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|Biodegradable packaging for solid thin film batteries for transient medical applications|
|Keywords: biodegradable packaging, energy sources, solid thin film battery|
|Over the past few years, we are witnessing to an increasing interest to biodegradable electronics especially for research in a variety of medical applications. Specific examples include the monitoring and treatment of transient diseases, such as wound healing, bone fracture, traumatic brain injury, and drug delivery systems. The emergent healthcare strategies for shorter-term medical applications preconize the recourse to implantable biodegradable electronic devices, as such devices offer the potential to provide therapeutic functions for limited and predetermined periods without need of secondary surgical removal. On the other way, the biodegradable electronics offer a new challenge in the field of energy sources by providing an alternative means to power active transient devices. Thus, the solid Thin Film Batteries (TFB) offer a unique combination of mechanical, electrochemical, electrical and safety properties that are attractive as an energy-storage option for medical applications. However, there are a number of future challenges that face biodegradable TFB including active and passive materials. In this respect, the ideal solid TFB should be biocompatible, and comprise biodegradable materials that would satisfy device power requirements and benignly degrade thereafter. Indeed, the effectiveness of biodegradable TFB is considerably based on the judicious design of packaging strategies that can ensure an equilibrium between device stability and bioabsoprtion time lines. This work introduces materials and fabrication strategies for fully biodegradable packaging for energy- storage source. To date, the common way to provide energy storage devices with higher performances and more advance functions, suggests the use of standard microelectronics techniques to fabricate solid TFB (<20µm). However, the active compounds of TFB including cathodes and electrolyte layers are usually fabricated at high temperature range (>400°C) which requires de facto the use of thick and rigid substrates (e.g. glass, silicon). As these substrates cannot be bioabsorbed normally either in metabolic or natural pathway, the principle idea presented in this work consist to slimming down these substrates up to the seed layer. With that in mind, the proposal concept for the biodegradable packaging is primarily based on the combination of thin barrier film (5-300 nm) with relatively thick polymer (10-30µm). The foundational role of barrier films is to protect solid thin film battery from moisture in vivo environment while the polymer provides a mechanical rigidity to the TFB device to compensate the slimmed substrate. The material list includes a series of inorganic candidates (e.g. alumina, titanium oxide, silicon nitride, silicon oxide) deposited by ALD (Atomic Layer Deposition) at 80°C or PECVD (Plasma Enhanced Chemical Vapor Deposition) at 150°C and organic candidates (e.g. chitosan, PLA/PLGA) obtained by spin coating or microdispensing techniques. In vitro degradation tests were performed using various vehicle tests (substrates with and without battery active materials) for several weeks at different temperatures in Phosphate Buffer Saline (PBS) and Hank’s Balanced Salt Solution (HBSS). The different films have been characterized by infrared spectroscopy, scanning electron microscopy (SEM) and complementary analysis (spectroscopic ellipsometry, X-ray photoelectron spectroscopy, Electrochemical Impedance Spectroscopy) before and after exposition to PBS and HBSS mediums. The first results seem to indicate that ALD materials (Al2O3 and TiO2) have slight stability in PBS solution at 37°C and 67°C with an excellent behavior as gas barrier. In contrast, films of chitosan, PLA/PLGA, Al2O3, SiO2 and SiN show relative chemical reactivity in both mediums. Thus, multilayer of alumina and chitosan or PLA/PLGA offers the best solution as biodegradable packaging for solid thin film batteries.|
|Messaoud BEDJAOUI, PhD Engineer