Mode1 Sensor: SoC with embedded wireless power reception and external device with wireless power transmission

• Since SoC with the wireless power reception is located inside the implant sensor, it has a size limitation. It is designed to receive power wirelessly with a 0.5 mm x 8 mm chip size to provide a stable power required to drive the sensor.
• The power transmission circuit of the external device is also designed to supply the power required for the implanted sensor, and it includes rectifier, LDO, overvoltage protector, low voltage circuit breaker, analog-to-digital converter (ADC) and LSK for SWIPT communication, & etc.

MCU-based wireless power transfer algorithm for SWIPT implementation (refer to Figure 2)

• In order to simultaneously transmit power and communicate by LSK, we developed a power control algorithm after designing a block of a hardware platform with MCU first and then commercial devices.
• We designed on-chip MCU circuit with high memory efficiency and low power consumption with ARM's 32-bit RISC Cortex-M0 core.

SoC circuit design for ultra-small size and ultra-low power Mode-1 sensor

• Using the SoC the S-parameter characteristics of the EM-based mode-1 sensor is measured and converted to digital data. Then it is transmitted to the external module outside of the body
• After analyzing the voltage and power for each block, the power and band required performance of each circuit block constituting the subcutaneous insertion sensor interface SoC are determined.

Determination of required performance for each circuit and design with system-level analysis

• We design the circuits according to the required performance, such as Low-noise Amplifier (LNA), Subcutaneously Inserted Sensor Interlocking Frequency-Selective Filter, Envelope Detector, Base-Band Amplifier (BB AMP), and analog-to-digital converter (ADC) circuit design for digital conversion of sensor S-parameter values as the main building blocks that make up the SoC sensor interface (refer to Figure 3)

Mode-2 (single-mode) sensor development based on resonance moment analysis

• Mode-2 sensor is based on a high Q-factor sensor. Multipolar moment is analyzed from the surface current induced in the EM sensor that confirms a sub-radiative resonance to improve Q-factor (Figure 4).
• Representative sub-radiative moments include magnetic dipoles, quadrupoles, and octa-poles. We implemented magnetic dipole resonance by applying a stacked asymmetric ring structure.

Mode-3 (array mode) sensor to improve coupling strength and penetration depth

• The EM array mode sensor is made of multiple sensors that can control distribution of the near-field in the body, to penetrate until the depth of blood vessels. We developed suitable sensors that satisfies above characteristics in parallel operation.
• We reviewed various types and shapes of sensors, and selected the optimal prototype for Mode 3 sensor. We analyzed the sensor performance by EM modeling and simulation.

In vitro biocompatibility evaluation according to ISO standards

• The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay is to evaluate the efficacy and physiological activity of a specific substance or to develop a new drug, The MTT assay for the packaging material of the continuous glucose measurement system is necessary to evaluate the toxicity, cell activity changes, ability to suppress tumor cell growth before the animal experiment and in-vivo application stage.
• The most direct way to measure living cells is to measure the number of surviving cells with a microscope and a hemocytometer after staining cells by treating trypan blue. However, it takes a lot of time and effort to measure and it can be inaccurate when the number of samples is large. Therefore, Tetrazolium salt assay is performed as an alternative method since it is difficult to use a direct method.
• Tetrazolium salt is a generic name for a heterocyclic compound in which three aromatic rings are connected, and it is easily reduced by a reduced compound or enzyme such as dehydrogenase to form formazan and causes color reaction. The solubility and absorbance of the aqueous solution vary depending on the type of aromatic ring structure bonded to the basic structure of tetrazolium salt. Cells that are metabolically active produce energy necessary for survival through the electron transport system process of mitochondria. The assay using tetrazolium salt demonstrates the principle that dehydrogenases present in the electron transfer system decompose tetrazolium salt to produce formazan. It can be used to quantitatively evaluate living cells.
• The absorbance is measured at 540 nm with an ELISA plate reader (VERSAmax, Molecular Devices), and the percentage value of the absorbance for the test substance-treated group is calculated and biocompatibility is evaluated by comparing the proportion of living cells.

Analysis of biocompatible packaging materials for subcutaneous insertion

• Packaging that ensures the stability of the Mode-1 sensor including SoC in the body is essential in order to suppress the immune response caused by the implant sensor. It is important to select a package material that is suitable for toxicity evaluation and at the same time does not block electromagnetic waves. It is also essential to have waterproof function in order to protect electric circuit of SoC.
• According to preliminary analysis of materials for suppressing the human immune response, COC (Cyclic Olefin Copolymer) can be used as a material that satisfies all these conditions, and Dextran-based polymers that can reduce reaction with fat or cholesterol in the interstitial fluid.
• We developed a packaging by analyzing membrane structures that can be manufactured with composite materials. It also increases specificity for glucose at the same time have good water-proof performance. It ensures stable performance in the body without shielding electromagnetic waves (Figure 6).
• It is possible to use Boronic acid derivatives that have excellent body compatibility and can reversibly detect glucose without enzymes or reagents to distinguish similarly sized ascorbic acid and acetaminophen among glucose-like substances.