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Diamond encapsulated photovoltaics for transdermal power delivery
Author’s Accepted Manuscript Diamond encapsulated transdermal power delivery
photovoltaics
for
A. Ahnood, K.E. Fox, N.V. Apollo, A. Lohrmann, D.J. Garrett, D.A.X. Nayagam, T. Karle, A. Stacey, K.M. Abberton, W.A. Morrison, A. Blakers, S. Prawer www.elsevier.com/locate/bios
PII: DOI: Reference:
S0956-5663(15)30486-3 http://dx.doi.org/10.1016/j.bios.2015.10.022 BIOS8058
To appear in: Biosensors and Bioelectronic Received date: 21 July 2015 Revised date: 3 October 2015 Accepted date: 8 October 2015 Cite this article as: A. Ahnood, K.E. Fox, N.V. Apollo, A. Lohrmann, D.J. Garrett, D.A.X. Nayagam, T. Karle, A. Stacey, K.M. Abberton, W.A. Morrison, A. Blakers and S. Prawer, Diamond encapsulated photovoltaics for transdermal power delivery, Biosensors and Bioelectronic, http://dx.doi.org/10.1016/j.bios.2015.10.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Diamond encapsulated implantable photovoltaics cell
Diamond encapsulated photovoltaics for transdermal power delivery 1†
1,2
,1,3
1
1, 3
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A. Ahnood , K.E. Fox , N.V. Apollo , A. Lohrmann ,D.J. Garrett , D.A.X. Nayagam , T. 1,5 1 6,7,8 6,7,8 9 1 Karle , A. Stacey , K.M. Abberton , W.A. Morrison , A. Blakers , S. Prawer 1
School of Physics, University of Melbourne, Victoria, Australia. School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Victoria, Australia. 3 The Bionics Institute, Victoria, Australia. 4 Department of Pathology, University of Melbourne, Australia. 5 The Florey Institute of Neuroscience and Mental Health, University of Melbourne, Victoria, Australia. 6 O’Brien Institute, Victoria, Australia. 7 Faculty of Health Sciences, Australian Catholic University, Melbourne, Australia. 2
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Department of Surgery, St. Vincent's Hospital, University of Melbourne, Melbourne, Australia.
Centre for Sustainable Energy Systems, College of Engineering and Computer Science, The Australian National University, Canberra, Australia.
[email protected]
A safe, compact and robust means of wireless energy transfer across the skin barrier is a key requirement for implantable electronic devices. One possible approach is photovoltaic (PV) energy delivery using optical illumination at near infrared (NIR) wavelengths, to which the skin is highly transparent. In the work presented here, a subcutaneously implantable silicon PV cell, operated in conjunction with an external NIR laser diode, is developed as a power delivery system. The biocompatibility and long-term biostability of the implantable PV is ensured through the use of an hermetic container, comprising a transparent diamond capsule and platinum wire feedthroughs. A wavelength of 980nm is identified as the optimum operating point based on the PV cell’s external quantum efficiency, the skin’s transmission spectrum, and the wavelength dependent safe exposure 2
limit of the skin. In bench-top experiments using an external illumination intensity of 0.7W/cm , a peak output power of 2.7mW is delivered to the implant with an active PV cell dimension of 1.5 × 1.5 × 3
0.06mm. This corresponds to a volumetric power output density of ~20mW/mm , significantly higher than power densities achievable using inductively coupled coil-based approaches used in other medical implant systems. This approach paves the way for further ministration of bionic implants. Keywords: Photovoltaics, Energy harvesting, Transdermal power delivery
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I) 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 [1] and retina stimulators [2] to low power prostheses such as spinal cord stimulators [3] and cardiac pacemakers [4]. 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 100s of µW or less [5], [6], 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 [7], [8], 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 [9], one major constraint when using a coil-based approach is the implanted coil dimensions. For instance, Ram Rakhyani et. al. [9] recently reported a high efficiency inductively-coupled resonance system, using implant coils with a diameter of 22mm and 2.5mm thickness capable of delivering up to 180mW of power to the prosthesis. This corresponds to a 3
volumetric output power density of 1.9mW/mm . 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 [10], [11] and imaging [12]. 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 [13]. Indeed optically based methods such as particle assisted pulsed laser ablation and photoacoustic tomography have been demonstrated for brain tumour removal [14] and imaging through the skull [15]. 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-
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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 [16]–[18] has been demonstrated in earlier works. However these devices without a suitable non-toxic and hermetic coating are not viable as chronic implants [19] 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 [20], [21], and chemical inertness [21], [22], 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 [23], [24] which offers a desired NIR transparency [25] and long term moisture resistance. A key challenge in adopting such approach is in difficulties in achieving impermeable moisture barrier [26]. 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
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joint [27]. 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.
Fig. 1 Schematic of wireless power delivery system based on the photovoltaic (PV) effect. a) An illustration of the illumination path from the NIR laser diode across the epidermis and dermis layers before reaching the diamond encapsulated PV cell implanted in the subcutaneous pocket. b) Key components of the implantable diamond encapsulated PV cell: transparent single crystal diamond, used as the optical window layer, in combination with the polycrystalline diamond form the full diamond capsule. Pt/Ir wires and Au-ABA is used to form hermetic feedthroughs across the diamond. c) An indication of a candidate implant location site at a subcutaneous pocket above the temporal bone behind the ear.
Earlier work has demonstrated the concept of using a PV cell for energy delivery to implantable medical devices [16]–[18], where in one of the works epoxy was used as the transparent encapsulation [17]. 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 minimallyinvasive and implantable power delivery system. II) Experimental Design and Methods a) Optimum Optical Window Therapeutic photomedicine, at wavelength ranges of 320~800nm has been used as a mean of in vivo photoactivation of medicines, drug delivery, and manipulation of host to maximize therapeutic ~ 4 / 18 ~
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index[28]. Exposure to light at below this wavelength range results in injurey 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 quantum efficiency (EQE) of the PV cell, ii) the skin’s transmission spectrum, and iii) the safe exposure limit of the skin to illumination. Diamond has a constant optical transmission across the wavelengths considered in this work and therefore does not influence the optimum operating wavelength of the device and as such is not considered in the calculations. The transmission at these wavelengths is limited to ~71% due to reflection losses stem from high refractive index of the diamond (2.38) [29]. Although not adopted in this work, the reflection loss at the diamond surface could be mitigated by the use of an anti-reflective coating as well as micro-patterning. Fig 2 a) illustrates the EQE of a typical commercially available c-Si PV cell, with a peak response at wavelengths of ~700nm, where the cell is most efficient, without accounting for light transmission through the skin. A typical transmission spectrum across a 0.5mm thick skin section is adapted from [30] and shown in Fig 2 a). In the range considered in this work, transmission through the skin was highest at a wavelength of ~900nm. The EQE of the c-Si solar cell can be recalculated to account for the skin’s transmission spectrum as depicted in Fig 2 a). As expected, the presence of skin in the optical path of the cell results in an overall reduction in the EQE. Furthermore the higher skin transmission at longer wavelengths results in a slight red shift and broadening of the EQE spectrum of an implanted PV cell. Excessive exposure of the skin to illumination can result in physiological damage [31]. This damage is dependent on the wavelength, duration and intensity of the illumination. Figure 2 b) illustrates the maximum permissible exposure (MPE) limit of continuous wave laser illuminated on the skin over an extended period of time [32]. MPE increases with wavelength with a ratio of approximately 4 between wavelengths of 400nm and 950nm. Based on this exposure intensity limit, a safe photon flux rate at the skin can be calculated as shown in Fig 2 b). The lower photon energy at longer wavelengths results in an increased difference between the short and long wavelengths, with a ratio of approximately 10 between the safe photon flux rate at 400nm and 950nm. The safe photon flux rate, and the EQE spectrum of the implanted PV cell can be used to calculate the carrier generation rate of the PV cell at the safe exposure limit as shown in Fig 2 c) and d). The
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high permissible photon flux at the longer wavelengths, combined with the high skin transmission at longer wavelengths and the EQE spectrum of the c-Si PV results in a sharp peak at the NIR wavelengths of approximately 950nm. Given that the carrier generation rate is directly proportional to the output power of the implanted cell, the optimum operating wavelength of the implantable PV can thus be seen to be around 950nm.
Fig 2 Identification of the optimum operating wavelength of the implantable PV cell. a) External quantum efficiency (EQE) of a typical c-Si PV cell before and after accounting for transmission across skin. b) Maximum permissible exposure limit (MPE) as a function of wavelength for prolonged exposure to continuous wave light source. Based on the MPE, the safe photon flux rate is calculated. c) the carrier generation rate at the MPE limit as a function of the wavelength. d) Scaled plot of carrier generation rate at the MPE limit in the NIR range. The peak generation rate is at 950nm, with 980nm laser identified as a suitable light source.
In this work, 980nm is adopted as the operating wavelength of the energy delivery system based on the availability of a portable and high efficiency InGaAs laser diode source at this wavelength [33]. The MPE in the wavelength range of 700-1050nm can be calculated based on Equation 1, where denotes wavelength [32]. (1) 2
Based on equation 1, MPE of 0.73 W/cm at 980nm is deduced. Fig 2 (a) shows an EQE of 35.5% after accounting for skin optical losses across the skin and the 71% transmission across diamond 2
capsule. Using EQE of 35.5% and MPE of 0.73 W/cm a calculated short circuit current (ISC) of
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~200mA/cm can be derived. Using a typical c-Si PV cell having an open circuit voltage of 0.7V and a 2
Fill Factor of 70%, a theoretical implant peak power is in the order of ~0.1W/cm . 2
It should be noted that the standard solar illumination has an intensity of 100mW/cm . This intensity is spread across a wide range of wavelengths, from UV to visible and inferred and based on 2
the 1.5AM standard solar spectrum only 17.4mW/cm of the solar power is within the 800~1050nm 2
wavelength range. This solar power is 42 times lower than the MPE of 0.73W/cm , suggesting the use of photodiode as the illumination source is far better than solar illumination source. The potential 2
electrical power output targeted in this work is in the range of 100mW/cm , compared with 10uW/cm
2
reported in works using ambient illumination [18]. Furthermore using a narrow wavelength band creates the opportunity for further enhancement of the PV cell at the targeted wavelengths using
nanostructures [34]. Device Fabrication The PV cells were manufactured as 5cm long strips with a width of 1.5mm and thickness of 60µm using SLIVER technology [35]. This technology allows the fabrication of high efficiency silicon solar cells with a thin form factor, ideal for integration with medical implants. Furthermore, the contacts of the PV cell are formed along two edges of the strip facilitating compact integration using flip chip bonding onto interconnects embedded within diamond [36]. The PV strips were diced into smaller cells with geometry of 1.5 mm ×1.5mm × 60µm suitable for use in this work using a 532nm Nd:YAG laser cutter (Alpha series - Oxford Lasers). Commercially available, chemical vapour deposition (CVD) grown diamond plates were used in this work (SC Plate CVD 3.0x3.0mm, 0.30mm thick, <100>, PL, Element Six Ltd.). Optically transparent single crystal diamond plates with dimensions of 3×3×0.3mm were used as the window half of the capsule. The electrical contacts to the solar cell were embedded in the window layer using a fabrication process illustrated in Fig 3 a). Here, laser micromachining was used to create grooves on the surface of the diamond using 532nm Nd:YAG laser cutter (ii). Two 140µm feedthrough holes were laser milled out across the thickness of the diamond (iii). After laser milling, graphitic cutting residues were removed from the substrate in a boiling mixture of 10 mL H2SO4 (conc) containing 1g of NaNO3 for 60 min. Two platinum/iridium (Pt/Ir 90% / 20%) 120 µm diameter wires were inserted into
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the feedthrough holes followed by the application of silver active braze alloy (Ag-ABA, Wesgo metals, Morgan Technical Ceramics) in paste form directly into the groves and over the back of the Pt/Ir feedthroughs (iv). Silver-ABA was melted into the groves and feedthroughs by heating the diamond substrate with an electron beam (Thermionics VE-180 Coating System) to a temperature of 930C -6
under vacuum with pressure of 1×10 bar (v). Once the Ag-ABA melted, the liquidus metal flowed across the diamond surfaces, with the titanium in the Ag-ABA reacting with the diamond to form titanium carbide [27]. The sample was further heated to allow the evaporation of the silver from the Ag- ABA as detected using a quartz crystal thickness monitor. Once the evaporation rate reached zero, the sample was cooled down and removed from the vacuum furnace. Gold ABA (Au-ABA, Wesgo metals, Morgan Technical Ceramics) in paste-form was added and the sample was returned to the vacuum furnace and heated to ~1030C to melt the Au-ABA. The Au-ABA reacted with the residue of the Ag-ABA metallisation thus allowing it to flow uniformly across the sample. This resulted in the formation of a hermetic interface between the Au-ABA and diamond, as well as the diamond/Au-ABA/Pt-Ir wire. Finally, excess braze was removed from the array by mechanical polishing with a 1200 grit diamond impregnated steel wheel on a Coborn PL3 polisher (vi) thus achieving flat gold interconnects embedded within the single crystal diamond. A polycrystalline diamond plate (TM100, Element Six Ltd.) with dimensions of 10×10×0.25mm was used as the backside of the diamond capsule. The fabrication steps for this device are illustrated in Fig 3. b). The diamond plate was laterally cut into a suitable size of 3×3mm (i). A 2×2×0.1mm cube was milled out at the centre of the sample using a laser micromachining to create the necessary cavity for the PV cell encapsulation (ii).
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Fig 3 Fabrication steps of the diamond capsule consisting of two diamond halves. a) single crystal diamond (i) is laser mircomachined to mill out two microchannels (ii) and to cut two feedthrough holes (iii). Pt/Ir wires are placed in the feedthrough holes and braze paste is applied around the platinum wire and in the microchannel (iv). The sample is then heated under vacuum to melt the braze (v) and then mechanically polished to remove excess metal and reveal the contacts forming within the microchannels (vi). b) polycrystalline diamond (i) is laser micromachined to mill out a cavity for the PV cell (ii). c) The two diamond plates are joined to form the final capsule.
The single crystal diamond window with embedded interconnects, PV cell and PCD back of the box were integrated as shown in Fig 4. A Bi (58%)/Sn (42%) solder (CR11 – EDSYN) was used to make electrical connections between the PV cell and the interconnects on the window layer. For proof of concept work epoxy resin (EpoFix, Struers) is used to seal the interface between back of the box and window diamonds in this instance. However we have recently published the technology for making a chronically stable interface using Au-ABA laser welding method [27]; In the future, it is envisioned that laser welding of Au-ABA will instead be used as this approach is compatible with the fabrication process outlined here.
Fig 4 cross sectional diagram of the diamond encapsulated PV cell indicating various materials used within the capsule. Only diamond, platinum and gold braze are exposed to the body. ~ 9 / 18 ~
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Measurement Setup The electrical measurements were performed using a Keithley 2400 Sourcemeter. The illumination source during the measurement was a 980nm Fibre Bragg grating wavelength-stabilised laser (Thorlabs PL980P330J). This NIR excitation was delivered via a 50µm core multimode fibre. The power delivered to the sample was calibrated using a Thorlabs power meter with a Si detector head. In order to test the feasibility of the solar sensor for transdermal power transfer, the PV cell was placed sub-dermal in the anterior pinna of a deceased porcine model (QVM, Melbourne, Victoria). Porcine ear skin is recognised as a trustworthy equivalent for human skin as pigs have comparable structure to humans [37] (i.e. stratum corneum thickness, epidermal thickness, dermal vasculature, collagen arrangement and follicular density) and similar optical properties [38]. Using a razor blade, the full thickness skin layer of the pinna was separated from the cartilage to form a pocket. The porcine pinna was kept hydrated during the study to ensure that the skin and cartilage remained elastic and the dermal thickness constant. The PV cell was positioned in the pocket with the sensor facing the anterior surface of the skin. The pocket was sized to allow the insertion of the entire device. The thickness of the skin overlaying the sensor was measured to be 2mm. The porcine pinna was glued onto a glass frame and vertically positioned on an x-y-z stage. The illumination was delivered from a 980nm laser source to the PV cell as a collimated free space beam through the tissue. The stability of Ag- and Au-ABA braze metals in the body was assessed using a guinea pig animal model. 8 mm diameter diamond disks containing inlaid rings of either Ag-ABA or Au-ABA braze (50µm width and 6mm diameter) on one side were surgically inserted into the back muscle of guinea pigs for a period of 12 weeks, , following an established chronic biocompatibility testing procedure
[39] (St Vincent’s Hospital Melbourne Animal Ethics Committee # 010/11r2). The diamond discs were sequestered into a silicone housing to cover the sharp edges of the diamond discs, leaving only the braze metal exposed to the tissue. The sample’s braze line was designed to better represent a welded diamond capsule which would only have a thin annulus of braze exposed. Au-ABA samples were prepared using silver-ABA adhesion layers as described above. At the 12 week timepoint the
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animals were euthanized and perfused. Samples were removed for scanning electron microscopy (SEM) characterisation. Results and Discussion 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 be seen. The outer dimension of the device is 3×3×0.55 mm, housing the 1.5x1.5x0.06mm active PV cell within it. Given the inherent properties of the diamond, it is perfectly feasible to use a smaller diamond capsule while still maintaining the biocompatibility and biostability of the implant. The device was measured using 980nm illumination through the optically transparent diamond side. The current-voltage characteristics of the PV cell under the safe exposure illumination intensity 2
of 0.7W/cm is shown in Fig 5 c). The PV cell under this illumination archives an open circuit voltage (VOC) of 0.64V, a short circuit current (ISC) of 6.38mA and a fill factor (FF) of 66%. This corresponds to a peak power output of 2.7mW, which when normalised to the total volume of the device yields a 3
volumetric power density of 0.6W/mm . The power conversion efficiency here is ~17%. In this work, readily available commercial diamond plate has been used, with thicknesses in excess of what is required for hermetic encapsulation. Indeed, the PV cell with dimensions of 1.5×1.5×0.06 mm, only occupies 3% of the total capsule volume. This presents a significant opportunity to further increase 3
the volumetric energy density of the implant, up to the maximum limit of ~20mW/mm , by fabricating a diamond capsule that matches the dimensions of the PV cell more closely. Fig 5 d) illustrates the variation of ISC and VOC with illumination intensity. At lower intensities, there 2
is a sharp increase in the VOC as the light intensity is raised up to ~0.6W/cm .Beyond this intensity, the increase in the VOC with the illumination intensity is fairly constant. The linear increase in the ISC with the illumination intensity is expected for this type of solar cell. However, at illumination intensities
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above ~1W/cm , the series resistance of the PV cell starts to limit the rate of increase in the ISC increase in the illumination intensities. Indeed, as illustrated in the insert of Fig 5 d) the increase in the VOC with ISC follows the expected logarithmic behaviour as expected in conventional c-Si PV cells [40]. It is expected that the laser cutting process results in the creation of defects along the sidewalls of the solar cell [41]. The large sidewall area relative to the PV cell’s lateral surface area further exacerbates the recombination losses. This will lead to a reduction in the PV cell’s efficiency. Further reduction in the generated defects or their passivation is possible by adopting suitable fabrication processes. Further improvements in the cell efficiency can be achieved through improved coupling of the light into the diamond capsule. As discussed earlier, diamond’s high refractive index results in a 71% transmission into the capsule. Antireflective coatings or surface texturing have been used to improve the optical transmission across diamond plate where values as high as 82% have been reported [42]. Similar approaches maybe useful in this work, to reduce back reflection at the capsule wall and thus increase the illumination intensity at the PV cell.
Fig. 5. a) Cross sectional image of the PV cell soldered to the single crystal diamond prior to the placement of the PCD back of the capsule. b) Top view, from the transparent single crystal diamond window side of the capsule after the placement of the PCD. c) Current-voltage characteristics of the device exposed to 980nm wavelength 2 with an MEP intensity of ~0.7W/cm . d) Variation of the short circuit current and open circuit voltage with the . illumination intensity. Insert here demonstrates the expected logarithmic dependence of V OC to ISC
Cadaver Study
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A cadaver experiment facilitated a preliminary investigation of the functionality of the PV cell surrounded by skin tissue. Skin is a complex optical medium [30]. This experiment captures various optical effects of skin and the subsequent effect on the PV cell performance. Figure 6 a) illustrates the experimental schematic adopted in here. The PV cell was implanted in a subcutaneous pocket in porcine ear skin. The thickness of overlay tissue was measured to be approximately 2mm. The PV cell was measured both with the device covered with the overlay skin and without the skin cover. Fig 6 b) illustrates the short circuit current of the PV cell at a range of illumination intensities of 20 to 55 2
mW/cm . Here the presence of the overlay skin results in between 52~65% reduction in the I SC, suggesting that 35~48% of the illumination is absorbed by the skin. This value is significantly higher than ~15% reduction at 980nm, shown in Fig 2 a). One possible explanation is the thickness of the skin in the cadaver study (~2mm) compared to the plotted values in in Fig 2 a) (~0.5mm). This illumination intensity used in the cadaver study is below the MPE intensities. The lower illumination intensity proved necessary in order to prevent the rise in the tissue temperature and subsequent change in its properties. Nevertheless, heating effects were evident as observed by the increase in the optical transmission ratio, in line with earlier works [43]. At the highest illumination intensity in this measurement, the VOC were 444mV and 394mV for conditions of without and with overlay skin respectively.
Fig. 6. a) Schematic illustration of the cadaver experiment in which the PV cell was inserted into the subcutaneous pocket of porcine ear skin. The skin thickness here was ~2mm. b) Short-circuit current as a function of the optical illumination intensity measured with the PV cell covered with skin layer, or directly exposed without the skin layer. Here the skin overlay results in 52~65% reduction in the short circuit current. The rise in the transmission ratio with optical power can be attributed to the increase in the tissue temperature.
In this work, chronic implantation on the performance of the diamond encapsulated PV cell has not been investigated. The formation of any possible tissue over time could result in a change in optical ~ 13 / 18 ~
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coupling of the light in to the implant and subsequent change in the PV cell. SLIVER PV technology also offers the possibility of a flexible implant, which over larger dimension will minimise the effect of tissue scaring. Material Biostability Although diamond and gold braze both have a proven record of biostability [27], their combined use within a system and in particular the use of Au-ABA is an important consideration. In particular, the biostability of the interface between the diamond and Au-ABA, where a titanium carbide adhesion layer is formed, is considered. Fig. 7. shows the effect of chronic implantation on the diamond and brazing materials. Fig 7 a) illustrates the degradation of Ag-ABA and subsequent corrosion along the braze/diamond interface. If used within an implant, this would result in the loss of hermeticity and leakage of the body fluids. Fig 7 a) and b) illustrate Au-ABA braze before and after implantation respectively. Here, no corrosion of the brazing alloy was observed, suggesting that diamond/Au-ABA is a bio-stable material system suitable for chronic implantation. Histopathology of the tissue surrounding the implanted samples revealed minimal inflammatory and foreign body responses to the Au-ABA compared with standard medical-grade silicone control samples. Conversely, the Ag-diabraze-467 did evoke a significant inflammatory response after 12 weeks in vivo [27]. As such, the Au-ABA braze would be preferable for use in implantable devices.
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Fig 7. Stability of the braze metal on diamond substrate during a 12 week chronic implantation experiment. a) and b) illustrate Ag-ABA before and after implantation respectively. Here a corrosion of silver due to exposure to bodily fluids is evident. c) and d) illustrates Au-ABA prior and after implantation respectively. No evidence of corrosion is observed, suggesting the biostability of AuABA/diamond interface.
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 980nm 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 2
illumination intensity of 0.7W/cm , peak output power of 2.7mW is delivered to the implant with an active PV cell area of 1.5 × 1.5 × 0.06mm. This corresponds to a volumetric power output density of 3
~20mW/mm . The use of c-Si PV fabricated with SLIVER technology enabled a significantly higher volumetric power output density than those achievable using coil-based approaches. Although the focus of this work is power delivery, laser pulse modulation can be used to transmit data as well as power across the skin barrier [44]. The optical to electrical conversion efficiency demonstrated in this proof of principle work is 17%. NIR laser diodes with an electrical to optical efficiency of 64% have been demonstrated [45]. Based ~ 15 / 18 ~
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on this an overall system efficiency of 10.9% can be envisaged. Further improvement in implantable PV cell through reduction in the sidewall defects as well as increase in the optical light coupling is possible resulting in an increase in overall system efficiency. The PV cell is capable of generating or direct current output. This is likely to permit a simplification of the power management circuitry, compared with the ones used for inductively coupled resonance coils, with a potential gain in the system efficiency. Nevertheless, overall system efficiencies of 70% have been reported in earlier works using the inductively coupled resonance coils [46]. The lower overall system efficiency of the PV power delivery system, compared to that of coil-based approach can be addressed through the use of the emerging battery technologies with high power and energy storage density [47] as well as mobile energy harvesting systems [48]. The key advantage of energy delivery using photovoltaics is its high volumetric energy density, which paves the way for further ministration of bionic implants. Acknowledgment 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. References [1] [2] [3] [4] [5] [6] [7]
[8]
[9]
G. Clark, “Cochlear Implants: Fundamentals and Applications,” in Cochlear Implants: Fundamentals and Applications, Springer Science & Business Media, 2003, p. 459. J. D. Weiland, W. Liu, and M. S. Humayun, “Retinal Prosthesis,” Annu. Rev. Biomed. Eng., vol. 7, no. 1, pp. 361–401, 2005. T. Cameron, “Safety and efficacy of spinal cord stimulation for the treatment of chronic pain: a 20-year literature review,” J. Neurosurg. Spine, vol. 100, no. 3, pp. 254–267, Mar. 2004. V. S. Mallela, V. Ilankumaran, and N. S. Rao, “Trends in Cardiac Pacemaker Batteries,” Indian Pacing Electrophysiol. J., vol. 4, no. 4, pp. 201–212, Oct. 2004. M. Belleville and C. Condemine, “Energy Autonomous Micro and Nano Systems,” in Energy Autonomous Micro and Nano Systems, John Wiley & Sons, 2012, p. 2.2.3. M. Rasouli and L. S. J. Phee, “Energy sources and their development for application in medical devices,” Expert Rev. Med. Devices, vol. 7, no. 5, pp. 693–709, Sep. 2010. B. E. Swartz, J. R. Rich, P. S. Dwan, A. DeSalles, M. H. Kaufman, G. O. Walsh, and A. V. Delgado-Escueta, “The safety and efficacy of chronically implanted subdural electrodes: a prospective study,” Surg. Neurol., vol. 46, no. 1, pp. 87–93, Jul. 1996. W. S. Lee, J. K. Lee, S. A. Lee, J. K. Kang, and T. S. Ko, “Complications and results of subdural grid electrode implantation in epilepsy surgery,” Surg. Neurol., vol. 54, no. 5, pp. 346–351, Nov. 2000. A. K. RamRakhyani, S. Mirabbasi, and M. Chiao, “Design and Optimization of ResonanceBased Efficient Wireless Power Delivery Systems for Biomedical Implants,” IEEE Trans. Biomed. Circuits Syst., vol. 5, no. 1, pp. 48–63, Feb. 2011. ~ 16 / 18 ~
Diamond encapsulated implantable photovoltaics cell [10] A. Vogel and V. Venugopalan, “Mechanisms of Pulsed Laser Ablation of Biological Tissues,” Chem. Rev., vol. 103, no. 2, pp. 577–644, Feb. 2003. [11] D. P. O’Neal, L. R. Hirsch, N. J. Halas, J. D. Payne, and J. L. West, “Photo-thermal tumor ablation in mice using near infrared-absorbing nanoparticles,” Cancer Lett., vol. 209, no. 2, pp. 171–176, Jun. 2004. [12] M. Xu and L. V. Wang, “Photoacoustic imaging in biomedicine,” Rev. Sci. Instrum., vol. 77, no. 4, p. 041101, Apr. 2006. [13] R. Weissleder, “A clearer vision for in vivo imaging,” Nat. Biotechnol., vol. 19, no. 4, pp. 316– 317, Apr. 2001. [14] J. A. Schwartz, A. M. Shetty, R. E. Price, R. J. Stafford, J. C. Wang, R. K. Uthamanthil, K. Pham, R. J. McNichols, C. L. Coleman, and J. D. Payne, “Feasibility Study of Particle-Assisted Laser Ablation of Brain Tumors in Orthotopic Canine Model,” Cancer Res., vol. 69, no. 4, pp. 1659–1667, Feb. 2009. [15] L. Nie, X. Cai, K. Maslov, A. Garcia-Uribe, M. A. Anastasio, and L. V. Wang, “Photoacoustic tomography through a whole adult human skull with a photon recycler,” J. Biomed. Opt., vol. 17, no. 11, Nov. 2012. [16] C. Algora and R. Peña, “Recharging the battery of implantable biomedical devices by light,” Artif. Organs, vol. 33, no. 10, pp. 855–860, Oct. 2009. [17] K. Goto, T. Nakagawa, O. Nakamura, and S. Kawata, “An implantable power supply with an optically rechargeable lithium battery,” IEEE Trans. Biomed. Eng., vol. 48, no. 7, pp. 830–833, Jul. 2001. [18] S. Ayazian and A. Hassibi, “Delivering optical power to subcutaneous implanted devices,” Conf. Proc. Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. IEEE Eng. Med. Biol. Soc. Annu. Conf., vol. 2011, pp. 2874–2877, 2011. [19] H. Hämmerle, K. Kobuch, K. Kohler, W. Nisch, H. Sachs, and M. Stelzle, “Biostability of microphotodiode arrays for subretinal implantation,” Biomaterials, vol. 23, no. 3, pp. 797–804, Feb. 2002. [20] P. Bajaj, D. Akin, A. Gupta, D. Sherman, B. Shi, O. Auciello, and R. Bashir, “Ultrananocrystalline diamond film as an optimal cell interface for biomedical applications,” Biomed. Microdevices, vol. 9, no. 6, pp. 787–794, Dec. 2007. [21] W. Tong, K. Fox, K. Ganesan, A. M. Turnley, O. Shimoni, P. A. Tran, A. Lohrmann, T. McFarlane, A. Ahnood, D. J. Garrett, H. Meffin, N. M. O’Brien-Simpson, E. C. Reynolds, and S. Prawer, “Fabrication of planarised conductively patterned diamond for bio-applications,” Mater. Sci. Eng. C, vol. 43, pp. 135–144, Oct. 2014. [22] D. Zhou and E. Greenbaum, Implantable Neural Prostheses 2: Techniques and Engineering Approaches. Springer Science & Business Media, 2010. [23] E. M. Schmidt, J. S. Mcintosh, and M. J. Bak, “Long-term implants of Parylene-C coated microelectrodes,” Med. Biol. Eng. Comput., vol. 26, no. 1, pp. 96–101, Jan. 1988. [24] C. Hassler, R. P. von Metzen, P. Ruther, and T. Stieglitz, “Characterization of parylene C as an encapsulation material for implanted neural prostheses,” J. Biomed. Mater. Res. B Appl. Biomater., vol. 93B, no. 1, pp. 266–274, Apr. 2010. [25] Y. S. Jeong, B. Ratier, A. Moliton, and L. Guyard, “UV–visible and infrared characterization of poly(p-xylylene) films for waveguide applications and OLED encapsulation,” Synth. Met., vol. 127, no. 1–3, pp. 189–193, Mar. 2002. [26] G. Jiang and D. D. Zhou, “Technology Advances and Challenges in Hermetic Packaging for Implantable Medical Devices,” in Implantable Neural Prostheses 2, D. Zhou and E. Greenbaum, Eds. Springer New York, 2009, pp. 27–61. [27] S. G. Lichter, M. C. Escudié, A. D. Stacey, K. Ganesan, K. Fox, A. Ahnood, N. V. Apollo, D. C. Kua, A. Z. Lee, C. McGowan, A. L. Saunders, O. Burns, D. A. X. Nayagam, R. A. Williams, D. J. Garrett, H. Meffin, and S. Prawer, “Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys,” Biomaterials, vol. 53, pp. 464–474, Jun. 2015. [28] J. A. Parrish, “New Concepts in Therapeutic Photomedicine; Photochemistry, Optical Targeting and the Therapeutic Window,” J. Invest. Dermatol., vol. 77, no. 1, pp. 45–50, Jul. 1981. [29] S. E. Coe and R. S. Sussmann, “Optical, thermal and mechanical properties of CVD diamond,” Diam. Relat. Mater., vol. 9, no. 9–10, pp. 1726–1729, Sep. 2000. [30] A. N. Bashkatov, E. A. Genina, V. I. Kochubey, and V. V. Tuchin, “Optical properties of human skin, subcutaneous and mucous tissues in the wavelength range from 400 to 2000 nm,” J. Phys. Appl. Phys., vol. 38, no. 15, p. 2543, Aug. 2005. [31] A. J. Welch, “The thermal response of laser irradiated tissue,” IEEE J. Quantum Electron., vol. 20, no. 12, pp. 1471–1481, Dec. 1984. ~ 17 / 18 ~
Diamond encapsulated implantable photovoltaics cell [32] “ANSI Z136. 1-2000: for Safe Use of Lasers,” Publ. Laser Inst. Am., 2000. [33] M. Buda, J. Hay, H. H. Tan, J. Wong-Leung, and C. Jagadish, “Low loss, thin p-clad 980-nm InGaAs semiconductor laser diodes with an asymmetric structure design,” IEEE J. Quantum Electron., vol. 39, no. 5, pp. 625–633, May 2003. [34] A. Ahnood, H. Zhou, Q. Dai, Y. Vygranenko, Y. Suzuki, M. Esmaeili-Rad, G. Amaratunga, and A. Nathan, “Vertical CNT-Si Photodiode Array,” Nano Lett., vol. 13, no. 9, pp. 4131–4136, 2013. [35] P. J. Verlinden, A. W. Blakers, K. J. Weber, J. Babaei, V. Everett, M. J. Kerr, M. F. Stuckings, D. Gordeev, and M. J. Stocks, “Sliver® solar cells: A new thin-crystalline silicon photovoltaic technology,” Sol. Energy Mater. Sol. Cells, vol. 90, no. 18–19, pp. 3422–3430, Nov. 2006. [36] A. Ahnood, M. C. Escudie, R. Cicione, C. D. Abeyrathne, K. Ganesan, K. E. Fox, D. J. Garrett, A. Stacey, N. V. Apollo, S. G. Lichter, C. D. L. Thomas, N. Tran, H. Meffin, and S. Prawer, “Ultrananocrystalline diamond-CMOS device integration route for high acuity retinal prostheses,” Biomed. Microdevices, vol. 17, no. 3, pp. 1–11, Apr. 2015. [37] B. Godin and E. Touitou, “Transdermal skin delivery: predictions for humans from in vivo, ex vivo and animal models,” Adv. Drug Deliv. Rev., vol. 59, no. 11, pp. 1152–1161, Sep. 2007. [38] Y. Du, X. H. Hu, M. Cariveau, X. Ma, G. W. Kalmus, and J. Q. Lu, “Optical properties of porcine skin dermis between 900 nm and 1500 nm,” Phys. Med. Biol., vol. 46, no. 1, p. 167, Jan. 2001. [39] D. A. X. Nayagam, R. A. Williams, J. Chen, K. A. Magee, J. Irwin, J. Tan, P. Innis, R. T. Leung, S. Finch, C. E. Williams, G. M. Clark, and G. G. Wallace, “Biocompatibility of Immobilized Aligned Carbon Nanotubes,” Small, vol. 7, no. 8, pp. 1035–1042, Apr. 2011. [40] N. H. Reich, W. G. J. H. M. van Sark, E. A. Alsema, R. W. Lof, R. E. I. Schropp, W. C. Sinke, and W. C. Turkenburg, “Crystalline silicon cell performance at low light intensities,” Sol. Energy Mater. Sol. Cells, vol. 93, no. 9, pp. 1471–1481, Sep. 2009. [41] M. Abbott, P. Cousins, F. Chen, and J. Cotter, “Laser-induced defects in crystalline silicon solar cells,” in Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005, 2005, pp. 1241–1244. [42] D. S. Hobbs, “Study of the environmental and optical durability of AR microstructures in sapphire, ALON, and diamond,” 2009, vol. 7302, p. 73020J–73020J–12. [43] H. Abdussamad Abbas and G. E. Triplett, “Infrared Transmission Characteristics and Laser Tissue Interaction of Albino Pigskin Using Pulsed NIR Laser Light,” Int. Sch. Res. Not., vol. 2013, p. e451208, Jan. 2013. [44] F. R. Gfeller and U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” Proc. IEEE, vol. 67, no. 11, pp. 1474–1486, Nov. 1979. [45] V. Rossin, E. Zucker, M. Peters, M. Everett, and B. Acklin, “High-power high-efficiency 910- to 980-nm broad-area laser diodes,” 2004, vol. 5336, pp. 196–202. [46] M. Pinuela, D. C. Yates, S. Lucyszyn, and P. D. Mitcheson, “Maximizing DC-to-Load Efficiency for Inductive Power Transfer,” IEEE Trans. Power Electron., vol. 28, no. 5, pp. 2437–2447, May 2013. [47] S. W. Lee, N. Yabuuchi, B. M. Gallant, S. Chen, B.-S. Kim, P. T. Hammond, and Y. Shao-Horn, “High-power lithium batteries from functionalized carbon-nanotube electrodes,” Nat. Nanotechnol., vol. 5, no. 7, pp. 531–537, Jul. 2010. [48] A. Ahnood and A. Nathan, “Flat-Panel Compatible Photovoltaic Energy Harvesting System,” J. Disp. Technol., vol. 8, no. 4, pp. 204–211, 2012.
Highlights Photovoltaic power delivery system for bioelectronics implants Superior volumetric power output density of ~20mW/mm3. Diamond encapsulation and packaging for long term implant stability.
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photovoltaics
for
A. Ahnood, K.E. Fox, N.V. Apollo, A. Lohrmann, D.J. Garrett, D.A.X. Nayagam, T. Karle, A. Stacey, K.M. Abberton, W.A. Morrison, A. Blakers, S. Prawer www.elsevier.com/locate/bios
PII: DOI: Reference:
S0956-5663(15)30486-3 http://dx.doi.org/10.1016/j.bios.2015.10.022 BIOS8058
To appear in: Biosensors and Bioelectronic Received date: 21 July 2015 Revised date: 3 October 2015 Accepted date: 8 October 2015 Cite this article as: A. Ahnood, K.E. Fox, N.V. Apollo, A. Lohrmann, D.J. Garrett, D.A.X. Nayagam, T. Karle, A. Stacey, K.M. Abberton, W.A. Morrison, A. Blakers and S. Prawer, Diamond encapsulated photovoltaics for transdermal power delivery, Biosensors and Bioelectronic, http://dx.doi.org/10.1016/j.bios.2015.10.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Diamond encapsulated implantable photovoltaics cell
Diamond encapsulated photovoltaics for transdermal power delivery 1†
1,2
,1,3
1
1, 3
3,4
A. Ahnood , K.E. Fox , N.V. Apollo , A. Lohrmann ,D.J. Garrett , D.A.X. Nayagam , T. 1,5 1 6,7,8 6,7,8 9 1 Karle , A. Stacey , K.M. Abberton , W.A. Morrison , A. Blakers , S. Prawer 1
School of Physics, University of Melbourne, Victoria, Australia. School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Victoria, Australia. 3 The Bionics Institute, Victoria, Australia. 4 Department of Pathology, University of Melbourne, Australia. 5 The Florey Institute of Neuroscience and Mental Health, University of Melbourne, Victoria, Australia. 6 O’Brien Institute, Victoria, Australia. 7 Faculty of Health Sciences, Australian Catholic University, Melbourne, Australia. 2
8
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†
Department of Surgery, St. Vincent's Hospital, University of Melbourne, Melbourne, Australia.
Centre for Sustainable Energy Systems, College of Engineering and Computer Science, The Australian National University, Canberra, Australia.
[email protected]
A safe, compact and robust means of wireless energy transfer across the skin barrier is a key requirement for implantable electronic devices. One possible approach is photovoltaic (PV) energy delivery using optical illumination at near infrared (NIR) wavelengths, to which the skin is highly transparent. In the work presented here, a subcutaneously implantable silicon PV cell, operated in conjunction with an external NIR laser diode, is developed as a power delivery system. The biocompatibility and long-term biostability of the implantable PV is ensured through the use of an hermetic container, comprising a transparent diamond capsule and platinum wire feedthroughs. A wavelength of 980nm is identified as the optimum operating point based on the PV cell’s external quantum efficiency, the skin’s transmission spectrum, and the wavelength dependent safe exposure 2
limit of the skin. In bench-top experiments using an external illumination intensity of 0.7W/cm , a peak output power of 2.7mW is delivered to the implant with an active PV cell dimension of 1.5 × 1.5 × 3
0.06mm. This corresponds to a volumetric power output density of ~20mW/mm , significantly higher than power densities achievable using inductively coupled coil-based approaches used in other medical implant systems. This approach paves the way for further ministration of bionic implants. Keywords: Photovoltaics, Energy harvesting, Transdermal power delivery
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Diamond encapsulated implantable photovoltaics cell
I) 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 [1] and retina stimulators [2] to low power prostheses such as spinal cord stimulators [3] and cardiac pacemakers [4]. 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 100s of µW or less [5], [6], 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 [7], [8], 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 [9], one major constraint when using a coil-based approach is the implanted coil dimensions. For instance, Ram Rakhyani et. al. [9] recently reported a high efficiency inductively-coupled resonance system, using implant coils with a diameter of 22mm and 2.5mm thickness capable of delivering up to 180mW of power to the prosthesis. This corresponds to a 3
volumetric output power density of 1.9mW/mm . 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 [10], [11] and imaging [12]. 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 [13]. Indeed optically based methods such as particle assisted pulsed laser ablation and photoacoustic tomography have been demonstrated for brain tumour removal [14] and imaging through the skull [15]. 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-
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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 [16]–[18] has been demonstrated in earlier works. However these devices without a suitable non-toxic and hermetic coating are not viable as chronic implants [19] 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 [20], [21], and chemical inertness [21], [22], 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 [23], [24] which offers a desired NIR transparency [25] and long term moisture resistance. A key challenge in adopting such approach is in difficulties in achieving impermeable moisture barrier [26]. 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
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joint [27]. 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.
Fig. 1 Schematic of wireless power delivery system based on the photovoltaic (PV) effect. a) An illustration of the illumination path from the NIR laser diode across the epidermis and dermis layers before reaching the diamond encapsulated PV cell implanted in the subcutaneous pocket. b) Key components of the implantable diamond encapsulated PV cell: transparent single crystal diamond, used as the optical window layer, in combination with the polycrystalline diamond form the full diamond capsule. Pt/Ir wires and Au-ABA is used to form hermetic feedthroughs across the diamond. c) An indication of a candidate implant location site at a subcutaneous pocket above the temporal bone behind the ear.
Earlier work has demonstrated the concept of using a PV cell for energy delivery to implantable medical devices [16]–[18], where in one of the works epoxy was used as the transparent encapsulation [17]. 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 minimallyinvasive and implantable power delivery system. II) Experimental Design and Methods a) Optimum Optical Window Therapeutic photomedicine, at wavelength ranges of 320~800nm has been used as a mean of in vivo photoactivation of medicines, drug delivery, and manipulation of host to maximize therapeutic ~ 4 / 18 ~
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index[28]. Exposure to light at below this wavelength range results in injurey 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 quantum efficiency (EQE) of the PV cell, ii) the skin’s transmission spectrum, and iii) the safe exposure limit of the skin to illumination. Diamond has a constant optical transmission across the wavelengths considered in this work and therefore does not influence the optimum operating wavelength of the device and as such is not considered in the calculations. The transmission at these wavelengths is limited to ~71% due to reflection losses stem from high refractive index of the diamond (2.38) [29]. Although not adopted in this work, the reflection loss at the diamond surface could be mitigated by the use of an anti-reflective coating as well as micro-patterning. Fig 2 a) illustrates the EQE of a typical commercially available c-Si PV cell, with a peak response at wavelengths of ~700nm, where the cell is most efficient, without accounting for light transmission through the skin. A typical transmission spectrum across a 0.5mm thick skin section is adapted from [30] and shown in Fig 2 a). In the range considered in this work, transmission through the skin was highest at a wavelength of ~900nm. The EQE of the c-Si solar cell can be recalculated to account for the skin’s transmission spectrum as depicted in Fig 2 a). As expected, the presence of skin in the optical path of the cell results in an overall reduction in the EQE. Furthermore the higher skin transmission at longer wavelengths results in a slight red shift and broadening of the EQE spectrum of an implanted PV cell. Excessive exposure of the skin to illumination can result in physiological damage [31]. This damage is dependent on the wavelength, duration and intensity of the illumination. Figure 2 b) illustrates the maximum permissible exposure (MPE) limit of continuous wave laser illuminated on the skin over an extended period of time [32]. MPE increases with wavelength with a ratio of approximately 4 between wavelengths of 400nm and 950nm. Based on this exposure intensity limit, a safe photon flux rate at the skin can be calculated as shown in Fig 2 b). The lower photon energy at longer wavelengths results in an increased difference between the short and long wavelengths, with a ratio of approximately 10 between the safe photon flux rate at 400nm and 950nm. The safe photon flux rate, and the EQE spectrum of the implanted PV cell can be used to calculate the carrier generation rate of the PV cell at the safe exposure limit as shown in Fig 2 c) and d). The
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high permissible photon flux at the longer wavelengths, combined with the high skin transmission at longer wavelengths and the EQE spectrum of the c-Si PV results in a sharp peak at the NIR wavelengths of approximately 950nm. Given that the carrier generation rate is directly proportional to the output power of the implanted cell, the optimum operating wavelength of the implantable PV can thus be seen to be around 950nm.
Fig 2 Identification of the optimum operating wavelength of the implantable PV cell. a) External quantum efficiency (EQE) of a typical c-Si PV cell before and after accounting for transmission across skin. b) Maximum permissible exposure limit (MPE) as a function of wavelength for prolonged exposure to continuous wave light source. Based on the MPE, the safe photon flux rate is calculated. c) the carrier generation rate at the MPE limit as a function of the wavelength. d) Scaled plot of carrier generation rate at the MPE limit in the NIR range. The peak generation rate is at 950nm, with 980nm laser identified as a suitable light source.
In this work, 980nm is adopted as the operating wavelength of the energy delivery system based on the availability of a portable and high efficiency InGaAs laser diode source at this wavelength [33]. The MPE in the wavelength range of 700-1050nm can be calculated based on Equation 1, where denotes wavelength [32]. (1) 2
Based on equation 1, MPE of 0.73 W/cm at 980nm is deduced. Fig 2 (a) shows an EQE of 35.5% after accounting for skin optical losses across the skin and the 71% transmission across diamond 2
capsule. Using EQE of 35.5% and MPE of 0.73 W/cm a calculated short circuit current (ISC) of
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Diamond encapsulated implantable photovoltaics cell 2
~200mA/cm can be derived. Using a typical c-Si PV cell having an open circuit voltage of 0.7V and a 2
Fill Factor of 70%, a theoretical implant peak power is in the order of ~0.1W/cm . 2
It should be noted that the standard solar illumination has an intensity of 100mW/cm . This intensity is spread across a wide range of wavelengths, from UV to visible and inferred and based on 2
the 1.5AM standard solar spectrum only 17.4mW/cm of the solar power is within the 800~1050nm 2
wavelength range. This solar power is 42 times lower than the MPE of 0.73W/cm , suggesting the use of photodiode as the illumination source is far better than solar illumination source. The potential 2
electrical power output targeted in this work is in the range of 100mW/cm , compared with 10uW/cm
2
reported in works using ambient illumination [18]. Furthermore using a narrow wavelength band creates the opportunity for further enhancement of the PV cell at the targeted wavelengths using
nanostructures [34]. Device Fabrication The PV cells were manufactured as 5cm long strips with a width of 1.5mm and thickness of 60µm using SLIVER technology [35]. This technology allows the fabrication of high efficiency silicon solar cells with a thin form factor, ideal for integration with medical implants. Furthermore, the contacts of the PV cell are formed along two edges of the strip facilitating compact integration using flip chip bonding onto interconnects embedded within diamond [36]. The PV strips were diced into smaller cells with geometry of 1.5 mm ×1.5mm × 60µm suitable for use in this work using a 532nm Nd:YAG laser cutter (Alpha series - Oxford Lasers). Commercially available, chemical vapour deposition (CVD) grown diamond plates were used in this work (SC Plate CVD 3.0x3.0mm, 0.30mm thick, <100>, PL, Element Six Ltd.). Optically transparent single crystal diamond plates with dimensions of 3×3×0.3mm were used as the window half of the capsule. The electrical contacts to the solar cell were embedded in the window layer using a fabrication process illustrated in Fig 3 a). Here, laser micromachining was used to create grooves on the surface of the diamond using 532nm Nd:YAG laser cutter (ii). Two 140µm feedthrough holes were laser milled out across the thickness of the diamond (iii). After laser milling, graphitic cutting residues were removed from the substrate in a boiling mixture of 10 mL H2SO4 (conc) containing 1g of NaNO3 for 60 min. Two platinum/iridium (Pt/Ir 90% / 20%) 120 µm diameter wires were inserted into
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the feedthrough holes followed by the application of silver active braze alloy (Ag-ABA, Wesgo metals, Morgan Technical Ceramics) in paste form directly into the groves and over the back of the Pt/Ir feedthroughs (iv). Silver-ABA was melted into the groves and feedthroughs by heating the diamond substrate with an electron beam (Thermionics VE-180 Coating System) to a temperature of 930C -6
under vacuum with pressure of 1×10 bar (v). Once the Ag-ABA melted, the liquidus metal flowed across the diamond surfaces, with the titanium in the Ag-ABA reacting with the diamond to form titanium carbide [27]. The sample was further heated to allow the evaporation of the silver from the Ag- ABA as detected using a quartz crystal thickness monitor. Once the evaporation rate reached zero, the sample was cooled down and removed from the vacuum furnace. Gold ABA (Au-ABA, Wesgo metals, Morgan Technical Ceramics) in paste-form was added and the sample was returned to the vacuum furnace and heated to ~1030C to melt the Au-ABA. The Au-ABA reacted with the residue of the Ag-ABA metallisation thus allowing it to flow uniformly across the sample. This resulted in the formation of a hermetic interface between the Au-ABA and diamond, as well as the diamond/Au-ABA/Pt-Ir wire. Finally, excess braze was removed from the array by mechanical polishing with a 1200 grit diamond impregnated steel wheel on a Coborn PL3 polisher (vi) thus achieving flat gold interconnects embedded within the single crystal diamond. A polycrystalline diamond plate (TM100, Element Six Ltd.) with dimensions of 10×10×0.25mm was used as the backside of the diamond capsule. The fabrication steps for this device are illustrated in Fig 3. b). The diamond plate was laterally cut into a suitable size of 3×3mm (i). A 2×2×0.1mm cube was milled out at the centre of the sample using a laser micromachining to create the necessary cavity for the PV cell encapsulation (ii).
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Fig 3 Fabrication steps of the diamond capsule consisting of two diamond halves. a) single crystal diamond (i) is laser mircomachined to mill out two microchannels (ii) and to cut two feedthrough holes (iii). Pt/Ir wires are placed in the feedthrough holes and braze paste is applied around the platinum wire and in the microchannel (iv). The sample is then heated under vacuum to melt the braze (v) and then mechanically polished to remove excess metal and reveal the contacts forming within the microchannels (vi). b) polycrystalline diamond (i) is laser micromachined to mill out a cavity for the PV cell (ii). c) The two diamond plates are joined to form the final capsule.
The single crystal diamond window with embedded interconnects, PV cell and PCD back of the box were integrated as shown in Fig 4. A Bi (58%)/Sn (42%) solder (CR11 – EDSYN) was used to make electrical connections between the PV cell and the interconnects on the window layer. For proof of concept work epoxy resin (EpoFix, Struers) is used to seal the interface between back of the box and window diamonds in this instance. However we have recently published the technology for making a chronically stable interface using Au-ABA laser welding method [27]; In the future, it is envisioned that laser welding of Au-ABA will instead be used as this approach is compatible with the fabrication process outlined here.
Fig 4 cross sectional diagram of the diamond encapsulated PV cell indicating various materials used within the capsule. Only diamond, platinum and gold braze are exposed to the body. ~ 9 / 18 ~
Diamond encapsulated implantable photovoltaics cell
Measurement Setup The electrical measurements were performed using a Keithley 2400 Sourcemeter. The illumination source during the measurement was a 980nm Fibre Bragg grating wavelength-stabilised laser (Thorlabs PL980P330J). This NIR excitation was delivered via a 50µm core multimode fibre. The power delivered to the sample was calibrated using a Thorlabs power meter with a Si detector head. In order to test the feasibility of the solar sensor for transdermal power transfer, the PV cell was placed sub-dermal in the anterior pinna of a deceased porcine model (QVM, Melbourne, Victoria). Porcine ear skin is recognised as a trustworthy equivalent for human skin as pigs have comparable structure to humans [37] (i.e. stratum corneum thickness, epidermal thickness, dermal vasculature, collagen arrangement and follicular density) and similar optical properties [38]. Using a razor blade, the full thickness skin layer of the pinna was separated from the cartilage to form a pocket. The porcine pinna was kept hydrated during the study to ensure that the skin and cartilage remained elastic and the dermal thickness constant. The PV cell was positioned in the pocket with the sensor facing the anterior surface of the skin. The pocket was sized to allow the insertion of the entire device. The thickness of the skin overlaying the sensor was measured to be 2mm. The porcine pinna was glued onto a glass frame and vertically positioned on an x-y-z stage. The illumination was delivered from a 980nm laser source to the PV cell as a collimated free space beam through the tissue. The stability of Ag- and Au-ABA braze metals in the body was assessed using a guinea pig animal model. 8 mm diameter diamond disks containing inlaid rings of either Ag-ABA or Au-ABA braze (50µm width and 6mm diameter) on one side were surgically inserted into the back muscle of guinea pigs for a period of 12 weeks, , following an established chronic biocompatibility testing procedure
[39] (St Vincent’s Hospital Melbourne Animal Ethics Committee # 010/11r2). The diamond discs were sequestered into a silicone housing to cover the sharp edges of the diamond discs, leaving only the braze metal exposed to the tissue. The sample’s braze line was designed to better represent a welded diamond capsule which would only have a thin annulus of braze exposed. Au-ABA samples were prepared using silver-ABA adhesion layers as described above. At the 12 week timepoint the
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animals were euthanized and perfused. Samples were removed for scanning electron microscopy (SEM) characterisation. Results and Discussion 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 be seen. The outer dimension of the device is 3×3×0.55 mm, housing the 1.5x1.5x0.06mm active PV cell within it. Given the inherent properties of the diamond, it is perfectly feasible to use a smaller diamond capsule while still maintaining the biocompatibility and biostability of the implant. The device was measured using 980nm illumination through the optically transparent diamond side. The current-voltage characteristics of the PV cell under the safe exposure illumination intensity 2
of 0.7W/cm is shown in Fig 5 c). The PV cell under this illumination archives an open circuit voltage (VOC) of 0.64V, a short circuit current (ISC) of 6.38mA and a fill factor (FF) of 66%. This corresponds to a peak power output of 2.7mW, which when normalised to the total volume of the device yields a 3
volumetric power density of 0.6W/mm . The power conversion efficiency here is ~17%. In this work, readily available commercial diamond plate has been used, with thicknesses in excess of what is required for hermetic encapsulation. Indeed, the PV cell with dimensions of 1.5×1.5×0.06 mm, only occupies 3% of the total capsule volume. This presents a significant opportunity to further increase 3
the volumetric energy density of the implant, up to the maximum limit of ~20mW/mm , by fabricating a diamond capsule that matches the dimensions of the PV cell more closely. Fig 5 d) illustrates the variation of ISC and VOC with illumination intensity. At lower intensities, there 2
is a sharp increase in the VOC as the light intensity is raised up to ~0.6W/cm .Beyond this intensity, the increase in the VOC with the illumination intensity is fairly constant. The linear increase in the ISC with the illumination intensity is expected for this type of solar cell. However, at illumination intensities
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Diamond encapsulated implantable photovoltaics cell 2
above ~1W/cm , the series resistance of the PV cell starts to limit the rate of increase in the ISC increase in the illumination intensities. Indeed, as illustrated in the insert of Fig 5 d) the increase in the VOC with ISC follows the expected logarithmic behaviour as expected in conventional c-Si PV cells [40]. It is expected that the laser cutting process results in the creation of defects along the sidewalls of the solar cell [41]. The large sidewall area relative to the PV cell’s lateral surface area further exacerbates the recombination losses. This will lead to a reduction in the PV cell’s efficiency. Further reduction in the generated defects or their passivation is possible by adopting suitable fabrication processes. Further improvements in the cell efficiency can be achieved through improved coupling of the light into the diamond capsule. As discussed earlier, diamond’s high refractive index results in a 71% transmission into the capsule. Antireflective coatings or surface texturing have been used to improve the optical transmission across diamond plate where values as high as 82% have been reported [42]. Similar approaches maybe useful in this work, to reduce back reflection at the capsule wall and thus increase the illumination intensity at the PV cell.
Fig. 5. a) Cross sectional image of the PV cell soldered to the single crystal diamond prior to the placement of the PCD back of the capsule. b) Top view, from the transparent single crystal diamond window side of the capsule after the placement of the PCD. c) Current-voltage characteristics of the device exposed to 980nm wavelength 2 with an MEP intensity of ~0.7W/cm . d) Variation of the short circuit current and open circuit voltage with the . illumination intensity. Insert here demonstrates the expected logarithmic dependence of V OC to ISC
Cadaver Study
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A cadaver experiment facilitated a preliminary investigation of the functionality of the PV cell surrounded by skin tissue. Skin is a complex optical medium [30]. This experiment captures various optical effects of skin and the subsequent effect on the PV cell performance. Figure 6 a) illustrates the experimental schematic adopted in here. The PV cell was implanted in a subcutaneous pocket in porcine ear skin. The thickness of overlay tissue was measured to be approximately 2mm. The PV cell was measured both with the device covered with the overlay skin and without the skin cover. Fig 6 b) illustrates the short circuit current of the PV cell at a range of illumination intensities of 20 to 55 2
mW/cm . Here the presence of the overlay skin results in between 52~65% reduction in the I SC, suggesting that 35~48% of the illumination is absorbed by the skin. This value is significantly higher than ~15% reduction at 980nm, shown in Fig 2 a). One possible explanation is the thickness of the skin in the cadaver study (~2mm) compared to the plotted values in in Fig 2 a) (~0.5mm). This illumination intensity used in the cadaver study is below the MPE intensities. The lower illumination intensity proved necessary in order to prevent the rise in the tissue temperature and subsequent change in its properties. Nevertheless, heating effects were evident as observed by the increase in the optical transmission ratio, in line with earlier works [43]. At the highest illumination intensity in this measurement, the VOC were 444mV and 394mV for conditions of without and with overlay skin respectively.
Fig. 6. a) Schematic illustration of the cadaver experiment in which the PV cell was inserted into the subcutaneous pocket of porcine ear skin. The skin thickness here was ~2mm. b) Short-circuit current as a function of the optical illumination intensity measured with the PV cell covered with skin layer, or directly exposed without the skin layer. Here the skin overlay results in 52~65% reduction in the short circuit current. The rise in the transmission ratio with optical power can be attributed to the increase in the tissue temperature.
In this work, chronic implantation on the performance of the diamond encapsulated PV cell has not been investigated. The formation of any possible tissue over time could result in a change in optical ~ 13 / 18 ~
Diamond encapsulated implantable photovoltaics cell
coupling of the light in to the implant and subsequent change in the PV cell. SLIVER PV technology also offers the possibility of a flexible implant, which over larger dimension will minimise the effect of tissue scaring. Material Biostability Although diamond and gold braze both have a proven record of biostability [27], their combined use within a system and in particular the use of Au-ABA is an important consideration. In particular, the biostability of the interface between the diamond and Au-ABA, where a titanium carbide adhesion layer is formed, is considered. Fig. 7. shows the effect of chronic implantation on the diamond and brazing materials. Fig 7 a) illustrates the degradation of Ag-ABA and subsequent corrosion along the braze/diamond interface. If used within an implant, this would result in the loss of hermeticity and leakage of the body fluids. Fig 7 a) and b) illustrate Au-ABA braze before and after implantation respectively. Here, no corrosion of the brazing alloy was observed, suggesting that diamond/Au-ABA is a bio-stable material system suitable for chronic implantation. Histopathology of the tissue surrounding the implanted samples revealed minimal inflammatory and foreign body responses to the Au-ABA compared with standard medical-grade silicone control samples. Conversely, the Ag-diabraze-467 did evoke a significant inflammatory response after 12 weeks in vivo [27]. As such, the Au-ABA braze would be preferable for use in implantable devices.
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Fig 7. Stability of the braze metal on diamond substrate during a 12 week chronic implantation experiment. a) and b) illustrate Ag-ABA before and after implantation respectively. Here a corrosion of silver due to exposure to bodily fluids is evident. c) and d) illustrates Au-ABA prior and after implantation respectively. No evidence of corrosion is observed, suggesting the biostability of AuABA/diamond interface.
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 980nm 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 2
illumination intensity of 0.7W/cm , peak output power of 2.7mW is delivered to the implant with an active PV cell area of 1.5 × 1.5 × 0.06mm. This corresponds to a volumetric power output density of 3
~20mW/mm . The use of c-Si PV fabricated with SLIVER technology enabled a significantly higher volumetric power output density than those achievable using coil-based approaches. Although the focus of this work is power delivery, laser pulse modulation can be used to transmit data as well as power across the skin barrier [44]. The optical to electrical conversion efficiency demonstrated in this proof of principle work is 17%. NIR laser diodes with an electrical to optical efficiency of 64% have been demonstrated [45]. Based ~ 15 / 18 ~
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on this an overall system efficiency of 10.9% can be envisaged. Further improvement in implantable PV cell through reduction in the sidewall defects as well as increase in the optical light coupling is possible resulting in an increase in overall system efficiency. The PV cell is capable of generating or direct current output. This is likely to permit a simplification of the power management circuitry, compared with the ones used for inductively coupled resonance coils, with a potential gain in the system efficiency. Nevertheless, overall system efficiencies of 70% have been reported in earlier works using the inductively coupled resonance coils [46]. The lower overall system efficiency of the PV power delivery system, compared to that of coil-based approach can be addressed through the use of the emerging battery technologies with high power and energy storage density [47] as well as mobile energy harvesting systems [48]. The key advantage of energy delivery using photovoltaics is its high volumetric energy density, which paves the way for further ministration of bionic implants. Acknowledgment 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. References [1] [2] [3] [4] [5] [6] [7]
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Highlights Photovoltaic power delivery system for bioelectronics implants Superior volumetric power output density of ~20mW/mm3. Diamond encapsulation and packaging for long term implant stability.
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