Energy Harvesting by Subcutaneous Solar Cells A L

Authors: L Bereuter S Williner F Pianezzi B Bissig S Buecheler J Burger R Vogel A Zurbuchen A Haeberlin

Publish Date: 2017/01/03

Volume: 45, Issue: 5, Pages: 1172-1180

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Abstract

Active electronic implants are powered by primary batteries which induces the necessity of implant replacement after battery depletion This causes repeated interventions in a patients’ life which bears the risk of complications and is costly By using energy harvesting devices to power the implant device replacements may be avoided and the device size may be reduced dramatically Recently several groups presented prototypes of implants powered by subcutaneous solar cells However data about the expected reallife power output of subcutaneously implanted solar cells was lacking so far In this study we report the first reallife validation data of energy harvesting by subcutaneous solar cells Portable light measurement devices that feature solar cells cell area = 36 cm2 and continuously measure a subcutaneous solar cell’s output power were built The measurement devices were worn by volunteers in their daily routine in summer autumn and winter In addition to the measured output power influences such as season weather and human activity were analyzed The obtained mean power over the whole study period was 67 µW =19 µW cm−2 which is sufficient to power eg a cardiac pacemakerElectronic implants are usually battery powered rarely with a rechargeable battery—which requires repeated recharging—or with a primary battery which requires an implant replacement when the battery is depleted In fact implant replacements due to battery depletion are common and account for approximately 25 of implantations of cardiac pacemakers which represent the majority of electronic implants12 These reinterventions cause costs and expose the patient to a risk of complications Moreover it may be a stressful intervention for the patient Finally the size of an electronic implant is mainly governed by the battery volume ie it could be designed smaller if not equipped with primary batteriesAs a promising alternative energy source ambient sunlight could be used Sunlight is a reliable and omnipresent energy source and a fraction of the ambient light penetrates the human skin In particular nearinfrared light features good skin penetration1 Thus an implant is irradiated yet implanted under the skin and the penetrating light could be converted into electrical energy by solar cellsRecently subcutaneously implantable pacemakers as well as sensors powered by solar cells have been proposed to overcome the batteryrelated limitations of contemporary devices The working principle of such devices has been proposed by several groups from a technical and biological point of view3671516 However precise knowledge of the actual light exposure and expectable power output of such an implant in everyday life is lacking so farTo investigate the reallife feasibility of an implant powered by solar cells a wearable measurement device was developed that gathers the output power of solar cells suitable for such implants A longterm validation study with 32 volunteers was performed for the duration of six months in Central Europe Switzerland The volunteers wore the measurement devices in their daily routine to determine the generated power as well as the influence of other factors such as weather or human behaviorA wearable measurement device that features solar cells was built The primary task of this measurement device is to continuously monitor the solar cells’ output power A key element is that the solar cells are covered by optical filters to simulate subcutaneously implanted solar cellsBlock diagram of the measurement device showing the ambient light 1 which is attenuated by optical filters 2 that mimic the optical properties of human skin The attenuated light 3 irradiates the solar cells 4 which are connected to a maximum power point tracker 7 The solar cells’ output power is monitored by a current and voltage measurement circuit 5 6 A microcontroller 8 controls the measurement and stores the data onto a memory card 9

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