Flexible optical fiber sensing: Materials, methodologies, and applications

Intrinsically soft and elastic polymer matrix is the key to the stretchability of flexible optical fiber sensors, flexible optical fibers are mainly directly prepared or encapsulated by elastic materials to acquire flexible and biocompatible properties. In addition, the requirement for optical performance has led to the selection of materials focusing on several classes of polymers, including thermoset or thermoplastic elastomers, highly biocompatible hydrogels, and environmentally friendly natural polymers. In this review, the authors summarize and analyze the advantages and disadvantages of the major classes of materials, provide classification and suggestions on material selection from the aspects of performance and application requirements. Finally, the performances of flexible optical fiber sensors reported in related materials are summarized and compared.

The design of flexible optical fiber sensors adopts different strategies and technologies, which are mainly divided into two categories. One is based on structural design. Macrobending structures (such as single ring, u-shape, etc.) or optical microstructures (such as micro-nano optical fiber, fiber Bragg grating, etc.) are intentionally designed or encapsulated, the characteristics of sensitivity to stimulation are obtained by reversible deformation. Another category is based on the composite of sensing materials (including but not limited to organic dyes, inorganic fluorescent materials, or other light-absorbing materials). Polymer optical fibers are modified by dopants, and the sensors’ performance is highly correlated with the characteristics of dopants. Flexible optical fiber sensing is flexible in design and rich in mechanism. The representative theories and advanced progress are introduced in this review.

The recent development in the utilization of flexible optical fiber sensors and the prospective application scenarios were then summarized, which encompass human activity monitoring and healthcare, biomedical diagnosis and therapy, soft robots, and human-machine interfaces. Differing traditional photonic devices, the exceptional mechanical flexibility and high biocompatibility of flexible optical fiber sensors enable the dynamic tracking of a wide range of strain in vivo and in vitro. Moreover, optical fibers deepen the depth of penetration of light signals into tissues, which meets the requirement for deep tissue monitoring. Thereby demonstrating the superiority and potential value of flexible optical fiber sensing.

This review summarizes the latest advancements in flexible optical fiber sensors concerning materials, mechanisms, and applications. These sensors, characterized by miniaturization, lightweight, high flexibility, and electromagnetic interference resistance, cater to the market's demand for novel, comfortable, and safe sensing solutions. The continuous emergence of novel materials, such as ultra-stretchable polymers, self-healing materials, and optoelectronically conductive polymers, has brought unprecedented performance enhancements and functional diversification to flexible optical fiber sensing technology. Regarding sensing mechanisms, while current approaches mostly rely on optical loss or fluorescent effects, the future trajectory lies in achieving long-distance distributed sensing with high sensitivity and selectivity using fully flexible optical fibers. This will significantly expand the sensing range and application scenarios. Flexible optical fiber sensing technology has demonstrated immense potential in areas like human monitoring, healthcare, biomedical applications, soft robotics, and human-machine interfaces. With advancements in materials science and sensing technology, these applications are poised to mature, offering users a richer range of functionalities. Nevertheless, building high-performance, long-term stable flexible optical fiber sensing systems necessitates addressing various technical challenges, including micro-light sources, flexible circuits, data processing, and wireless transmission. Despite these challenges, the promising outlook for flexible optical fiber sensing technology warrants sustained investment and exploration.