Soft Robotics

Multi-material fluidic actuators

Soft fluidic actuators consisting of elastomeric matrices with embedded flexible materials (e.g. cloth, paper, fiber, particles) are of particular interest to the robotics community because they are lightweight, affordable and easily customized to a given application. These actuators can be rapidly fabricated in a multi-step molding process and can achieve combinations of contraction, extension, bending and twisting with simple control inputs such as pressurized fluid. In our approach is to use new design concepts, fabrication approaches and soft materials to improve the performance of these actuators compared to existing designs. In particular, we use motivating applications (e.g. heart assist devices, soft robotic gloves) to define motion and force profile requirements. We can then embed mechanical intelligence into these soft actuators to achieve these performance requirements with simple control inputs.

Modeling of soft actuators

Characterizing and predicting the behavior of soft multi-material actuators is challenging due to the nonlinear nature of both the hyper-elastic material and the large bending motions they produce. We are working to comprehensively describe the principle of operation of these actuators through analytical, numerical and experimental approaches and characterize their outputs (motion and force) as a function of input pressure as well as geometrical and material parameters. Both models and experiments offer insight into the actuator behavior and the design parameters that affect it. We envision this work will lead to improved predictive models that will enable us to rapidly converge on new and innovative applications of these soft actuators.

Sensing and control

In order to control soft actuators, we need means of monitoring their kinematics, interaction forces with objects in the environment and internal pressure. We accomplish this through the use of fully soft sensors, developed with collaborators, and miniature or flexible sensors that can be incorporated into the actuator design during the manufacturing process. For power and control, we use off the shelf components such as electronic valves, pumps, regulators, sensors, and control boards etc. to rapidly modulate the pressure inside the chambers of the actuators using feedback control of pressure, motion and force. In addition, we can use the analytical models we develop to estimate state variables that may be difficult to measure directly.

Translational applications

There are approximately four million chronic stroke survivors with hemiparesis in the US today and another six million in developed countries globally. In addition, there are millions of other individuals suffering from similar conditions. For the majority of these cases, loss of hand motor ability is observed, and whether partial or total, this can greatly inhibit activities of daily living (ADL) and can considerably reduce one’s quality of life. To address these challenges, we are developing a modular, safe, portable, consumable, at-home hand rehabilitation and assistive device that aims to improve patient outcomes by significantly increasing the quantity (i.e. time) and quality of therapy at a reduced cost while also improving independence of users with chronic hand disabilities by enabling them to perform activities of daily living.

In the United States, the lifetime risk of developing heart failure is roughly 20%. The current clinical standard treatment is implantation of a ventricular assist device that contacts the patient’s blood and is associated with thromboembolic events, hemolysis, immune reactions and infections. We are applying the field of soft robotics to develop a benchtop cardiac simulator and a Direct Cardiac Compression (DCC) device employing soft actuators in an elastomeric matrix. DCC is a non-blood contacting method of cardiac assistance for treating heart failure involving implantation of a device that surrounds the heart and contracts in phase with the native heartbeat to provide direct mechanical assistance during the ejection phase (systole) and the relaxation phase (diastole) of the cardiac cycle.

Associated Papers

Z. Wang, P. Polygerinos, J. T. B. Overvelde, K. C. Galloway, K. Bertoldi, and C. J. Walsh, “Interaction Forces of Soft Fiber Reinforced Bending Actuators,” IEEE/ASME Transactions on Mechatronics, vol. PP, no. 99, 2016. Publisher's VersionAbstract

Soft bending actuators are inherently compliant, compact, and lightweight. They are preferable candidates over rigid actuators for robotic applications ranging from physical human interaction to delicate object manipulation. However, characterizing and predicting their behaviors are challenging due to the material nonlinearities and the complex motions they can produce. This paper investigates a soft bending actuator design that uses a single air chamber and fiber reinforcements. Additionally, the actuator design incorporates a sensing layer to enable real-time bending angle measurement for analysis and control. In order to study the bending and force exertion characteristics when interacting with the environment, a quasistatic analytical model is developed based on the bending moments generated from the applied internal pressure and stretches of the soft materials. Comparatively, a finite-element method model is created for the same actuator design. Both the analytical model and the finite-element model are used in the fiber reinforcement analysis and the validation experiments with fabricated actuators. The experimental results demonstrate that the analytical model captures the relationships of supplied air pressure, actuator bending angle, and interaction force at the actuator tip. Moreover, it is shown that an off-the-shelf bend angle sensor integrated to the actuator in this study could provide real-time force estimation, thus eliminating the need for a force sensor.

F. Connolly, C. J. Walsh, and K. Bertoldi, “Automatic design of fiber-reinforced soft actuators for trajectory matching,” Proceedings of the National Academy of Sciences (PNAS), vol. 114, no. 1, pp. 51-56, 2017. Publisher's VersionAbstract

Soft actuators are the components responsible for producing motion in soft robots. Although soft actuators have allowed for a variety of innovative applications, there is a need for design tools that can help to efficiently and systematically design actuators for particular functions. Mathematical modeling of soft actuators is an area that is still in its infancy but has the potential to provide quantitative insights into the response of the actuators. These insights can be used to guide actuator design, thus accelerating the design process. Here, we study fluid-powered fiber-reinforced actuators, because these have previously been shown to be capable of producing a wide range of motions. We present a design strategy that takes a kinematic trajectory as its input and uses analytical modeling based on nonlinear elasticity and optimization to identify the optimal design parameters for an actuator that will follow this trajectory upon pressurization. We experimentally verify our modeling approach, and finally we demonstrate how the strategy works, by designing actuators that replicate the motion of the index finger and thumb.

D. P. Holland, et al., “The Soft Robotics Toolkit: Strategies for Overcoming Obstacles to the Wide Dissemination of Soft-Robotic Hardware,” IEEE Robotics and Automation Magazine, Special Issue on Open Source and Widely Disseminated Robot Hardware, vol. 24, no. 1, pp. 57-64, 2017. Publisher's VersionAbstract

The Soft Robotics Toolkit (SRT) is an open-access website containing detailed information about the design, fabrication, and characterization of soft-robotic components and systems (Figure 1). Soft robotics is a growing field of research concerned with the development of electromechanical technology composed of compliant materials or structures. The SRT website hosts design files, multimedia fabrication instructions, and software tutorials submitted by an international community of soft-robotics researchers and designers. In this article, we describe the development of the SRT and some challenges in developing widely disseminated robotic-hardware resources. Our attempts to overcome these challenges in the development of the toolkit are discussed by focusing on strategies that have been used to engage participants ranging from K-12 grade students to robotics research groups. A series of design competitions encouraged people to use and contribute to the toolkit. New fabrication methods requiring only low-cost and accessible materials were developed to lower the entry barriers to soft robotics and instructional materials and outreach activities were used to engage new audiences. We hope that our experiences in developing and scaling the toolkit may serve as guidance for other open robotic-hardware projects.

Soft robotic sleeve supports heart function
E. T. Roche, et al., “Soft robotic sleeve supports heart function,” Science Translational Medicine, vol. 9, no. 373, 2017. Publisher's VersionAbstract

There is much interest in form-fitting, low-modulus, implantable devices or soft robots that can mimic or assist in complex biological functions such as the contraction of heart muscle. We present a soft robotic sleeve that is implanted around the heart and actively compresses and twists to act as a cardiac ventricular assist device. The sleeve does not contact blood, obviating the need for anticoagulation therapy or blood thinners, and reduces complications with current ventricular assist devices, such as clotting and infection. Our approach used a biologically inspired design to orient individual contracting elements or actuators in a layered helical and circumferential fashion, mimicking the orientation of the outer two muscle layers of the mammalian heart. The resulting implantable soft robot mimicked the form and function of the native heart, with a stiffness value of the same order of magnitude as that of the heart tissue. We demonstrated feasibility of this soft sleeve device for supporting heart function in a porcine model of acute heart failure. The soft robotic sleeve can be customized to patient-specific needs and may have the potential to act as a bridge to transplant for patients with heart failure.

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