Flexible Strain Sensors as Real-Time Stretch Receptors for Biometric Muscle Applications in Robotics

Rochelle Jubert, a recent graduate from Portland State University, collaborated with Liquid Wire on research using stretchable strain gauges to measure real-time length-sensing for biomimetic Braided Pneumatic Actuators (BPAs) in soft robotics artificial proprioception applications.  

Leveraging the low-profile, modular design of Liquid Wire’s strain sensors, Rochelle developed a reliable method for sensing the changes in length of an artificial muscle for use as feedback in a BPA-actuated robot. This feedback enables the robot to "sense" its own muscle activity with precision, closely mimicking the behavior of biological systems. This research project is representative of a growing trend in robotics, where flexible electronics are enabling more sophisticated and biomimetic systems. 

The research collaboration between Liquid Wire and Rochelle highlights the broader potential of flexible electronics to transform not only robotics but also healthcare, wearables, and industrial automation by offering precision, enhanced adaptability, and novel functionality. 

The Research: Combining Bio-Inspired Robotics and Flexible Electronics to Study Artificial Proprioceptive Feedback

Rochelle performed her research in the Agile and Adaptive Robotics Lab at Portland State University, which focuses on using bio-inspired robots to study artificial proprioception. Proprioception refers to a living creature’s ability to intuitively sense the layout and positioning of their body - artificial proprioception mimics this function in robots. She recognized the potential of applying Liquid Wire’s sensors, which are engineered for biocompatibility to bend and stretch like biological tissues, to artificial muscle proprioception.  

Liquid Wire’s sensors feature channels filled with the company’s proprietary Metal Gel™ conductor encapsulated within Thermoplastic Polyurethane (TPU). The selected TPU offers high elasticity, is capable of more than 1 million strain cycles at over 30 percent elongation, and demonstrates rapid recovery, thereby minimizing hysteresis (the delay between strain and recovery).

The unique composition of Metal Gel™ conductor endows it with both solid and fluid characteristics, allowing it to stretch within the TPU while electrical resistance increases linearly as the conductive channels lengthen.

Upon relaxation, both the channel lengths and the electrical resistance rapidly return to their original form and its original resistance. Thus, the circuit displays very little hysteresis and gives a researcher a baseline they can trust even in high cycling applications. 

Reflecting on the adaptability of these sensors, Rochelle noted:

"Liquid Wire’s flexible strain sensors are like ready-to-go, ready-to-use solutions. Typically, strain sensors or flexible strain gauges need to be custom-manufactured or refined in-house, but that's where Liquid Wire stands out in the world of soft robotics. Anyone can use them—they’re simple, precise, and efficient. And since time is money, I believe that’s something many people can appreciate."  

She prioritized durability, modularity, biomimicry, and a low profile when determining how to attach the Liquid Wire sensor to a BPA artificial muscle. After testing various methods, she found that heat-pressing the sensor onto nylon fabric, then sewing it into a flexible cuff, was the most effective solution. Rochelle designed the cuff to be easily mounted onto the BPA, with the strain gauge positioned perpendicular to the muscle’s movement for accurate measurement. Her solution successfully met all the design priorities, making it an ideal fit for this soft-robotics application. 

Challenges of Traditional Sensors in Robotics and Liquid Wire’s Advantage  

Before discovering Liquid Wire’s strain sensors, Rochelle encountered challenges with traditional sensors. Conventional encoders lack the precision needed to replicate the fine-tuned movements of biological systems. 

Liquid Wire’s sensors addressed these challenges by offering a modular (easily integrated into different systems), accessible, and highly sensitive solution. Made from materials that can flex and stretch, Liquid Wire’s sensors are lightweight, highly customizable, and designed to adapt seamlessly to both natural and engineered movements.  

Rochelle elaborated on this, saying: 

“One of the main reasons I wanted to pursue the Liquid Wire sensor is because it's a modular option that makes artificial muscle research more accessible. There are other sensing methods where you try to integrate the sensors directly into the muscles, but those are very costly, both in terms of money and time, and they're quite complicated. With Liquid Wire, you can take a standard BPA off the shelf, attach the sensor, calibrate it, and integrate it into the design. I’ve worked with other resistive sensors in research, but with Liquid Wire, I was able to achieve much higher accuracy.”

Collaboration with Dr. Mike Hopkins 

Throughout the project, Rochelle worked closely with Dr. Mike Hopkins, the Vice President of Research and Development at Liquid Wire. Dr. Hopkins provided ongoing guidance, with regular email exchanges and monthly in-person meetings. His insights were instrumental in helping Rochelle integrate Liquid Wire’s sensors into her robotics design. One of the key strengths of Liquid Wire’s technology is its customizability, and together, Rochelle and Dr. Hopkins developed a sensor specifically tailored to her research needs.

“Dr. Hopkins played a huge part in me being able to accomplish this project,” Rochelle noted. “He gave me feedback on my research paper and even co-authored it. His support, along with the technology from Liquid Wire, was essential to the success of the project." 

This collaboration not only enhanced Rochelle’s research but also contributed to expanding knowledge in the field of soft robotics. Specifically, the project explored new ways to attach flexible strain sensors to soft robotic actuators—a topic that had not been extensively covered in previous research. 

The Broader Impact: From Soft Robotics to Assistive Devices 

Liquid Wire’s flexible strain sensors have far-reaching applications beyond robotics. Rochelle’s work demonstrates how this technology can be applied to assistive devices and other areas where precision and adaptability are critical. Liquid Wire’s sensors are not only accessible and easy to use, but they also have the potential to revolutionize the development of prosthetics, orthotics, and wearable technology. 

Rochelle’s interest in bio-inspired robotic engineering continues to grow, and her experience with Liquid Wire has opened new career opportunities.  

“This research opened all the doors,” she said, highlighting how her collaboration with Liquid Wire directly contributed to her new role at Biomotum, a company whose transforming mobility rehabilitation with their own robotic exoskeleton systems. “I wouldn’t be the engineer I am now without this experience.”

The Future of Stretchable Sensors in Robotics and Beyond 

As the fields of soft robotics and biomimicry continue to evolve, the integration of flexible, stretchable sensors like those developed by Liquid Wire marks a game-changing advancement in technology. These sensors are not just incremental improvements—they’re revolutionizing how systems can sense and respond in dynamic environments. 

Liquid Wire’s flexible sensors have already proven their value in real-world applications, showcasing versatility and reliability where traditional systems fall short. Tested over 1 million cycles at 30% elongation, they consistently deliver high performance with minimal degradation, underscoring their potential for long-term use in demanding settings. 

More than just an evolution in materials, Liquid Wire’s stretchable electronics have the potential to reshape entire industries, setting new standards for adaptability, precision, and durability. As these technologies continue to advance, Liquid Wire remains at the forefront, offering solutions that are as flexible as the challenges they’re designed to meet. 

Whether you’re innovating in robotics, wearable technology, or other pioneering fields, Liquid Wire’s sensors are built to adapt, ensuring a customized, high-performance solution for every application.