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

Perovskite Photodiode for Wearable Electronics

Photodetectors are sensing devices that have been used for a broad range electromagnetic wave sensing applications. We are currently investigating the use of photovoltaic cells for implantable and wearable applications [1] [2]. In this work, we have demonstrated the use of CH3NH3PbI3-xClx perovskite materials for photo sensing applications in wearable electronic devices. Our photodetectors were fabricated from two different structures. The first involves the formation of a thin film perovskite material that is sandwiched between bottom and top contact electrodes, while the second involves using hole and electron transport layers between the bottom and top electrodes. Despite a poorer device stability, our experimental results confirmed that devices without an interlayer yield superior performance. Furthermore, AFM results show that the perovskite film formed on top of the PEDOT: PSS layer is non-uniform with more crystalline domains, while it has better surface coverage on top of bare ITO substrates [3] [4]

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

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

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

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

Power Approaches for Implantable Medical Devices.

Implantable medical devices have been implemented to provide treatment and to assess in vivo physiological information in humans as well as animal models for medical diagnosis and prognosis, therapeutic applications and biological science studies. The advances of micro/nanotechnology dovetailed with novel biomaterials have further enhanced biocompatibility, sensitivity, longevity and reliability in newly-emerged low-cost and compact devices. Close-loop systems with both sensing and treatment functions have also been developed to provide point-of-care and personalized medicine. Nevertheless, one of the remaining challenges is whether power can be supplied sufficiently and continuously for the operation of the entire system. This issue is becoming more and more critical to the increasing need of power for wireless communication in implanted devices towards the future healthcare infrastructure, namely mobile health (m-Health). In this review paper, methodologies to transfer and harvest energy in implantable medical devices are introduced and discussed to highlight the uses and significances of various potential power sources

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

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

Energy-Efficient Start-up Power Management for Batteryless Biomedical Implant Devices

This paper presents a solar energy harvesting power management using the high-efficiency switched capacitor DC-DC converter for biomedical implant applications. By employing an on-chip start-up circuit with parallel connected Photovoltaic (PV) cells, a small efficiency improvement can be obtained when compared with the traditional stacked photodiode methodology to boost the harvested voltage while preserving a single-chip solution. The PV cells have been optimised in the PC1D software and the optimal parameters modelled in the Cadence environment. A cross-coupled circuit with level shifter loop is also proposed to improve the overall step up voltage output and hybrid converter increases the start-up speed by 23.5%. The proposed system is implemented in a standard 0.18-μm CMOS technology. Simulation results show that the 4-phase start-up and cross coupled with level-shifter can achieve a maximum efficiency of 60%