Femtosecond laser programmed artificial musculoskeletal systems

Abstract

Natural musculoskeletal systems have been widely recognized as an advanced robotic model for designing robust yet flexible microbots. However, the development of artificial musculoskeletal systems at micro-nanoscale currently remains a big challenge, since it requires precise assembly of two or more materials of distinct properties into complex 3D micro/nanostructures. In this study, we report femtosecond laser programmed artificial musculoskeletal systems for prototyping 3D microbots, using relatively stiff SU-8 as the skeleton and pH-responsive protein (bovine serum albumin, BSA) as the smart muscle. To realize the programmable integration of the two materials into a 3D configuration, a successive on-chip two-photon polymerization (TPP) strategy that enables structuring two photosensitive materials sequentially within a predesigned configuration was proposed. As a proof-of-concept, we demonstrate a pH-responsive spider microbot and a 3D smart micro-gripper that enables controllable grabbing and releasing. Our strategy provides a universal protocol for directly printing 3D microbots composed of multiple materials.

Fig. 1. Scheme of the successive on-chip TPP strategy and a proof-of-concept spider microbot.

a Schematic illustration for the femtosecond laser programmable fabrication of the musculoskeletal systems. (i) TPP fabrication of the 3D SU-8 body and skeleton structure for eight legs; (ii) in situ development of the spider body and skeleton; (iii) secondary integration of the pH-responsive BSA muscles via TPP. b, c SEM images of the micro-spider before and after the integration of BSA muscles, respectively. The insets correspond to their optical microscopy images, and the blue parts in the latter one indicates the presence of BSA muscles at the joints of the legs. d–f Detailed SEM images of the joints where the BSA muscles were well integrated. g The actuation of the micro-spider when the pH value was switched from 13 to 5. The profiles of the micro-spider in this actuation are marked in red, yellow, and green, respectively. The superposition of the contour profiles (right image) at different time is provided for comparison.

Fig. 2. pH-responsive properties of the active BSA muscle and the inert SU-8 skeleton.

a Schematic illustration for the pH-responsive property of BSA molecular chains. Typical arginine, aspartate, histidine, glutamate, and lysine fragments were used for demonstrating their electrostatic interaction under pH values of 5, 1, and 13, respectively. At the isoelectric point (pH = 5), the BSA molecular chains are electrically neutral, thus exhibiting the smallest volume. At pH = 1, the amino groups of the BSA molecular chains are protonated, whereas the carboxyl groups are electrically neutral, so the molecular chains are positively charged. At pH = 13, the amino groups are electrically neutral, whereas the carboxyl groups are deprotonated, so the molecular chains are negatively charged. Due to the electrostatic repulsion of the BSA molecular chains, the volume of BSA structure increased. b The contrast of the swelling ratio of SU-8 and BSA blocks (10 × 10 μm in size). c Dynamic tuning of the size of BSA blocks by 200 times of pH switching. Error bars denote the standard deviation of the measurements. Source data are provided as a Source data file.

Fig. 3. Dynamic pH actuation of two typical musculoskeletal systems.

a Arm-muscle system: Top images: the model (left) and SEM image (right) of the SU-8 skeleton. Middle images: SEM image (left) of the arm-muscle after BSA integration and SEM image of the BSA and SU-8 interface (right). Bottom images: SEM images of the arm-muscle (left) and the two materials interface (right) after storage for 45 days. b Optical microscopic images of the pH actuation of the arm-muscle and that after 45 days. c Dynamic pH-responsive properties of the arm-muscle structure. d The crab claw-muscle system: Top images: Optical microscopic image (left) and SEM image of the SU-8 skeleton (right); bottom images: Optical microscopic image (left) and SEM image of the claw after BSA integration (right). e Optical microscopic images of the pH actuation of the claw. f Dynamic pH-responsive properties of the crab claw for 4 cycles. Error bars denote the standard deviation of the measurements. Source data are provided as a Source data file.

Fig. 4. A pH-driven micro-gripper.

a The 3D model of the micro-gripper. b, c SEM images of the micro-gripper before and after the integration of BSA muscles, respectively. d Dependence of the swelling ratio of a BSA micro-block (10 μm × 10 μm) on the laser scanning step length (50, 100, 150, and 200 nm) when the pH of the surrounding solution was switched between 5 and 13. The insets are corresponding optical microscopic images of the BSA blocks. e The dependence of the folding angle (one arm of the micro-gripper) on the laser step lengths for the fabrication of BSA muscles. f–i The pH-responsive grapping performance of the micro-grippers with BSA muscles fabricated under the laser step lengths of 50, 100, 150, and 200 nm, respectively. Error bars denote the standard deviation of the measurements. Source data are provided as a Source data file.

Fig. 5. Flexible manipulation of the smart micro-gripper.

a The folding angle of the micro-grippers at different pH values. b The pH-responsive folding dynamics of the micro-grippers. c The actuation process of the micro-gripper in an aqueous solution with pH = 13. d SEM image of the target cargo, a SU-8 block attached to a base. e Schematic procedure of the catching and releasing of the SU-8 cargo using the pH-responsive micro-gripper. f The real process of the manipulation: (i) positioning, the micro-gripper that is integrated with a glass cantilever gradually approached the target and precisely aligned with it with the help of a 3D moving stage; (ii) actuation, the micro-gripper was stimulated by the solution with pH = 13, which triggered the folding of the micro-gripper. In this way, the target block was tightly gripped; (iii) transporting, the micro-gripper holding the object can be transferred to any desired location under the precise control of the motion system; (iv) releasing, the micro-gripper was stimulated by a solution of pH 5, which caused the unfolding of the arms, thus allowing the release of the object. Error bars denote the standard deviation of the measurements. Source data are provided as a Source data file.