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Deformation Sensing in Soft Bio-Surrogate Materials

Accurate measurement of deformations occurring within or on soft materials has recently generated interest for its benefits to the fields of soft robotics and wearable biomedical sensors.

Deformation sensing in and on soft materials has garnered increased interest with the advancement of emerging technologies such as soft robotics, wearable computing, and biomedical applications. These applications have a need for quantification of stretching-contraction deformations along a single-axis as well as multi-directional deformation quantifications (i.e. bending, pressure, membrane stretch, planar and torsional shear).

These measurement devices must conform to the movements of the device or component under test, which can be complex, involving multiple degrees-of-freedom and dynamic rates, while closely matching the mechanical properties of the system they are embedded in, such as skin, tissue, textiles, and soft actuators. For example, soft actuators are increasingly used in the fields of bio-fidelic robotics and aerospace, however, there is an absence of reliable positional and force feedback, which is necessary to provide a soft touch as well as accurate and controllable behavior. Soft sensors are being implemented to provide this feedback, however, embedding them without hindering the actuator’s functionality has been a significant challenge.

Traditional strain gages, typically made of a resistive metal alloy element, cannot measure large strains (typically limited to strains < 1%). They are well suited to measure strain on stiffer materials (e.g., metals, composites, plastics), but cannot be used for strain measurements on soft materials, whose stiffness is much lower than the gage itself and whose strains may be greater than 20%, sometimes 250%+. In designing a soft strain sensor, the following requirements (application dependent) need to be considered:

  1. Reliable, accurate quantification of small, intermediate, and large strains (20% +);

  2. Type of quantification desired (i.e. normal or shear strain due to axial, bending, pressure, shear, or torsional loading);

  3. Impedance matching of the sensor with the material system for accuracy as well as unimpeded movement;

  4. Biomechanically relevant strain rates and frequency responses.

The objective of this research was to develop and characterize materials and sensors for measuring normal and shear strains in soft materials. Of particular interest was developing impedance matched soft strain sensors for use in anatomically correct, bio-surrogate injury assessments in visually obscured threat protection applications.

There are three types of electromechanical, soft sensors: piezoresistive, piezoelectric, and capacitive sensors.

Piezoresistive sensors exhibit a change of resistance under an applied deformation. The resistance change typically occurs as a result of deformation induced changes in the sensing element’s physical dimensions.

Piezoelectric sensors measure strain by transducing a stress-induced electric field. They are susceptible to temperature changes, however, and flexible piezoelectric materials, such as polymers, have a low piezoelectric coefficient.

Capacitive type sensors measure change in capacitance between a set or sets of embedded electrodes. Although capacitive type sensors can accommodate large deformations, they typically have very low sensitivity (gage factors < 1). For these reasons, the focus of this research was on piezoresistive type sensors.

This work was done by Christopher C. Rudolph for the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Sensors category. NRL-0076

Topics:
Aerospace Automation Biosensors Consumer electronics Data Acquisition Data Acquisition Detectors Digital Health Electronic Components Electronic equipment Electronics Electronics & Computers Energy Energy Harvesting Energy Storage Fabrics Health Monitors and Activity Trackers Materials Materials properties Mechanical Components Nanomaterials Nanotechnology Polymers Pressure Robotics Robotics Sensors Sensors and actuators Smart materials Wearables
This Brief includes a Technical Support Package (TSP).
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Deformation Sensing in Soft Bio-Surrogate Materials

(reference NRL-0076) is currently available for download from the TSP library.

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    Aerospace & Defense Technology Magazine

    This article first appeared in the April, 2020 issue of Aerospace & Defense Technology Magazine (Vol. 5 No. 2).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Deformation Sensing in Soft Bio-Surrogate Materials," authored by Christopher C. Rudolf from the Naval Research Laboratory, presents a comprehensive study aimed at advancing deformation sensing technologies in soft materials. The report outlines several key areas of research and development, focusing on the integration of conductive liquids and matrix materials to create impedance-matched resistive sensors.

    The report begins with an introduction to the significance of soft sensors in various applications, particularly in bioengineering and robotics. It emphasizes the need for accurate deformation sensing in soft materials, which can mimic biological tissues. The research investigates the selection of conductive liquids and sensor matrix materials that enhance the performance of these sensors.

    A significant portion of the document details the design and development of a custom electronics package capable of resolving minute changes in resistance due to deformation. This electronics setup is crucial for the effective operation of the sensors, allowing for precise measurements in real-time.

    The report also describes three innovative fabrication techniques for creating embedded channels within the sensors. These methods include 3D printing, direct-laser patterning, and laser cutting of flash tape. Each technique is characterized in terms of its effectiveness in producing the necessary channel geometry for the liquid metal sensors. The document provides experimental results from cyclic stretching tests conducted on prototype soft strain sensors, demonstrating the performance and reliability of the fabricated sensors.

    Additionally, the report discusses the assembly method for the sensors, which includes the encapsulation and filling of the embedded channels with conductive liquids. The use of a rosette-style gauge for full two-dimensional strain analysis is also highlighted, showcasing the versatility of the developed sensors.

    In conclusion, the report summarizes the findings and offers recommendations for future research directions. It emphasizes the potential applications of these soft sensors in various fields, including healthcare, robotics, and materials science. The document serves as a valuable resource for researchers and engineers interested in the development of advanced sensing technologies in soft materials, providing insights into both the methodologies employed and the experimental outcomes achieved.

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