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

Thermal and Mechanical Energy Harvesting Using Lead Sulfide Colloidal Quantum Dots

The human body is an abundant source of energy in the form of heat and mechanical movement. The ability to harvest this energy can be useful for supplying low-consumption wearable and implantable devices. Thermoelectric materials are usually used to harvest human body heat for wearable devices; however, thermoelectric generators require temperature gradient across the device to perform appropriately. Since they need to attach to the heat source to absorb the heat, temperature equalization decreases their efficiencies. Moreover, the electrostatic energy harvester, working based on the variable capacitor structure, is the most compatible candidate for harvesting low-frequency-movement of the human body. Although it can provide a high output voltage and high-power density at a small scale, they require an initial start-up voltage source to charge the capacitor for initiating the conversion process. The current methods for initially charging the variable capacitor suffer from the complexity of the design and fabrication process. In this research, a solution-processed photovoltaic structure was proposed to address the temperature equalization problem of the thermoelectric generators by harvesting infrared radiations emitted from the human body. However, normal photovoltaic devices have the bandgap limitation to absorb low energy photons radiated from the human body. In this structure, mid-gap states were intentionally introduced to the absorbing layer to activate the multi-step photon absorption process enabling electron promotion from the valence band to the conduction band. The fabricated device showed promising performance in harvesting low energy thermal radiations emitted from the human body. Finally, in order to increase the generated power, a hybrid structure was proposed to harvest both mechanical and heat energy sources available in the human body. The device is designed to harvest both the thermal radiation of the human body based on the proposed solution-processed photovoltaic structure and the mechanical movement of the human body based on an electrostatic generator. The photovoltaic structure was used to charge the capacitor at the initial step of each conversion cycle. The simple fabrication process of the photovoltaic device can potentially address the problem associated with the charging method of the electrostatic generators. The simulation results showed that the combination of two methods can significantly increase the harvested energy

Flexible Wirelessly Powered Implantable Device

Brain implantable devices have various limitations in terms of size, power, biocompatibility and mechanical properties that need to be addressed. This paper presents a neural implant that is powered wirelessly using a flexible biocompatible antenna. This delivers power to an LED at the end of the shaft to provide a highly efficient demonstration. The proposed design in this study combines mechanical properties and practicality given the numerous constraints of this implant typology. We have applied a modular structure approach to the design of this device considering three modules of antenna, conditioner circuit and shank. The implant was fabricated using a flexible substrate of Polyimide and encapsulated by PDMS for chronic implantation. In addition, finite element method COMSOL Multiphysics simulation of mechanical forces acting on the implant and shank was carried out to validate a viable shank conformation-encapsulation combination that will safely work under operational stress with a satisfactory margin of safety

Self-powered implantable CMOS photovoltaic cell with 18.6% efficiency

Harvesters for implantable medical applications need to generate enough energy to power their loads, but their efficiency is reduced when implanted under the tissue. Conventional photovoltaic (PV) cell harvesters made with CMOS technology stack cells in series, which raises output voltage but lowers power conversion efficiency. In addition, it is difficult to assess harvester performance prior to fabrication. To address these challenges, we developed a novel parallel PV cell configuration that fully utilizes all triple-well diodes and responds efficiently to near-infrared light. Using an optimized structure, the PV cells were fabricated through standard TSMC 65-nm CMOS technology, achieving an efficiency of 18.6%, open circuit voltage of 0.45 V, and short circuit current of 1.9 mA cm −2 . These results confirm the ability of the device to generate sufficient energy even when implanted beneath the tissue. Multiphysics finite element modeling (FEM) was used to optimize the stacking structure of the CMOS PV cell, and experimental results showed a successfully delivered power density of 1.2 mW cm −2 (single cell 1.04 mm 2 ) when placed 2 mm below porcine skin. Different array configurations of six PV cells were also experimentally studied using external wire switching, demonstrating the flexibility of the PV array in delivering different output energy for various implantable devices

Study of systems powered by triboelectric generators for bioengineering applications

Treballs Finals de Grau d'Enginyeria Biomèdica. Facultat de Medicina i Ciències de la Salut. Universitat de Barcelona. Curs: 2020-2021. Director: Pere Lluís Miribel Català. Co-director: Manel Puig i Vida

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

Biointegrated and wirelessly powered implantable brain devices: a review

Implantable neural interfacing devices have added significantly to neural engineering by introducing the low-frequency oscillations of small populations of neurons known as local field potential as well as high-frequency action potentials of individual neurons. Regardless of the astounding progression as of late, conventional neural modulating system is still incapable to achieve the desired chronic in vivo implantation. The real constraint emerges from mechanical and physical diffierences between implants and brain tissue that initiates an inflammatory reaction and glial scar formation that reduces the recording and stimulation quality. Furthermore, traditional strategies consisting of rigid and tethered neural devices cause substantial tissue damage and impede the natural behaviour of an animal, thus hindering chronic in vivo measurements. Therefore, enabling fully implantable neural devices, requires biocompatibility, wireless power/data capability, biointegration using thin and flexible electronics, and chronic recording properties. This paper reviews biocompatibility and design approaches for developing biointegrated and wirelessly powered implantable neural devices in animals aimed at long-term neural interfacing and outlines current challenges toward developing the next generation of implantable neural devices

NIR light transmission through skin and muscle

Light has been used extensively in the medical field for both therapeutic and diagnostic applications. Tissue optical window or therapeutic window defines the range of wavelengths where the light has the maximum transmittance through tissue. In this range, absorption and scattering effects are relatively lower when compared to the visible or middle infrared wavelengths. Knowledge of the transmittance through tissue can help determine the effective light intensities in medical applications. The objective of this thesis is to determine the NIR light transmission through different thicknesses of animal tissue and its spatial spread due to the scattering effect. Primarily pork skin and muscle tissues are used due to their similar optical properties to human tissue. Tissue thicknesses range from 4 mm to 20 mm. A NIR LED array with the wavelength of 875 nm serves as the light source. A commercial photodiode is used for measurements of the transmitted light intensities. The results demonstrate a transmittance of 18% for 4 mm tissue thickness and 3% for 20 mm and vary exponentially in between. Scattering increases the spatial spread of the light beam and makes it very difficult to focus inside the tissue. In addition to the transmittance measurements, temperature elevation due to the NIR light illumination is investigated. Thermocouple measurements show a temperature increase of 1.2 °C on the surface of the tissue slab at the light intensities tested in this project

SkinnySensor: Enabling Battery-Less Wearable Sensors Via Intrabody Power Transfer

Tremendousadvancement inultra-low powerelectronics and radiocommunica tionshas significantly contributed towards the fabrication of miniaturized biomedical sensors capable of capturing physiological data and transmitting them wirelessly. However, most of the wearable sensors require a battery for their operation. The battery serves as one of the critical bottlenecks to the development of novel wearable applications, as the limitations offered by batteries are affecting the development of new form-factors and longevity of wearable devices. In this work, we introduce a novel concept, namely Intra-Body Power Transfer (IBPT), to alleviate the limitations and problems associated with batteries, and enable wireless, batteryless wearable devices. The innovation of IBPT is to utilize the human body as the medium to transfer power to passive wearable devices, as opposed to employingon-boardbatteries for each individual device. The proposed platform eliminates the on-board rigid battery for ultra-low power and ultra-miniaturized sensors such that their form-factor can be flexible, ergonomically designed to be placed on small body parts. The platform also eliminates the need for battery maintenance (e.g., recharging or replacement) for multiple wearable devices other than the central power source. The performance of the developed system is tested and evaluated in comparison to traditional Radio Frequency based solutions that can be harmful to human interaction. The system developed is capable of harvesting on average 217µW at 0.43V and provides an average sleep/high impedance mode voltage of 4.5V

Human Heat Energy Harvesting Using Thermoelectric Cooler

An extensive research in renewable energy harvesting is increasing due to the limitation of energy resources. The known leading renewable energy harvesting sources is such as hydroelectricity, wind and solar. This research will focus on the human heat energy harvesting which will convert human waste heat to electricity. It focuses on converting the waste heat to electricity from five area of human body such as human palm, top palm, wrist, top wrist and leg. Thermoelectric cooler is used to convert the human body heat to electricity. In this research, a booster circuit is developed to boost the small voltage to higher voltage in order to power up an LED as output indicator. Based on experimental results, the maximum output voltage from Peltier module is obtained from human palm which is 0.1 V. The voltage is able to be boosted up to 2.9 V at the voltage booster output. The output voltage generated at load is 2.47 V and the power output is 24.7 mW. The prototype board built is able to generate human heat as electricity. Human palm is the most suitable location to power-up the LED