Diamond encapsulated photovoltaics for transdermal power delivery
Introduction
There has been significant progress in the development of electronic medical prostheses, ranging from the high power consumption devices such as the cochlear implant (Clark, 2003) and retina stimulators (Weiland et al., 2005) to low power prostheses such as spinal cord stimulators (Cameron, 2004) and cardiac pacemakers (Mallela et al., 2004). Delivery of electrical power in a safe, robust and minimally invasive manner is a critical aspect of electronic prosthesis design. Whilst low power prostheses, with consumption of 100 s of µW or less (Belleville and Condemine, 2012; Rasouli and Phee, 2010), use implanted batteries to meet their energy needs over extended periods of time, higher power consumption prostheses (e.g. cochlear or visual implants) require continuous power delivery from external sources. The two widely used methods for continuous power delivery are (i) percutaneous plug transmission and (ii) inductively coupled resonance coils. Percutaneous plugs, where lead wires permanently cross the skin barrier (Swartz et al., 1996; Lee et al., 2000), carry a risk of infection and thus make such an approach unfavourable. In comparison, power transmission using inductively coupled resonance coils allows wireless power delivery across the skin and therefore minimising infection risk. Despite their high power transmission efficiency (RamRakhyani et al., 2011), one major constraint when using a coil-based approach is the implanted coil dimensions. For instance, RamRakhyani et al. (2011) recently reported a high efficiency inductively-coupled resonance system, using implant coils with a diameter of 22 mm and 2.5 mm thickness capable of delivering up to 180 mW of power to the prosthesis. This corresponds to a volumetric output power density of 1.9 mW/mm3. Despite the advantages of coil-based approaches, these methods may not be suitable for applications where there exists a geometrical constraint on the implant, such as retinal prostheses that are designed to fit inside the eyeball.
Optical methods have widely been used in medicine as a mean for treatment and diagnoses. Wavelength dependent optical properties of tissue allows light to be used for laser ablation of tissues (Vogel and Venugopalan, 2003; O’Neal et al., 2004) and imaging (Xu and Wang, 2006). The high optical transparency of biological tissues at certain wavelengths allows these methods to be used at depths of least 10 cm through breast tissue, and 4 cm of skull/brain tissue (Weissleder, 2001). Indeed optically based methods such as particle assisted pulsed laser ablation and photoacoustic tomography have been demonstrated for brain tumour removal (Schwartz et al., 2009) and imaging through the skull (Nie et al., 2012). In this work we investigate wireless power transfer across the skin barrier using the photovoltaic effect. The system, illustrated in Fig. 1(a), consists of two components: i) a diamond-encapsulated crystalline silicon (c-Si) photovoltaic (PV) cell, and ii) an external laser diode powered using a battery pack. The PV implant supplies electrical power to the prosthesis (e.g. visual or cochlear) via lead wires under the skin. As discussed in the following section, the combination of a) skin transparency, b) high safe illumination intensity, and c) photoresponse of the PV cell makes the near infrared (NIR) range an ideal operating wavelength window for a subcutaneously implanted PV energy delivery system.
The concept of using photovoltaic type devices for energy delivery to implantable medical devices (Algora and Peña, 2009; Ayazian and Hassibi, 2011, Goto et al., 2001) has been demonstrated in earlier works. However these devices without a suitable non-toxic and hermetic coating are not viable as chronic implants (Hämmerle et al., 2002) due to their tendency to leach toxic elements into the surrounding biological fluid (resulting in adverse body reaction) and physically degrade in physiological conditions (resulting in device degradation and failure). This raises the need for a durable and biocompatible encapsulation and packaging strategy for any c-Si PV cells. In this work, a c-Si PV cell is protected from body fluids with a transparent diamond capsule. The wide transmission spectrum of diamond makes it suitable for use as an optical window for PV implants, while the inherent properties of the diamond, such as its mechanical robustness, biocompatibility (Bajaj et al., 2007; Tong et al., 2014), and chemical inertness (Tong et al., 2014; Zhou and Greenbaum, 2010), make it ideal for use as a long lasting clinical implant. Polymer encapsulation is an alternative approach for biocompatible encapsulation of the PV cell. Parylene-C is a benchmark thin-film coating for biomedical implants (Schmidt et al., 1988; Hassler et al., 2010) which offers a desired NIR transparency (Jeong et al., 2002) and long term moisture resistance. A key challenge in adopting such approach is in difficulties in achieving impermeable moisture barrier (Jiang and Zhou, 2009). This is particularly the case at feedthroughs were lead wires exit the implant. Here break in the encapsulation is required to allow lead wires to cross the package which create a potential for moisture ingress at the parylene/metal interface and subsequent package failure.
Fig. 1(b) illustrates the various components of the diamond encapsulated implantable PV cell. The capsule consists of two free-standing diamond plates, the first an optically transparent single crystal diamond, and the second an opaque polycrystalline diamond (PCD). Hermetically sealed electrical feedthroughs across the diamond capsule are achieved using platinum/iridium (Pt/Ir) wires and gold active brazing alloy (Au-ABA). In earlier work, we have demonstrated the laser welding of Au-ABA annuli for joining two diamond capsule halves to create a biocompatible, hermetically sealed joint (Lichter et al., 2015). Fig. 1(c) shows a candidate location for the implant in a subcutaneous pocket above the temporal bone behind the ear. The minimal movement of the skin, and hair follicles as well as the thin dermis at this location makes for a viable surgical site.
Earlier work has demonstrated the concept of using a PV cell for energy delivery to implantable medical devices (Algora and Peña, 2009; Goto et al., 2001; Ayazian and Hassibi, 2011), where in one of the works epoxy was used as the transparent encapsulation (Goto et al., 2001). Given the permeability of epoxy, we aim to extend this work by demonstrating a diamond encapsulated PV cell with the biostability and biocompatibility required for a chronically implanted prosthesis. We have systematically calculated the optimum operating wavelength of the system in order to deliver the maximum output power. Despite the higher efficiency of coil-based approaches, we demonstrate a higher volumetric power density, creating the potential for a minimally-invasive and implantable power delivery system.
Section snippets
Optimum optical window
Therapeutic photomedicine, at wavelength ranges of 320–800 nm has been used as a mean of in vivo photoactivation of medicines, drug delivery, and manipulation of host to maximise therapeutic index (Parrish, 1981). Exposure to light at below this wavelength range results in injury to the tissue and is typically avoided. At wavelengths above this range, the interaction of the light with tissue is minimal. Three factors determine the optimum operating wavelength of the PV implant: i) external
Current voltage characteristics
A diamond encapsulated PV cell was fabricated using the method outlined above. Fig. 5(a) shows a side-view of the device prior to the placement of the opaque PCD. The PV cell was flip chip bonded to the diamond embedded interconnects using solder bumps. Fig. 5(b) illustrates a top view of the device after the placement of PCD, and therefore shows the complete device. The image is taken from the optically transparent diamond side so the PV cell connected to the diamond embedded interconnects can
Conclusions
A safe, compact and robust mean of wireless power delivery using the photovoltaic effect is demonstrated. Transparent diamond encapsulation is used to ensure the long term biostability and biocompatibility of the c-Si PV cell. Wavelength of 980 nm is identified as the optimum operating point based on a) the PV cell's external quantum efficiency, b) the skin's transmission spectrum, and c) the wavelength-dependent safe exposure limit of the skin. In bench top experiments using external
Acknowledgement
This research was supported by the Melbourne Materials Institute's seed grant, University of Melbourne. Lisa Cardamone and Alexia Saunders assisted with the chronic experiments. N.V.A. is supported by a MMI-CSIRO Materials Science PhD scholarship. D.J.G. is supported by an Australian Research Council (ARC) DECRA grant DE130100922. The Bionics Institute acknowledges the support received from the Victorian Government through its Operational Infrastructure Program.
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