In a study published in Nature, a team led by Prof. SU Yewang from the Institute of Mechanics of the Chinese Academy of Sciences, along with Dr. LI Shuang from Tsinghua University and Prof. YU Xinge from City University of Hong Kong, developed a soft, biodegradable, wireless sensing device which can monitor multiple signals from inner body over long distances (e.g.,16 cm), while maintaining accuracy across varying positions and angles.

Monitoring internal physiological signals is essential for effective medical care. Most current technologies rely on external measurements or imaging systems which cannot capture enough deep-tissue dynamics. Implantable devices offer a solution, but their designs often require batteries or magnets, carrying risks during removal.

Biodegradable devices use passive inductor-capacitor resonant circuits to simplify the circuit design and avoid batteries. Their standard readout systems require very close readout distances. Systems which can increase the readout distance to some extent require strict control over the distance and angle between the sensor and the reader, which is difficult to be achieved in clinical conditions.

In this study, the researchers analyzed coupled multi-oscillator equations in a passive wireless system, and revealed the regulatory mechanism of the pole-zero frequency separation and the imaginary part of the pole on signal response. They proposed a 'pole-moving sweeping' readout system which is distinct from systems with stationary poles during a frequency sweep, and solves the issues of short readout distance and low robustness in passive wireless sensing systems.

Moreover, the researchers designed an integrated folded structure by combining mechanics and electromagnetics for the sensors. Through multiple cycles of localized plastic twisting and folding, a multilayer flexible serpentine inductor-capacitor circuit with identical geometry and current direction was formed, which increases the inductance and capacitance while avoiding non-degradable soldering.

The multi-neutral-axis serpentine mechanical structure design further improved the flexibility and stretchability of the sensor, and it significantly reduced the skin effect and proximity effect of high-frequency currents. This structure addressed the challenge of simultaneously achieving softness, biodegradability and high electromagnetic function.

In vivo tests in the abdominal cavity of horses reliably captured deep-tissue pressure and temperature, and ex vivo measurements demonstrated accurate strain monitoring without strict positional control. The long-distance and wide-angle readout of soft biodegradable implants holds great promise for accessing deep-tissue physiological signals.