Untethered Soft Pneumatic Actuators with Embedded Multiple Sensing Capabilities

Abstract

Pneumatic soft robot attracts extensive attention because of its own characteristics. It has great application potential in medical and other fields. Although the recent improvement of the soft robot shows great potentials for delicate manipulations, the development of completely untethered pneumatic intelligent soft robots remains challenging. This article introduces a novel type of untethered soft pneumatic actuator with embedded multiple sensing capabilities. The untethered drive of the soft pneumatic actuator is achieved by near-infrared-induced liquid-gas phase transition. In addition, a soft conductive resin was developed to make flexible sensors. Embedded flexible sensors enable bending and temperature sensing of soft actuators. With Digital Light Processing three-dimensional printing, the rapid fabrication of soft actuators and flexible sensors was realized. This article demonstrates the potential of the proposed untethered soft actuators with embedded multiple sensing capabilities as an important contribution to the research of completely untethered intelligent soft robots.

Introduction

Pneumatic soft robots have become a hot topic in the field of soft robotics because of their huge application potential.^1–10 However, traditional soft pneumatic actuators require auxiliary equipment such as air compressors and pressure regulators.¹¹ The complex and bulky drive system in practical applications severely affects their application in unstructured environments.^12,13

To promote the process of completely untethered soft robots, new types of soft actuators with untethered drives have been developed in recent years.^14–16 The driving mechanism is based on thermal-guided volume expansion, including magnetocaloric drive,¹⁷ electrothermal drive,^18–20 and photothermal drive.^21–27 The untethered near-infrared (NIR)-driven actuator allows the soft actuator to achieve bending actuation without any tether connection. It has advantages in terms of safety, controllability, harmlessness, and great human–machine interaction.

In addition, to promote the completely untethered process of intelligent soft robots, real-time wireless sensing of soft robot is needed, which requires highly stretchable flexible sensors that match it.^28–30 Traditional electronic sensors are not compatible with soft robots. Their production materials have higher stiffness than soft robots, which will affect the compliance of soft robots and are not conducive to the completely untethered process of soft robots.^31–33 Therefore, there is a need to develop new flexible sensors that are made of the same low-stiffness material as the soft robot.

In this study, an untethered soft pneumatic actuator with embedded multiple sensing capabilities was proposed. As shown in Figure 1, the soft actuator achieves bending actuation by NIR-induced liquid-gas phase transition of the internal encapsulated liquid. To realize real-time temperature and bending detection of soft actuators, embedded flexible bending sensors and temperature sensors were developed. Soft pneumatic actuators and flexible sensors were made by Digital Light Processing (DLP) three-dimensional (3D) printing. The printing materials were soft resin and soft conductive resin. Soft conductive resin was made by mixing soft resin with conductive filler. Through the characterization of electrical properties and mechanical properties, the optimal ratio of soft resin to conductive filler and the optimal operating temperature of the flexible bending sensor were established.

FIG. 1.

Schematic diagram of the fabrication of soft actuators, untethered drive, and embedded multiple sensing. Soft actuators of various structures. DMD, digital micromirror device; DLP, Digital Light Processing; NIR, near-infrared; PC, personal computer.

The untethered soft pneumatic actuator with embedded multiple sensing capabilities proposed in this study promotes research on completely untethered soft robots and has broad application prospects. For example, when the affected area is pressed during medical surgery, the flexible bending sensor can be used to adjust the force of the pressing operation in real time, while the flexible temperature sensor can detect and provide feedback physiological signals such as the temperature of the affected area. In the detection of unknown complex environments, the flexible bending sensor detects and provides feedback on the attitude of the actuator body, while the flexible temperature sensor detects and provides feedback on the temperature of the detection environment in real time.

Materials and Methods

Material synthesis

Supplementary Figure S1 shows the preparation process of the soft conductive resin required to make flexible sensors; the soft resin was purchased from Zhongshan Huayu Yuanxing Electronic Technology Co., Ltd. The main components of the 3D printing photosensitive resin used in this project are acrylic oligomers and initiators. The viscosity is 245 mPa.s (25°C), the density is 58.1 g/cm³, the volume shrinkage speed is 4.24%, the solid density is 1.22 g/cm³, and graphite (purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) is added to a tin foil-wrapped beaker in proportion. A suitable size magnet is added to the magnetic stirrer (purchased from JOANLAB Equipment CO., Ltd.) and stirred evenly (2 h).

Fabrication of soft actuators

The soft actuator is fabricated by DLP 3D printing (DLP light curing bio-printer was purchased from Hefei Zhongjian Lipu Medical Technology Co., Ltd.). First, the 3D structure of the soft actuator is designed and optimized by 3D modeling software. The model is imported into the slicing software in the format of stereolithrography (STL) file. The printing parameters (light intensity: 4.0, exposure time: 4 s, layer thickness: 75 μm) are adjusted. The soft resin was added to the tank to complete the 3D printing of the soft actuator.

Fabrication of flexible sensors

The flexible sensor is prepared by DLP 3D printing. First, the 3D structure of the flexible sensor is designed and optimized by 3D modeling software. The model is imported into the slicing software in the format of STL file. The printing parameters are adjusted (light intensity: 4.0, exposure time: 5 s, layer thickness: 70 μm). The soft resin and conductive graphite were mixed proportionally (10:1, 7.5:1, 5:1) to prepare the soft conductive resin. The mixed flexible conductive resin is added to the tank to complete the 3D printing of the flexible sensor.

The mechanical tensile properties are tested using a mechanical testing machine (purchased from Shenzhen Reger Instrument Co., Ltd.), as shown in Supplementary Figure S2. The test results are recorded and exported by the software. The mechanical bending properties were tested using custom fixtures and digital push-pull force meters (purchased from Yueqing Handpi Instrument Co., Ltd.), as shown in Supplementary Figure S3.

As shown in Supplementary Figure S4, the bending (tensile) response and temperature response of flexible sensors are tested using custom fixtures, electrochemical workstations (purchased from Shanghai Chin Instrument Co., Ltd.), and NIR lamps (purchased from Heng Ming Medical Co., Ltd.). The test results are recorded and exported by software.

Ethical statement

This study was conducted with approval from the Biomedical Ethics Committee of Anhui Medical University (20210317), and was performed in accordance with established guidelines.

Results

Untethered drive

In this study, the bending actuation of soft actuators is realized by a heated phase transition of the encapsulated liquid (as shown in Supplementary Video S1). As shown in Figure 2a, the liquid-gas phase transition of the internal encapsulated liquid and bending actuation of the soft actuator were realized by NIR irradiation. The overall system is shown in the Supplementary Figure S16. In this study, NH₄CO₃ solution is used as the encapsulated liquid. Considering the influence of temperature on the service life and the response speed of the soft actuator, the soft actuator is placed at 70 mm from the light source in this article.

FIG. 2.

Mechanism and performance of the untethered NIR drive. (a) Driving mechanism and process. Scale bar, 10 mm. (b) The bending angle and temperature of the actuator at each stage of the driving process. (c) Comparison of the infrared response speed of soft actuators under different encapsulated liquids. a: NH₄CO₃ (solid), b: NH₄CO₃:MXene = 10:1 (liquid), c: NH₄CO₃ 0.11 g/mL, d: NH₄CO₃:Graphite = 10:1 (liquid), e: NH₄CO₃:Fe₃O₄ = 3:1 (solid), f: NH₄CO₃:Fe₃O₄ = 1:1 (solid), g: NH₄CO₃:Fe₃O₄ = 1:2 (solid), h: NH₄CO₃:Fe₃O₄ = 1:2 (liquid). Scale bar, 10 mm. (d) Comparison of the NH₄CO₃ heating speed at different infrared powers.

The NH₄CO₃ solution begins to decompose at 36°C and completes decomposition at 60°C. As the temperature reaches and exceeds the decomposition temperature of NH₄CO₃ in solution, the encapsulated liquid gradually translates from liquid to gas. There should be three stages in the untethered NIR driving process, corresponding to the three states of the encapsulated liquid. As shown in Figure 2b, they are the prebending stage in the liquid state, the stable bending stage in the liquid-gas coexistence state, and the dynamic balance stage in the gaseous state. In this study, bending actuation of the soft actuator is realized by NIR-induced liquid-gas phase transition.

However, the low thermal conductivity of the shell of the soft actuator and the low photothermal efficiency of the NH₄CO₃ solution result in long response times for actuators. The infrared response speed of the soft actuator could be improved by enhancing the photothermal efficiency of the liquid encapsulated inside the soft actuator and the power of the external NIR light. To improve the photothermal efficiency of the internally encapsulated NH₄CO₃ solution and the decomposition speed of NH₄CO₃, the NH₄CO₃ solution was mixed with various substances with high photothermal efficiency. As shown in Figure 2c, the infrared response speed of the actuator is the fastest when NH₄CO₃ solution is mixed with Fe₃O₄ in a ratio of 1:2 (Fe₃O₄ has great photothermal efficiency^34,35). Figure 2d shows the heating curve of the NH₄CO₃ solution under different infrared light powers. Higher NIR light power leads to faster decomposition of NH₄CO₃ and faster response speed of the soft actuator.

Soft actuators can simulate the movement of human fingers. The detection of their somatosensory position is quite important. The soft actuator in this study provides heat input for untethered drive by NIR light. Since the liquid-gas phase transition of the internally encapsulated liquid is inspired by rising temperature, the temperature detection of the soft actuator is equally important.

In this study, embedded flexible temperature and bending multiple sensing were realized. As shown in Figures 1 and 3a, the flexible bending sensor and temperature sensor are bonded to the upper and lower top surfaces of the soft actuator using very high bond double-sided adhesive. The embedded flexible sensor improves the infrared response speed of the actuator due to its high photothermal efficiency when sensing. The flexible sensor was made by DLP printing of soft conductive resin. The soft conductive resin was made of a mixture of soft resin and graphite.

FIG. 3.

Embedded flexible bending sensing. (a) Sensing mechanism of the flexible bending sensor. Scale bar, 10 mm. (b) Curvature-resistance curve of the flexible bending sensor. Scale bar, 10 mm. (c) Tensile displacement-impedance curves of flexible bending sensors with different mixing ratios. (d) Relative strain-resistance change curve of flexible bending sensors with different mixing ratios.

Embedded bending sensing

The component of the soft sensor—graphite—has both conductive and photothermal properties.^36,37 The flexible sensor in this article can improve the photothermal performance of the soft actuator while carrying out sensing and detection. As shown in Figure 3a, the soft bending sensor is attached to the bottom of the soft actuator. As the actuator bends, the length of the soft sensor decreases, resulting in a larger cross-sectional area, and the resistance of the soft sensor is decreased.³⁸ Therefore, the bending degree of the soft actuator could give feedback through the resistance value of the soft bending sensor. The compression bending test of the flexible bending sensor was re-conducted to obtain the relationship between its bending degree and resistance value. The relationship between the curvature of the flexible bending sensor and resistance value is finally obtained as shown in the Figure 3b. The bending degree of the soft actuator is reflected by the curvature. It can be seen from the figure that as the bending degree of the sensor increases, its resistance value decreases.

The bending deformation of the flexible bending sensor can be decomposed into countless small segments of tensile deformation. To explore the influence of the graphite proportion on the tensile response, the tensile response of the flexible sensor was tested. As shown in Figure 3c, the higher the proportion of graphite, the denser the conductive network formed, resulting in better conductivity.

To explore the influence of the graphite proportion on the sensitivity of the tensile response, the sensitivity of the tensile response is calculated from Figure 3c. Figure 3d shows that when the ratio of flexible resin to graphite is between 7.5:1 and 5:1, the tensile response sensitivity of the flexible sensor is the highest.

Embedded temperature sensing

The soft actuator performs bending actuation mainly through the expansion deformation of the side wall of the deformed layer, while the side of the chamber is larger than the top of the chamber (as shown in Supplementary Fig. S15). The Finite element method (COMSOL Multiphysics) was also used to simulate the deformation of the soft actuator, and the result shows that the top surface of the cavity could generate less stress when the actuator expands, as shown in the Supplementary Figure S14.

Graphite is mixed with soft resin. Carbon nanoparticles are combined with acrylate polymer by covalent bonding, as shown in Figure 4a. The increase in temperature frees the carbon nanoparticles from the polymer chain, forming a conductive network with better conductivity between them. Thus, the resistance value of the flexible sensor changes when the temperature changes.³⁹ Figure 4b shows the infrared response of the flexible temperature sensor using an NIR lamp. To explore the influence of the graphite proportion on the temperature response, a temperature response test of different proportions of flexible temperature sensors was conducted, as shown in Figure 4c. Figure 4d shows the sensitivity of the temperature response of the flexible temperature sensor. The higher the proportion of graphite is, the higher the sensitivity of the temperature response of the sensor.

FIG. 4.

Embedded flexible temperature sensing. (a) Schematic diagram of the internal microscope of the flexible temperature sensor. (b) Infrared response test of the flexible temperature sensor. (c) Temperature-impedance curve of flexible temperature sensors with different mixing ratios. (d) Temperature response sensitivity curve of flexible temperature sensors with different mixing ratios.

Characterization

In this study, a soft conductive resin was made by mixing soft resin with a conductive filler. To explore the influence of the conductive filler proportion on the electrical and mechanical properties of flexible sensors and DLP printing, a variety of different proportions were tested. The printing performance is the viscosity of the printing material. Too high of a viscosity will prevent the material from being reflowed and thus not being able to form in 3D. The electrical properties are manifested as the stability of the tensile response and temperature response. The mechanical properties are manifested as tensile and bending properties. The better the mechanical properties are, the longer the service life.

To explore the influence of the graphite proportion on the mechanical properties, mechanical tensile tests (as shown in Supplementary Fig. S2) and mechanical bending tests (as shown in Supplementary Fig. S3) were performed on flexible sensors. Figure 5a shows that the higher the proportion of graphite is, the stronger its tensile resistance. Figure 5b shows that graphite reduces the bending resistance of the sensor. Figure 5c shows a comparison of the elongation, bending modulus, and tensile modulus of flexible sensors with different proportions. The characterization of mechanical properties includes stability, so a cyclic tensile test of the flexible bending sensor is conducted. Figure 5d shows that the mechanical properties of flexible bending sensors with different mixing ratios can remain stable during multiple cyclic stretching.

FIG. 5.

Comparison of the mechanical properties. (a) Stress-strain curves of flexible sensors with different mixing ratios. (b) Displacement-bending stress curves of flexible bending sensors with different mixing ratios. (c) Comparison of the mechanical properties of flexible sensors and soft actuators of different proportions. (d) Cyclic tensile test of flexible bending sensors with different mixing ratios.

The electrical properties of flexible bending sensors are characterized by the stability of the tensile response, and the electrical properties of flexible temperature sensors are characterized by the stability of the temperature response. To test the stability of the tensile response of flexible bending sensors, three mixed ratios of flexible bending sensors were tested for cyclic tensile response. Figure 6a–c shows that the three proportions of flexible bending sensors have great tensile response stability. The degree of response to different degrees of stretching is clearly distinguished. To test the stability of the temperature response, a cycle temperature response test (from 23°C to 80°C) of different proportions of flexible temperature sensors is carried out. Figure 6d shows that the stability of the temperature response of three proportions of flexible temperature sensors is still high after multiple cycles of temperature increase.

FIG. 6.

The performance of the sensor response. (a) Cyclic tensile response test of the flexible bending sensor with a mixing ratio of 5:1. (b) Cyclic tensile response test of the flexible bending sensor with a mixing ratio of 7.5:1. (c) Cyclic tensile response test of the flexible bending sensor with a mixing ratio of 10:1. (d) Cyclic temperature response test.

The tensile response of the flexible bending sensor in this study is affected by the external temperature (NIR). To explore the influence of temperature on the tensile response, the tensile response of the flexible bending sensor at different temperatures was tested. Figure 7a shows that temperature influences the overall resistance of the sensor. As the temperature rises, the resistance decreases. To explore the influence of temperature on the sensitivity and the linearity of the tensile response, relative resistance with strain of the flexile bending sensor at different temperatures was calculated. Figure 7b shows that the flexible bending sensor has the highest sensitivity of its tensile response at ∼55°C. At 40°C, goodness-of-fit R2 reaches 0.95731844 higher than other temperature conditions. It is suitable for measuring situations where linearity is required. The linear range is between 6.7% strain and 40% strain.

FIG. 7.

(a) Tensile response of flexible bending sensors at different temperatures. (b) Variation of relative resistance with strain and goodness-of-fit R2 at different temperatures. (c) Cyclic mechanical performance test of flexible sensor. (d) Hysteresis test of flexible temperature sensor.

The viscoelasticity of materials affects the mechanical and electrical properties of the sensor. Cyclic mechanical properties of the flexible sensor are tested. The test results are shown in Figure 7c. It can be seen from the figure that the flexible sensor has stable mechanical properties and high repeatability during the 20-cycle stretching process. The positive stroke and reverse stroke temperature input tests of the flexible temperature sensor can be seen from Figure 7d that the flexible temperature sensor has hysteresis, which may be related to the material and manufacturing method of the sensor.

As shown in Supplementary Figure S5, the output performance of the soft actuator printed by DLP was tested (bending angle and end force). As shown in Supplementary Figure S6 and Supplementary Video S3, two- and three-finger object grasping experiments of the soft actuator were performed. As shown in Supplementary Figure S7, DLP^40–42 3D printing is suitable for actuators of a variety of structures and facilitates the rapid realization of structural optimization of actuators.

Discussion

Advantage of the drive system is the untethered part. This active noncontact driving concept serves as an important reference for the development of untethered soft robots. The most significant point of this article is the pneumatic soft actuator driven by the system. It has the advantages of being untethered and meets the smaller size requirements, as shown in Supplementary Figure S16. Compared to other drive systems, the soft actuator presented in this study can be detached from the drive system. By optimizing the system size from the perspective of infrared light sources, such as infrared lasers⁴³ or components⁴⁴ with high energy conversion efficiency, the drive system offers a promising solution for future soft robotics applications (as shown in Supplementary Fig. S17). Before it could be of practical use, compared to existing mature drive methods, the repeatability, energy efficiency, and size of the photothermal drive need to be further optimized. For example, the use of collimators to concentrate NIR light and the use of highly efficient photothermal material mixing solutions^35,45 are expected to further improve their energy efficiency.

Water, ethanol, and NH₄CO₃ were used as encapsulated liquid to realize the infrared untethered drive of the soft actuator. NH₄CO₃ was used as encapsulated liquid because it is easy to decompose under heat. Water could be used as a reversible actuated encapsulated liquid. NH₄CO₃ could be used as a fast actuated encapsulated liquid. As shown in Supplementary Figure S8, the photothermal phase transition speed of NH₄CO₃ solution is faster compared with water. Ethanol is more dangerous and harmful than NH₄CO₃. The main component of the soft photosensitive resin in this subject is acrylic resin. It will be dissolved in high concentration alcohol. When the soft actuator printed by DLP is exposed to ethanol, its structure would be greatly affected. As shown in Supplementary Figure S9, the soft actuator soaked in ethanol will become embrittled and finally cannot perform bending actuation.

As shown in Supplementary Video S2 and Supplementary Figure S13, the repeatable untethered cycle actuating of the soft actuator has been achieved when water is used (mixed with a high photothermal effect subject-Mxene) as the encapsulated liquid. The time required for the soft actuator to return to its original state when it is cooled is affected by many factors. On the one hand, the speed of reverse liquid-gas transition of the internal encapsulated liquid directly affects the recovery speed of the soft actuator (as shown in Supplementary Fig. S12); on the other hand, the thermal conductivity and heat dissipation performance of the soft actuator material itself will have a great impact on the recovery speed of the soft actuator. Therefore, the recovery speed of the actuator could be improved by increasing the speed of the liquid-gas phase transition of the encapsulated liquid and the heat dissipation performance of the soft actuator material.

The infrared response speed of the soft actuator could be improved by improving the photothermal efficiency of the liquid encapsulated inside the soft actuator and the power of the external NIR light. The response speed could be improved by adding materials with higher photothermal efficiency^35,45 and increasing the external light intensity. Recently, it is also reported that the response speed of the soft actuator could be improved by the design of 3D printed soft polymers.⁴⁶ The response speed of viscous materials under pressures or stresses could be improved by the design of 3D printed soft polymers.

The cyclic actuating test using water as the encapsulated liquid was performed, as shown in Supplementary Video S2. The test results are shown in Supplementary Figure S13. It can be concluded from the experiment results that the actuation performance of the soft actuator does not show obvious attenuation after 20 cycles. The life span and the maximum number of operations of the manufactured actuator are affected by many factors, such as printing materials, 3D printing, and the temperature. Therefore, to improve the repeatability of liquid-gas phase transition of encapsulated liquid, the following aspects could be considered: (1) Adopt 3D printing materials with higher quality and higher temperature resistance.^47,48 (2) Optimize and adjust 3D printing parameters (layer height, light intensity, and illumination time) and optimize the internal structure of the actuator.^46,49 (3) Set the temperature of the NIR lamp and the distance between the actuator and the light source reasonably. Recently, it is also reported that design of 3D printed soft polymers could reduce the leakage of the actuator.⁴⁶

The external energy input of soft actuators could be realized through collimating infrared light. The operation of a single actuator without affecting adjacent actuators could be realized by a single actuator corresponding to a single collimating infrared light source, as shown in the Supplementary Figure S14.

Conclusion

In conclusion, an untethered soft pneumatic actuator with embedded multiple sensing capabilities was proposed. The infrared response speed of the soft actuator was improved by adding photothermal materials to the encapsulated liquid and increasing the NIR power. The soft conductive resin mixed with soft resin and graphite realizes the embedded multiple sensing of soft actuators by DLP 3D printing to make flexible sensors and explores the optimal proportion of graphite through the mechanical property, electrical property, and response sensitivity of flexible sensors.

Experiments show that the sensitivity of the response of soft resin and graphite is the highest when mixed in a ratio ranging from 5:1 to 7.5:1. The optimal operating temperature of the flexible bending sensor is ∼40°C. As shown in the Supplementary Figure S11, a coin-sized measurement board (Seeeduino XIAO) can be used to measure and wirelessly transmit the resistance value of the flexible sensor. While the main power for the drive could be untethered supply by infrared, the measurement board only requires a little energy for untethered measurement. In our study, the measurement board could continuously be powered by a 90mAH battery. The power consumption of the measurement circuit is relatively low. At the same time, the drive module in this article can pass noncontact visual detection in view of special situations such as the elimination of electrical signals in some applications. As shown in Supplementary Figure S10, the vision system can be built with OpenMV, to obtain a vision based measurement of deformation. Wireless deformation measurement can be achieved through image processing.

This untethered NIR-induced liquid-gas phase transition-driven soft actuator with embedded multiple sensing could provide important references when they encounter rigid machinery. It has enormous potential for future medical, rescue, and harsh environment applications.

Footnotes

Authors' Contributions

X.F.: Conceptualization, formal analysis, methodology, writing—original draft. K.W.: Data curation, investigation, validation. R.Y.: Writing—review and editing.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the National Natural Science Foundation of China (Grant Nos. 62373004 and 61973003), the Outstanding Youth Research Project in Universities of Anhui Province (Grant No. 2022AH030077), the Key Program in the Youth Elite Support Plan in Universities of Anhui Province (Grant No. gxyqZD2020012), the Basic and Clinical Collaborative Research Improvement Project of Anhui Medical University (Grant No. 2019xkjT017), the Research Foundation of Anhui Institute of Translational Medicine (Grant No. 2021zhyx-C27), and Technical development project of Anhui Medical University (Grant Nos. 2022100, 2022054, and 2023045).

Supplementary Material

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