Overview of Hydrogels in Soft Robots
Hydrogel-Based Soft Actuators
Other Stimuli-Responsive Actuators
Hydrogel-Based Soft Sensors
New Developments
Hydrogels as Soft Robots
References and Further Reading
In recent years, the emergence of soft robotics has spurred significant developments in smart materials. Hydrogels have long been considered suitable candidates for soft robotics owing to their elasticity, tunable smart properties, softness, and the ability to be modified with nanomaterials. This article explores how hydrogels are used to develop soft robots.
Image Credit: Vershinin89/Shutterstock
Overview of Hydrogels in Soft Robots
The field of robotics in which flexible soft materials such as elastomers and hydrogels are used to build robots that are relatively safer and more effective for complex tasks across diverse environments is known as soft robotics.1,2
Hydrogels most closely resemble biological tissues and exhibit various smart properties that can be tuned by adding nanomaterials. They are modeled after biological systems, which combine stiff and soft components. The softness of hydrogels reduces the possibility of harm during close human contact.1,2
The material contributes to two key components of soft robots, including sensors and actuators, which enable robots to provide more adaptable and safer services. Hydrogel-based soft robots are classified into environmental monitoring, industrial automation, and biomedical robotics based on applications.2
In soft robotics, hydrogels operate through diverse mechanisms, including responsive actuation, environmental adaptation, enhanced sensing, and biomimetic design. Collectively, these characteristics improve the adaptability and usefulness of hydrogel-based continuum soft robotics.2
Hydrogel-Based Soft Actuators
Soft actuators based on hydrogels enable soft robots to perform different actions or functions by regulating the deformation of morphing materials. For instance, hydrogel-based actuators enable soft robots to perform locomotive actions such as crawling, jumping, walking, and swimming. They also enable these robots to grasp delicate objects.1
Motivated by the emergence of soft robotics, several studies have focused on electroactive hydrogels in recent years. For instance, the walker robot, composed of two hydrogels with different electrical properties, can be used for underwater locomotion and robotic manipulation.1
Yet, the electroactive response of these actuators is slow. For instance, the largest propulsion velocity achieved by the walker robot was ∼2.5 mm min-¹, corresponding to moving half its body length in 1 min.1
Other Stimuli-Responsive Actuators
Apart from electricity, other stimuli-responsive mechanisms, such as light or magnetic fields, have also been investigated. However, these actuation methods are implemented as small-scale robots or proof of concept.1
Nanomaterials have played a major role in the development of hydrogel-based underwater actuators that respond to non-electrical stimuli. For instance, the single-walled carbon nanotube-loaded poly(N-isopropylacrylamide) (pNIPAM) hydrogel synthesized in a study is utilized to build underwater folding structures and responds to temperature when it remains immersed in water.1
Download the PDF of this page here
The loaded carbon nanotubes improved the thermal response by 5-fold compared to the pure pNIPAM hydrogel and enabled a fast optical response owing to their robust near-infrared absorption.1
In the case of out-of-water actuation, several studies investigated the encapsulation of hydrogels using the water required for their actuation. For instance, a study developed a light- and thermal-responsive actuator by encapsulating a pNIPAM hydrogel in water within a polyvinylidene chloride thin film.1
The hydrogel loses water and shrinks when heated, causing the actuator to bend. However, the released water remains in the package, enabling swelling when the hydrogel is cooled and thus allowing the actuator to operate repeatedly.1
Hydrogel-Based Soft Sensors
Soft sensors leverage material properties like capacitive, resistive, piezoresistive, and optical characteristics. Yet, unlike actuators, most applications of these sensors occur outside water, necessitating the prevention of water loss from hydrogels.1
For instance, a resistive sensor was sandwiched with a commercial elastomer in a study to reduce the water loss of the sensor.3 Hydrogels are increasingly used in resistive strain sensors due to the ease of obtaining a conductive stretchable hydrogel. Hydrogel-based resistive sensors can be obtained by conductive polymers or ionic networks. The addition of nanomaterials such as metal nanoparticles or carbon-based nanomaterials can impart conductivity to an electrically neutral hydrogel.1
Joint angle sensors and pressure and tactile sensors, which are used in robotic systems for human motion rehabilitation, smart skins, and manipulator robots, are common applications of resistive strain sensors.1
Capacitive sensors with hydrogels are typically developed by sandwiching two conductive hydrogels between a dielectric soft layer to form a variable-capacitance capacitor. These sensors have similar applicability to resistive strain sensors, and their conductivity can be enhanced using nanomaterials.1
Hydrogel-based capacitive sensors can also be developed by incorporating hydrogel into the dielectric layer. For instance, a study developed a sensor in which a hybrid hydrogel with graphene oxide, wrapped in an insulated ultrathin polyethylene film, serves as the dielectric.1
New Developments
Many studies have explored different approaches for actuation. For instance, hybrid hydrogels embedded with ferrous (iron oxide) nanoparticles were synthesized in a study published in the 2021 IEEE International Conference on Intelligence and Safety for Robotics to respond to magnetic fields.1,4
The hydrogel moved macroscopically when magnetic attraction was applied to the ferromagnetic nanoparticles. In these actuators capable of operating under air conditions, actuation is attributed to smart properties that can reside in the hydrogel's composition or network, or in nanoparticles added to the matrix.1,4
A recent study published in Advanced Robotics developed a cost-effective, disposable double-network gel-based soft inchworm robot with adjustable movement speeds and motion techniques controlled by varying the voltage applied to its legs.2,5
Fluidic actuators, primarily pneumatic, are used in soft manipulators and soft grippers. All hydrogel-based fluidic actuators are operated outside water to prevent swelling. Using hydrogels rather than other materials is beneficial for these actuators.1
For instance, a study published in Science Robotics proposed a classical pneumatic actuator composed of two hydrogels, including polyacrylamide (pAAM) and pNIPAM. This actuator benefits from autonomous perspiration owing to the chemo-mechanical response of the hydrogel materials, which open and close pores in the actuator's structure in response to temperature. This capacity prolongs the actuator's life cycle.1,6
Hydrogels as Soft Robots
Hydrogels are used to build soft robots because of their softness, tunable properties, and compatibility with nanomaterials. They function as actuators and sensors, enabling robots to crawl, swim, jump, grasp delicate objects, and detect movement or pressure. By responding to electrical, thermal, optical, and magnetic stimuli, hydrogel-based robots support biomedical applications, industrial automation, and environmental monitoring.
References and Further Reading
- López-Díaz, A., Vázquez, A. S., & Vázquez, E. (2024). Hydrogels in soft robotics: past, present, and future. ACS Nano, 18(32), 20817. DOI: 10.1021/acsnano.3c12200, https://pubs.acs.org/ancac3/article/18/32/20817/150974/Hydrogels-in-Soft-Robotics-Past-Present-and-Future
- Bhuyan, M. M., Hasan, N., & Jeong, J. H. (2025). Single- and Multi-Network Hydrogels for Soft Electronics - A Review. Gels, 11(7), 480. DOI: 10.3390/gels11070480, https://www.mdpi.com/2310-2861/11/7/480
- Wang, S., Sun, Z., Zhao, Y., & Zuo, L. (2021). A highly stretchable hydrogel sensor for soft robot multi-modal perception. Sensors and Actuators A: Physical, 331, 113006. DOI: 10.1016/j.sna.2021.113006, https://www.sciencedirect.com/science/article/abs/pii/S0924424721004714
- Bai, A. D., Zhang, B. X., & Dai, C. J. (2021). Preparation and property analysis of Fe3O4/PVA magnetic hydrogel base on soft robot matrix. 2021 IEEE International Conference on Intelligence and Safety for Robotics (ISR), 47-51. DOI: 10.1109/ISR50024.2021.9419553, https://ieeexplore.ieee.org/document/9419553
- Selvamuthu, M. G. et al. (2023). Development of soft inchworm robot with friction control of feet using double-network gel. Advanced Robotics, 37(6), 407-422. DOI: 10.1080/01691864.2022.2152291, https://www.tandfonline.com/doi/full/10.1080/01691864.2022.2152291
- Mishra, A. K. et al. (2020). Autonomic perspiration in 3D-printed hydrogel actuators. Science Robotics. DOI: 10.1126/scirobotics.aaz3918, https://www.science.org/doi/10.1126/scirobotics.aaz3918
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.