Researchers develop soft biodegradable implants for long-distance and wide-angle sensing

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 the City University of Hong Kong, developed a soft, biodegradable, wireless sensing device which can monitor multiple signals from inside the 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 that cannot capture enough deep-tissue dynamics. Implantable devices offer a solution, but their designs often require batteries or magnets, carrying risk during removal.

Biodegradable devices currently use passive inductor–capacitor resonant circuits to simplify the circuit design and avoid batteries. Furthermore, their standard readout systems require very short readout distances. For those systems that allow a longer readout distance, strict control over the distance and angle between the sensor and the reader is required, which is difficult to achieve in clinical conditions.

In this study, the researchers analyzed coupled multi-oscillator equations in a passive wireless system and explained how pole–zero frequency separation and the imaginary part of the pole affect the signal response. They proposed a "pole-moving sweeping" readout system, which differs from conventional frequency-sweep methods with fixed poles and improves readout distance and robustness in passive wireless sensing.

Moreover, the researchers designed an integrated folded sensor structure that combines mechanical and electromagnetic principles. Through repeated localized plastic twisting and folding, they formed a multilayer flexible serpentine inductor–capacitor circuit with identical geometry and current direction. This design increases both inductance and capacitance while eliminating the need for non-degradable soldering.

The multi-neutral-axis serpentine mechanical design further improved the sensor's flexibility and stretchability while significantly reducing the skin and proximity effects at high frequencies. This structure enables the simultaneous achievement of softness, biodegradability, and high electromagnetic performance.

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