A novel hybrid technique to fabricate silicon-based micro-implants with near defect-free quality for neuroprosthetics application.

This paper introduces a new hybrid microfabrication technique which combines ultra-precision micro-milling and a ductile sacrificial material deposition process to fabricate a silicon-based implant for neuroprosthetics applications with near defect-free quality at several hundreds of micrometres in thickness. The sacrificial materials can influence the quality of silicon during machining. The cutting mechanism and feasibility of the hybrid technique are studied by molecular dynamics (MD) simulations and experiments. Due to the complexity of modelling PMMA and SU-8 structures in MD environment, only copper was modelled as the simulation is intended to understand the performance of using a ductile sacrificial layer structure in silicon machining. MD analysis shows that the reduced stress intensity and subsurface damage were mainly attributed to workpiece plasticity enhancement, where its mechanism was contributed by better deformability of the ductile sacrificial layer and enhanced thermal softening from the heat generated by the high interfacial stress between the sacrificial layer and silicon substrate. Despite the MD simulation and experiment having different machining scale in terms of cutting parameters, phenomenal behaviours of the cutting performance when observed under the experimental conditions are in good agreement with simulation. Experimental verification shows that near defect-free quality was achieved at large cutting depth of 150 µm when silicon is coated either with PMMA or SU-8. An exemplary implant structure was also fabricated to better demonstrate the hybrid technique's capability. In addition, the hybrid technique will be beneficial for low volume high customisation applications as it is a serial process.

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

BACKGROUND: One 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. METHODS: To 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. RESULTS: The 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. CONCLUSIONS: We 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.

Opto-electro-thermal optimization of photonic probes for optogenetic neural stimulation.

Implantable photonic probes are of increasing interest to the field of biophotonics and in particular, optogenetic neural stimulation. Active probes with onboard light emissive elements allow for electronic multiplexing and can be manufactured through existing microelectronics methods. However, as the optogenetics field moves towards clinical practice, an important question arises as to whether such probes will cause excessive thermal heating of the surrounding tissue. Light emitting diodes typically produce more heat than light. The resultant temperature rise of the probe surface therefore needs to be maintained under the regulatory limit of 2°C. This work combines optical and thermal modelling, which have been experimental verified. Analysis has been performed on the effect of probe/emitter geometries, emitter, and radiance requirements. Finally, the effective illumination volume has been calculated within thermal limits for different probe emitter types and required thresholds.

Self-sensing of temperature rises on light emitting diode based optrodes.

OBJECTIVE: This work presents a method to determine the surface temperature of microphotonic medical implants like LEDs. Our inventive step is to use the photonic emitter (LED) employed in an implantable device as its own sensor and develop readout circuitry to accurately determine the surface temperature of the device. APPROACH: There are two primary classes of applications where microphotonics could be used in implantable devices; opto-electrophysiology and fluorescence sensing. In such scenarios, intense light needs to be delivered to the target. As blue wavelengths are scattered strongly in tissue, such delivery needs to be either via optic fibres, two-photon approaches or through local emitters. In the latter case, as light emitters generate heat, there is a potential for probe surfaces to exceed the 2 °C regulatory. However, currently, there are no convenient mechanisms to monitor this in situ. MAIN RESULTS: We present the electronic control circuit and calibration method to monitor the surface temperature change of implantable optrode. The efficacy is demonstrated in air, saline, and brain. SIGNIFICANCE: This paper, therefore, presents a method to utilize the light emitting diode as its own temperature sensor.

Experimental observation of negative capacitance in ferroelectrics at room temperature.

Effective negative capacitance has been postulated in ferroelectrics because there is a hysteresis in plots of polarization-electric field. Compelling experimental evidence of effective negative capacitance is presented here at room temperature in engineered devices, where it is stabilized by the presence of a paraelectric material. In future integrated circuits, the incorporation of such negative capacitance into MOSFET gate stacks would reduce the subthreshold slope, enabling low power operation and reduced self-heating.