SAW RFID Sensor for Hemodynamic Wearables
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Solution Overview
Problem
Current wearable devices for hemodynamic monitoring face challenges in accurately detecting ECG signals due to the gradient electric field distribution along the arm, which hinders signal detection with existing elbow and wrist devices, and there is a need for non-differential sensing techniques that can effectively monitor cardiovascular signals from the wrist.
Innovation Solution
A surface acoustic wave (SAW) RFID sensor chip based on a GaN/AlGaN heterostructure with a two-dimensional electron gas (2DEG) or hole gas (2DHG) conducting structure, utilizing interdigitated transducers and non-ohmic source and drain contacts for capacitive coupling, allowing for sensitive detection of electrical fields without requiring ohmic contact for DC readout, enabling remote and zero-power operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If differential ECG measurement is used with standard Ag/AgCl gel electrodes, then ECG signal can be detected from chest to elbow, but signal detection is hindered below the elbow due to gradient electric field distribution
Solution Approach 1:
The patent replaces traditional differential voltage measurement (mechanical/electrical contact method) with a non-differential sensing technique that detects the electric field directly using a SAW sensor. This substitution enables single-point measurement at the wrist without requiring multiple electrodes or ohmic contact, thereby maintaining measurement precision while significantly improving adaptability for wearable device placement.
2Adaptability or versatility
If non-differential sensing technique is used for single-point detection, then wearable device placement flexibility is improved, but detection sensitivity to electric field gradients is reduced
Solution Approach 1:
The patent changes the operating parameters of the SAW sensor by applying AC-frequency excitation instead of DC measurement. This parameter change enables the sensor to detect electric field variations through capacitive coupling, significantly improving detection sensitivity for non-differential single-point measurement at the wrist while maintaining wearable device placement flexibility.
Solution Approach 2:
The patent employs a GaN/AlGaN heterostructure with a two-dimensional electron gas (2DEG) or hole gas (2DHG) conducting structure. This composite material structure provides high sensitivity to electric fields while enabling operation in the AC-frequency regime, thereby resolving the contradiction between non-differential sensing capability and detection sensitivity.
3Measurement precision
If AC-frequency regime with capacitive coupling is used, then detection sensitivity is improved for wearable applications, but device complexity increases
Solution Approach 1:
The patent implements a zero-power RFID sensor design where the SAW sensor is remotely powered by an external RFID reader. The sensor structure itself provides the capacitive coupling mechanism needed for AC-frequency operation, and the 2DEG/2DHG channel automatically responds to electric field variations. This self-service approach improves detection sensitivity while avoiding the need for complex external power supply and signal processing circuits, thereby reducing overall device complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The SAW RFID sensor achieves high sensitivity and selectivity in detecting hemodynamic signals, overcoming limitations of existing devices by using AC-frequency regime and capacitive coupling, allowing for effective monitoring of cardiovascular activity with reduced power consumption and enhanced signal stability.
Implementation Method 1
at least one pair of metal interdigitated transducers (IDT) (100) mounted on said piezoelectric substrate (101), for receiving a radio frequency (RF) input signal, transducing said input signal into a surface acoustic wave (SAW)
Implementation Method 2
a piezoelectric substrate (101), said substrate comprising a piezoelectric layer
Implementation Method 3
at least one PC-HEMT-like structure (102) deposited on said piezoelectric substrate (101) for forming the pseudo-conducting 2DEG or 2DHG channel in said heterojunction structure at the interface between said buffer layer and said barrier layer; and electrical metallizations (not shown in the figure) capacitively-coupled to said IDTs (100) and to said HEMT-like structures (103) and/or PC-HEMT-like structures (102)
Data Source
Figure 1a~1c
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Figure 3
AI summary
The present application describes embodiments of a radio-frequency identification (RFID) sensor based on a combination of a surface acoustic wave (SAW) transducer and two-dimensional electron gas (2DEG) or two-dimensional hole gas (2DHG) conducting structure, and its use in hemodynamic wearable devices. The SAW RFID sensor chip contains a piezoelectric substrate, on which a multilayer heterojunction structure is deposited. The heterojunction structure comprises at least two layers, a buffer layer and a barrier layer, wherein the layers are grown from III-V single-crystalline or polycrystalline semi-conductor materials, such as Ga N/Al Ga N. Interdigitated transducers (IDTs) transducing SAWs are installed on top of the barrier layer. A 2DEG or 2DHG conducting channel is formed at the interface between the buffer and barrier layers and provides electron or hole current in the system between the non-ohmic (capacitively-coupled) source and drain contacts connected to the formed channel.