharp interface based on the creation of chemical bonds between Ti and O and C contained in polyimide [1]. Reactive metals like Ti are known to have low mobility in polymers and to form relatively sharp interfaces with them [2,3]. Titanium is believed to interact primarily with the oxygen double bonded to carbon in the carbonyl groups >C=O, which form part of the PI molecular chains, and to form bonding configurations of the type Ti-O-C. XPS results have given evidence for the existence of TiO2 particles [4] and Ti-C [5] bonds at such interfaces (obtained and examined under clean, vacuum conditions), and theoretical ab initio self-consistent-field calculatio
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