Thin Film Piezoelectric Energy Scavenging Systems for Long Term Medical Monitoring Elizabeth K. Reilly, Eric Carleton, and Paul K. Wright University of California Berkeley, Berkeley CA [email protected] Abstract For small, inexpensive, ubiquitous wireless sensors to be realized, all constituents of the device, including the power source, must be directly integratable. For long term application the device must be capable of scavenging power from its surrounding environment. An apparent solution lies in conversion of mechanical energy to electrical output via the growth and direct integration of piezoelectric thin films unimorphs with the wireless electronics. 1. Introduction The use of wireless sensor networks for long term medical monitoring and intervention devices represents a special set of challenges for the development of a renewable power source. The specificity of the medical application requires a power source that is virtually self sufficient and does not interact with its surroundings. Fixed-energy alternatives, such as batteries, are impractical for wireless devices with an expected lifetime of more than 10 years because the importance of the application of these devices precludes changing or recharging of batteries1. Current energy scavenging technology is highly focused on environmental phenomena such as solar and wind power, yet clearly these are not as applicable in the case of specialized monitoring devices. The body represents an excellence source of thermal as well as mechanical energy. Thermal gradients are present on the surface of the skin and may be used for external skin mounted sensors. However, for more ubiquitous monitoring, vibrational energy scavenging is a viable source of renewable energy. Piezoelectric materials are perfect candidates for vibrational energy scavenging as they can efficiently convert mechanical strain to an electrical charge without any additional power2. Several bulk piezoelectric generators have been developed using the d31 piezoelectric mode3-5. 2. Conversion design The energy conversion from mechanical vibration into electrical power can be described using the elements of linear spring mass system with electrical and mechanical damping terms (Eq. 1). ymkzzbbzmme&&&&&−=+++)( Where z is the output tip displacement, y the base displacement, m the lumped mass, k the spring constant, bm the mechanical damping coefficient, and be is the electrical damping coefficient6. Figure 1: Schematic of generic vibration converter Using this simple model the power output of this system at resonance is )(42meeAmPξξωξ+= k m be bm y(t) z(t) (1) (2) 0-7695-2547-4/06 $20.00 © 2006 IEEE 38Where b=2mξωn and represents the relative damping ratio, A the acceleration input of the input vibration, and ω the operation frequency. The system should be designed so that they mechanically resonate at a frequency tuned with ambient vibration source in order to generate maximum electrical power. The scavengaeable power decreases by half is the applied frequency deviates 2% from the resonant frequency and is almost completely diminished if the frequency deviation is more than 5% from resonance.5 Considering the characteristics of the source vibration, the power decreases as frequency increases because the decreasing input vibration amplitude dominates the increasing frequency contribution. A study done by Roundy et al.2 shows that most of the available environmental vibrations are at lower frequencies. Therefore in order to maximize the power output the energy scavenging device should resonate at a frequency lower that 1 kHz. 3. Device design considerations The ultimate goal of energy scavenging devices for medical monitoring is the realization of a completely integrated, microfabricated sensor node. To create an ubiquitous device of this nature requires careful consideration of a number of materials and processing difficulties, each of which predicate certain constraints on the materials selection and processing. Specifically, the concerns are: 1) the piezoelectric material must have sufficient material properties to produce a useable voltage under strain and have an intimate crystallographic and interfacial contact with the electrodes and growth substrate; 2) the growth, fabrication, and integration of the power source must consist solely of standard microfabrication processes; 3) the design of the energy scavenging device (with optimized piezoelectric layers) must be able to produce sufficient voltage under ambient vibrational excitation. Although many piezoelectric materials exist and are commercially available as thin films (thickness on the order of 1 micron), few of them exhibit large enough piezoelectric coefficients to generate appreciable voltage while under strain. The Pb(Zr,Ti)O3 (PZT) family of piezoelectrics in general have large piezoelectric coefficients and can be grown epitaxially with careful selection of growth substrate and electrodes (typically oxide electrodes, such as SrRuO3 (SRO) and LaSr0.5Co0.5O3 (LSCO)). Epitaxial, or pseudo-single crystal, thin films are attractive because the piezoelectric coefficients, mechanical constants, and dielectric properties of the films can be an order of magnitude higher than polycrystalline films of the same composition. Also, epitaxial growth processes result in a very intimate contact with the electrode which can reduce both mechanical fatigue and cyclic depolarization. The requirement for all growth, fabrication, and integration steps to utilize standard microfabrication techniques necessitates the use of silicon wafer substrates and fabrication processes. Although the literature points to many methods for achieving growth of PZT on Si, most of these processes involve elaborate methods and multiple depositions of exotic buffer layers, most of which are not necessarily compatible with standard post-growth fabrication processes such as selective wet etching. An exciting alternative has been recently developed by Motorola [US Patent 2004] and involves the deposition of only one buffer layer, SrTiO3, which is an excellent growth template and allows for the growth of epitaxial PZT thin films. Using SRO as an electrode also allows the use of typical wet and dry etch fabrication processes, thus permitting a design based completely on standard microfabrication processes. Even assuming that a complete microfabrication