Overview
Human skin is a complex sensory network composed of multiple receptors that can accurately sense and distinguish thermal and mechanical stimuli. Bionic tactile sensors that mimic skin functions are important for skin repair, assistive robotics, and health monitoring.
Researchers at Beihang University recently published a paper in ACS Applied Materials & Interfaces titled "Soft Biomimetic Fiber-Optic Tactile Sensors Capable of Discriminating Temperature and Pressure". The study reports a flexible bionic fiber-optic tactile sensor able to detect and discriminate temperature and pressure simultaneously.
Key concept
The sensor uses a bent fiber Bragg grating (FBG) as the sensing element, encapsulated in a droplet-shaped elastomer. It leverages the different sensitivities of the FBG and whispering gallery modes (WGMs) excited in the bent fiber to temperature and pressure to achieve complete decoupling of the two parameters. To simplify the signal demodulation system, the authors designed a single-cavity dual-comb fiber laser as the light source and used a single photodetector for rapid spectral sampling. The sensor demonstrates high sensitivity, stability, and low cost, and can sense contact pressure, temperature, and hardness in real time by touch, which is relevant for soft-tissue palpation and robotic sensing.
Figure 1 Schematic of the fiber-optic tactile sensor.
Source: ACS Applied Materials & Interfaces (2023)
Background
Tactile perception enables humans to interact with the environment and recognize physical properties of objects. Flexible tactile sensors can emulate skin sensation and have potential applications in intelligent robotics, medical monitoring, and human-machine interaction. Over the past decade, many electronic tactile sensors based on functional conductive materials have been developed to detect temperature, pressure, and vibration by converting external stimuli into electrical signals such as resistance, capacitance, or current. These approaches often suffer from limited sensing modalities and signal crosstalk, making it difficult to distinguish multiple stimuli. Hybrid systems combining different sensing units can address this, but they require heterogeneous integration, complex wiring, and increase fabrication complexity and cost. Thus, integrating detection and discrimination of thermal and mechanical stimuli within a single sensor remains challenging.
Fiber-optic sensors offer strong potential for multi-parameter tactile sensing by converting external stimuli into changes in light transmission properties. They provide high sensitivity, small size, immunity to electromagnetic interference, and intrinsic electrical isolation. Using structures such as fiber Bragg gratings, interferometers, and micro/nano fibers, various fiber-optic tactile sensors have been developed. However, thermo-optic and elasto-optic effects in fibers often cause cross-sensitivity between temperature and pressure. In addition, the high stiffness of standard fibers limits mechanical matching with soft biological systems, restricting biomedical applications.
Design and methods
This work proposes a flexible bionic fiber-optic tactile sensor that can detect and distinguish temperature and pressure stimuli. The sensor comprises a bent FBG encapsulated in polydimethylsiloxane (PDMS). The FBG and WGMs excited in the bent fiber exhibit different sensitivities to temperature and pressure, enabling full decoupling of those parameters. PDMS encapsulation imparts mechanical flexibility and deformability; its high thermo-optic coefficient and low elastic modulus significantly enhance sensitivity to temperature and pressure.
In transmission spectra, the sensor shows two resonance modes corresponding to the sensing FBG and WGMs along the bent fiber. For spectral demodulation, the authors built a polarization-multiplexed single-cavity dual-comb fiber laser with carbon nanotube mode-locking. Because the two comb outputs share the same cavity environment, common-mode noise is suppressed and complex frequency-locking is unnecessary, simplifying the dual-comb system and reducing cost. This setup converts optical frequency signals into radio-frequency signals for compact, rapid, high-resolution spectral measurement.
Figure 2 The tactile sensor and its spectral demodulation principle.
Source: ACS Applied Materials & Interfaces (2023)
Theoretical and numerical analysis
The sensing principle was analyzed using finite element modeling (FEM) and beam propagation methods. Under external load, elastomer deformation changes the FBG bending radius and axial strain, producing wavelength shifts in both the FBG and WGMs. Thermal effects including thermo-optic and thermal expansion produce temperature-dependent resonance shifts. Notably, with increasing temperature, WGM resonances shift in the opposite direction compared with the FBG resonances. This behavior arises because WGMs propagate along the interface between the fiber coating and PDMS, and both the coating and PDMS exhibit large negative thermo-optic coefficients. Numerical results show linear responses to pressure and temperature, with distinct sensitivity coefficients for FBG and WGMs. Experimental tests using the dual-comb spectral detection system measured sensor responses across temperatures and pressures, and the wavelength shift relationships were used to build a temperature-pressure sensitivity matrix. The sensor demonstrated good linearity and repeatability, consistent with simulations.
Figure 3 Pressure and temperature sensing characteristics obtained via dual-comb spectral demodulation.
Source: ACS Applied Materials & Interfaces (2023)
Decoupling and performance
Using the sensitivity matrix, the authors decoupled temperature and pressure responses based on the wavelength shifts of the FBG and WGMs. Experimental validation showed strong agreement between estimated and reference values, with root-mean-square errors (RMSE) of 0.8 mN and 0.2 °C, enabling identification of pressure and temperature stimuli with relative errors below 5%. The sensor also demonstrated stability and fatigue resistance over repeated tests without significant performance degradation.
Figure 4 Validation of dual-parameter decoupling for pressure and temperature.
Source: ACS Applied Materials & Interfaces (2023)
Results and application demonstration
The study presents a novel flexible fiber-optic tactile sensor capable of simultaneous sensing and discrimination of temperature and pressure. The PDMS-encapsulated bent FBG, combined with a single-cavity dual-comb fiber laser, provides compact rapid, high-resolution spectral measurements. Reported sensitivity coefficients are about -0.324 nm/°C for temperature and -14.737 nm/N for pressure. The sensor can distinguish temperature and pressure with precisions of 0.2 °C and 0.8 mN, respectively. The paper also demonstrates real-time sensing of pressure, temperature, and hardness during soft-tissue palpation, illustrating potential use in biomimetic robotics, prosthetics, and human-machine interfaces.
Figure 5 Demonstration of sensor applications.
Source: ACS Applied Materials & Interfaces (2023)
Author biographies
Shang Ce (co-first author) is a doctoral student at Beihang University. He received his BSc and MSc from Hebei Normal University in 2015 and 2018. His research focuses on novel flexible optical sensors and applications. He has published papers in journals including Applied Physics Reviews and ACS Applied Materials & Interfaces, and holds several invention patent applications.
Fu Bo (co-first author) is an associate professor at Beihang University and a doctoral supervisor. He received his PhD from Tsinghua University in 2015 and conducted postdoctoral research at the University of Cambridge. His research includes optical metrology and flexible photonic devices. He has published extensively and holds multiple patents.
Guo Jingjing (corresponding author) is an associate professor at Beihang University, with a PhD from Tsinghua University. Her research focuses on flexible photonic materials and devices. She has published in major journals and her work has been reported in academic media.
