Simulation of Photovoltaic Cells for Implantable Sensory Applications

Wireless biomedical implantable devices provide a variety of applications based on identification, health, and safety of mankind. Power harvesting and power generation methods through human tissues are still looming challenges because of low efficiency and energy instability. The minimum tissue loss at the optical transparency windows of 650 nm-1350 nm. Photovoltaic cells can be effectively used to provide the necessary power for these implantable devices. However, there have been no previous investigations into the optimum dimensions nor properties of these solar cells. In this case, we show an accurate multi-physics simulation of the performance of photovoltaic cells for implantable devices under the skin. A combination of semiconductor and optical simulations are developed in order to analyse the electro-optic behaviour of these cells. In addition, the efficiencies of 8.97 % and 0.26 % were evaluated under air and air-skin multilayer respectively

Modelling of Implantable Photovoltaic Cell based on Human Skin Types

Implantable electronic devices are emerging as important healthcare technologies due to their sustainable operation and low risk of infection. To overcome the drawbacks of the built-in battery in implantable devices, energy harvesting from the human body or another external source is required. Energy harvesting using appropriately sized and properly designed photovoltaic cells enable implantable medical devices to be autonomous and self-powered. Among the challenges in using PV cells is the small fraction of incident light that penetrates the skin. Thus, it is necessary to involve such physical properties in the energy harvesting system design. Consequently, we propose a novel photodiode model that considers skin loss in different ethnic groups. Our physical simulations have been implemented using COMSOL and MATLAB. Circuit and system modelling have been performed using Cadence 180nm TSMC technology. Our results show that the transmittance of near infrared light is almost the same in three skin types: Caucasian, Asian and African. Maximum power delivery of 12 μW (African skin) and 14 μW (Caucasian and Asian skin) were achieved at 0.45 V. With the help of a power management unit, an output voltage of 1.8–2 V was achieved using the PV cells

An Implantable Photovoltaic Energy Harvesting System With Skin Optical Analysis

Medical implantable devices can use photovoltaic (PV) energy harvesting to extend battery life span and increase their performance. The power conditioning and management circuitry is essential not only to regulate the voltage requirements of the load but also optimize the output power of PV cells. However, the optical losses due to the skin and the device characteristics of the PV cells are rarely analyzed before chip fabrication. This inevitably leads to sub-optimal system performance in in-vitro or in-vivo tests owing to the varying PV output characteristics. To address this problem, we use the finite-element-method (FEM) to analyze the optical and physical performance of the PV cell under the skin, and then export the model into the p-spice simulator for circuit-level implementation. We further demonstrate a 1:2 cross-coupled DC-DC converter using pulse density modulation for load regulation control to meet the loading requirement. In this work, the PV cell can achieve an 18% of efficiency, and the power conditioning circuit can provide an 84% of end-to-end efficiency

Power management using photovoltaic cells for implantable devices

This paper presents a novel inductor-less switched capacitor (SC) DC-DC converter, which generates simultaneous dual-output voltages for implantable electronic devices. Present dual output converters are limited to fixed ratio gain, which degrade conversion efficiency when the input voltage changes. The proposed power converter offers both step-up and step-down conversion with 4-phase reconfigurable logic. With an input voltage of 1 V provided by photovoltaic (PV) cells, the proposed converter achieves step-up, step-down and synchronised voltage conversions in four gain modes. These are 1.5 V and 0.5 V for Normal mode, 2 V and 1 V for High mode, 2 V for Double Boost mode, as well as 3 V and 2 V for Super Boost mode with the ripple variation of 14-59 mV. The converter circuit has been simulated in standard 0.18 μm CMOS technology and the results agree with state-of-the-art SC converters. However, our proposed monolithically integrated PV powered circuit achieves a conversion efficiency of 85.26% and provides extra flexibility in terms of gain, which is advantageous for future implantable applications that have a range of inputs. This research is therefore an important step in achieving truly autonomous implantable electronic devices

Photovoltaic power harvesting technologies in biomedical implantable devices considering the optimal location

here are still many challenges in effectively harvesting and generating power for implantable medical devices. Most of today's research focuses on finding ways to harvest energy from the human body to avoid the use of batteries, which require surgical replacement. For example, current energy harvesters rely on piezoelectricity, thermoelectricity and solar electricity to drive the implantable device. However, the majority of these energy harvesting techniques suffer from a variety of limitations such as low power output, large size or poor efficiency. Due to their high efficiency, we focus our attention on solar photovoltaic cells. We demonstrate the tissue absorption losses severely influence their performance. We predict the performance of these cells using simulation through the verified experimental data. Our results show that our model can obtain 17.20% efficiency and 0.675 V open-circuit voltage in one sun condition. In addition, our device can also harvest up to 15 mW/ cm2 in dermis and 11.84 mW/ cm2 in hypodermis by using 100 mW/ cm2 light source at 800 nm and 850 nm, respectively. We propose implanting our device in hypodermis to obtain a stable power output

Subdermal solar energy harvesting – A new way to power autonomous electric implants

Subdermal solar harvesting has the potential to obviate the need for the periodic battery replacements as required in patients with cardiac pacemakers. The achievable power output of the subdermal solar module depends on implantation depth, optical skin properties and to an important part on solar cell characteristics. Monte Carlo simulations of light distribution in human skin were used to estimate the power output of subdermal solar cells under midday sunlight exposure in geographical mid-latitudes as a function of implantation depth and solar panel size. For the darkest skin type, the daily energy demand of a modern cardiac pacemaker (0.864 J at a power demand of 10 uW) can be provided by a 2 cm2 solar cell implanted subdermally at a depth of 3 mm when exposed to just 11 min of midday, clear sky irradiance. Our study reveals that solar harvesting with relatively small solar cells if optimized for the spectral subdermal fluence has the potential to power cardiac pacemakers in all skin types within reasonable irradiation exposure times. Solar energy harvesting is very promising to power electronic implants

Power management systems based on switched-capacitor DC-DC converter for low-power wearable applications

The highly efficient ultra-low-power management unit is essential in powering low-power wearable electronics. Such devices are powered by a single input source, either by a battery or with the help of a renewable energy source. Thus, there is a demand for an energy conversion unit, in this case, a DC-DC converter, which can perform either step-up or step-down conversions to provide the required voltage at the load. Energy scavenging with a boost converter is an intriguing choice since it removes the necessity of bulky batteries and considerably extends the battery life. Wearable devices are typically powered by a monolithic battery. The commonly available battery such as Alkaline or Lithium-ion, degrade over time due to their life spans as it is limited by the number of charge cycles- which depend highly on the environmental and loading condition. Thus, once it reaches the maximum number of life cycles, the battery needs to be replaced. The operation of the wearable devices is limited by usable duration, which depends on the energy density of the battery. Once the stored energy is depleted, the operation of wearable devices is also affected, and hence it needs to be recharged. The energy harvesters- which gather the available energy from the surroundings, however, have no limitation on operating life. The application can become battery-less given that harvestable energy is sufficiently powering the low-power devices. Although the energy harvester may not completely replace the battery source, it ensures the maximum duration of use and assists to become autonomous and self-sustain devices. The photovoltaic (PV) cell is a promising candidate as a hypothetical input supply source among the energy harvesters due to its smaller area and high power density over other harvesters. Solar energy use PV harvester can convert ambient light energy into electrical energy and keep it in the storage device. The harvested output of PV cannot directly connect to wearable loads for two main reasons. Depending on the incoming light, the harvested current result in varying open-circuit voltage. It requires the power management circuit to deal with unregulated input variation. Second, depending on the PV cell's material type and an effective area, the I-V characteristic's performance varies, resulting in a variation of the output power. There are several works of maximum power point tracking (MPPT) methods that allow the solar energy harvester to achieve optimal harvested power. Therefore, the harvested power depends on the size and usually small area cell is sufficient for micro-watt loads low-powered applications. The available harvested voltage, however, is generally very low-voltage range between 0.4-0.6 V. The voltage ratings of electronics in standard wearable applications operate in 1.8-3 V voltages as described in introduction’s application example section. It is higher than the supply source can offer. The overcome the mismatch voltage between source and supply circuit, a DC-DC boost converter is necessary. The switch-mode converters are favoured over the linear converters due to their highly efficient and small area overhead. The inductive converter in the switch-mode converter is common due to its high-efficiency performance. However, the integration of the inductor in the miniaturised integrated on-chip design tends to be bulky. Therefore, the switched-capacitor approach DC-DC converters will be explored in this research. In the switched-capacitor converter universe, there is plenty of work for single-output designs for various topologies. Most converters are reconfigurable to the different DC voltage levels apart from Dickson and cross-coupled charge pump topologies due to their boosting power stage architecture through a number of stages. However, existing multi-output converters are limited to the fixed gain ratio. This work explores the reconfigurable dual-output converter with adjustable gain to compromise the research gap. The thesis's primary focus is to present the inductor-less, switched-capacitor-based DC-DC converter power management system (PMS) supplied by a varying input of PV energy harvester input source. The PMS should deliver highly efficient regulated voltage conversion ratio (VCR) outputs to low-power wearable electronic devices that constitute multi-function building blocks