Soft robotics is an emerging field of research that looks at new ways to build machines, computing systems, and biomedical devices out of materials that match the mechanical compliance and deformability of natural biological tissue. Unlike conventional robotic hardware, these systems don’t use electrical motors and instead use novel actuator materials that change their shape in response to controlled stimulation. Among the various types of actuators used in soft robotics, magnetically-responsive materials have become popular because they do not require significant on-board hardware and can instead be remotely controlled with a magnetic field.
However, soft robotic systems typically require more than the ability to actuate and move. This is especially true for biomedical applications, where devices should also be able to perform sensing, support bioelectronic or digital circuit capabilities, and harvest energy when needed in order to reduce dependency on batteries and recharging.
Recently, researchers from the School of Biomedical Engineering at Sun Yat-sen University, together with Zhejiang University and Carnegie Mellon University, have developed a hybrid magnetic-mechanical-electrical (MME) core-sheath fiber that combines actuation with sensing, electrical conductivity, and energy harvesting capabilities. Enabled by a coaxial printing method, the MME fiber can be printed into complex 2D/3D MME structures with integrated magnetoactive and conductive properties. Together, these materials allow for hybrid functions including programmable magnetization, somatosensory, and magnetic actuation along with simultaneous wireless energy transfer.
Coaxial printing of the hybrid MME structures
The MME has a core-sheath structure consisting of two functional components (Fig. 1), including a soft magnetoactive composite sheath (NdFeB particles suspended in a soft PDMS silicone matrix) and a flexible and electrically conductive core composed of liquid metal. The coaxial printing method was used to fabricate these core-sheath fibers into various desired geometries. A pulsed magnetic field Bmag (about 3 T) was then used to magnetically program the printed fibers so that they could change shape in a prescribed manner when an external field is applied.
Hybrid functionalities of magnetic driven deformation, sensing, and energy harvesting
The core-sheath MME structure can achieve hybrid functionalities of magnetically actuated deformation under an external magnetic field Bact, somatosensory and wireless power transmission. Figure 2a exhibits an electrical switch made of a 1D MME fiber that can be precisely actuated by an external magnetic field and provide electrical connection to the light LED pixels through the magnetic composite sheath and the liquid metal core respectively. Further, the 1D MME fiber can be constructed into a 2D MME coil structure by using numerically controlled coaxial printing. By altering the dimensions of the liquid metal core, the deformation of the 2D MME coil will induce the dynamic change of inductance ∆L in the 2D MME coil, which can be exploited to the somatosensory deformation of the MME structure (Fig. 2b). On the other side, a variable external actuation magnetic field Bact also induces a dynamic change in the magnetic flux of the 2D MME coil, generating an induced voltage E at the same output for inductance sensing, which can be utilized for wireless energy transmission (Fig. 2c).
Soft manipulation: self-sensing gripper
A soft somatosensory gripper made of 2D MME coils was implemented to demonstrate the ability to combine actuation with sensing. During the magnetically actuated gripping, the gripper can feel the touch/release of the object and detect the size of the object. Integrating with a robotic arm, this soft MME somatosensory gripper can perform manipulation tasks such as object identification or sorting (Fig. 2d).
Soft robotics: self-powered robot
Soft robots with 2D MME coil structures with hybrid functionality of actuation and energy harvesting were demonstrated. This soft robot can pass complex labyrinthine paths with translational/rotational motions under the remote control of the actuation magnetic field. Moreover, owing to the MME coil structures integrated with the soft robot, wireless and multimodal energy transmission/harvesting can be achieved along with robot motion/deformation (Fig. 2e). Our work thus provides a material design strategy for soft electromagnetic devices with unexplored hybrid functions.
Biomedical application: a catheter-style soft surgical tool
To show how the co-axial fiber can be used in a biomedical context, we create a catheter-style soft surgical tool for minimally invasive electro-ablation surgery in vivo on a rat heart (Fig. 3). Controlled by an actuation magnetic field, the flexible MME fiber catheter can be manipulated in an enclosed space for precise and minimally invasive surgery on rats. Compared with the operation by a rigid interventional device, the catheter-style soft surgical tool offers the flexibility of accessing a large operation area in a confined space through a small entrance hole, presenting a promising strategy for targeted and minimally invasive surgery.