A current-mode system to self-measure temperature on implantable optoelectronics

Abstract

BackgroundOne of the major concerns in implantable optoelectronics is the heat generated by emitters such as light emitting diodes (LEDs). Such devices typically produce more heat than light, whereas medical regulations state that the surface temperature change of medical implants must stay below + 2 °C. The LED’s reverse current can be employed as a temperature-sensitive parameter to measure the temperature change at the implant’s surface, and thus, monitor temperature rises. The main challenge in this approach is to bias the LED with a robust voltage since the reverse current is strongly and nonlinearly sensitive to the bias voltage.MethodsTo overcome this challenge, we have developed an area-efficient LED-based temperature sensor using the LED as its own sensor and a CMOS electronic circuit interface to ensure stable bias and current measurement. The circuit utilizes a second-generation current conveyor (CCII) configuration to achieve this and has been implemented in 0.35 μm CMOS technology.ResultsThe developed circuits have been experimentally characterized, and the temperature-sensing functionality has been tested by interfacing different mini-LEDs in saline models of tissue prior to in vivo operation. The experimental results show the functionality of the CMOS electronics and the efficiency of the CCII-based technique with an operational frequency up to 130 kHz in achieving a resolution of 0.2 °C for the surface temperature up to + 45 °C.ConclusionsWe developed a robust CMOS current-mode sensor interface which has a reliable CCII to accurately convey the LED’s reverse current. It is low power and robust against power supply ripple and transistor mismatch which makes it reliable for sensor interface. The achieved results from the circuit characterization and in vivo experiments show the feasibility of the whole sensor interface in monitoring the tissue surface temperature in optogenetics.

Design and Implementation of the Temperature Sensor for Health Care Monitoring Based on Optical Fiber Technology

“…The temperature-sensing functionality was tested by interfacing various mini-LEDs in saline tissue models prior to in vivo surgery, and the developed circuits were experimentally characterized. With an operating frequency of up to 130 kHz, the outcomes of the experiment demonstrate the versatility the reliability of the CCII-based technique and the efficiency of CMOS electronics achieving a 0.2°C resolution for surface temperatures up to +45°C [12].…”

Thermal management of wearable and implantable electronic healthcare devices: Perspective and measurement approach

“…F, Relationship among junction current, surface temperature, and junction temperature variation when the LED is driven using a 2.5 mA pulse in water and the base temperature is 37 C. G, Output current and voltage; the surface temperature of the LED is driven using a 7.5 mA pulse in water. H, Extracted surface temperature variation; LED with 2 mA and 4 mA currents for 1 min 104 [Colour figure can be viewed at wileyonlinelibrary.com] human body (43 C) when handling implantable devices. They developed a 2D axisymmetric transient equivalent model of an implantable device's radio energy transmission to simulate the temperature variation of the metal shell at the receiving end.…”

“…The temperature change limit for a device must be below +2 C to reduce the tendency of various internal and component damages. Dehkhoda et al 104 developed an area-efficient light-emitting diode (LED)-based temperature sensor by using LED to ensure stable bias and current measurement. They adopted a second-generation current conveyor configuration and realized a resolution of 0.2 C for a frequency of 130 kHz at a surface temperature of 45 C. The study was designed for monitoring tissue surface temperature in optogenetics, as shown in Figure 5, and can provide low power and a robust system to a device.…”

“…G, Output current and voltage; the surface temperature of the LED is driven using a 7.5 mA pulse in water. H, Extracted surface temperature variation; LED with 2 mA and 4 mA currents for 1 min 104 [Colour figure can be viewed at wileyonlinelibrary.com]…”