A high performance wearable strain sensor with advanced thermal management for motion monitoring

Resistance change under mechanical stimuli arouses mass operational heat, damaging the performance, lifetime, and reliability of stretchable electronic devices, therefore rapid thermal heat dissipating is necessary. Here we report a stretchable strain sensor with outstanding thermal management. Besides a high stretchability and sensitivity testified by human motion monitoring, as well as long-term durability, an enhanced thermal conductivity from the casted thermoplastic polyurethane-boron nitride nanosheets layer helps rapid heat transmission to the environments, while the porous electrospun fibrous thermoplastic polyurethane membrane leads to thermal insulation. A 32% drop of the real time saturated temperature is achieved. For the first time we in-situ investigated the dynamic operational temperature fluctuation of stretchable electronics under repeating stretching-releasing processes. Finally, cytotoxicity test confirms that the nanofillers are tightly restricted in the nanocomposites, making it harmless to human health. All the results prove it an excellent candidate for the next-generation of wearable devices.

Background:

Wearable and flexible/stretchable strain sensors have attracted great attention because they can convert mechanical deformations into electrical signals and are widely used in soft robotics, human–machine interaction, health-monitoring systems, and human motion monitoring and detection. Stretchable resistive strain sensors with high sensitivity are extremely desirable due to their simple structure and facile fabrication process. However, the resistance change, particularly large nanocontact resistance between nanofillers, generates a mass of heat. For stretchable electronics, this elevated heat negatively affects the performance, safety, and reliability of the devices, and may cause them to fail in structure and function. Materials with enhanced thermal conductivity are strongly recommended for dissipating this heat.

Nanocomposites composed of insulating polymer matrix and inorganic fillers are usually prepared to achieve both mechanical stability and high thermal conductivity. Among these fillers, boron nitride (BN) is considered one of the most well-known candidates for thermal dissipation materials due to its extremely high aspect ratio and outstanding thermal transport properties.

Methods:

A stretchable strain sensor was fabricated using a casted thermoplastic polyurethane-boron nitride nanosheets (TPU-BNNS) layer to provide enhanced thermal conductivity for rapid heat transmission, while a porous electrospun fibrous thermoplastic polyurethane membrane was used to provide thermal insulation. The sensor’s performance was tested by human motion monitoring to verify stretchability and sensitivity. Cytotoxicity tests were conducted to confirm that the nanofillers are tightly restricted in the nanocomposites, making the device harmless to human health. In-situ investigation of dynamic operational temperature fluctuation under repeating stretching-releasing processes was performed for the first time.

Results:

The stretchable strain sensor demonstrated high stretchability and sensitivity, as testified by human motion monitoring, along with long-term durability. The enhanced thermal conductivity from the casted TPU-BNNS layer enabled rapid heat transmission to the environment, while the porous electrospun fibrous TPU membrane provided thermal insulation. A 32% drop of the real-time saturated temperature was achieved. For the first time, the dynamic operational temperature fluctuation of stretchable electronics under repeating stretching-releasing processes was investigated in situ. Cytotoxicity tests confirmed that the nanofillers are tightly restricted in the nanocomposites, making it harmless to human health.

Data Summary:

A 32% drop of the real-time saturated temperature was achieved through the combination of the thermally conductive TPU-BNNS layer and the thermally insulating electrospun fibrous TPU membrane. The sensor exhibited high stretchability and sensitivity during human motion monitoring, along with long-term durability.

Conclusions:

All the results prove it an excellent candidate for the next-generation of wearable devices.

Practical Significance:

The stretchable strain sensor with advanced thermal management can be used for motion monitoring in wearable applications, providing both high performance and safety. Because the nanofillers are tightly restricted in the nanocomposites and the device is harmless to human health, it is suitable for direct skin contact in real-world wearable monitoring systems.