Soft robots have the appealing advantages of being highly flexible and adaptive to complex environments. However, the low‐stiffness nature of the constituent materials makes soft robotic systems incompetent in tasks requiring relatively high load capacity. Despite recent attempts to develop stiffness‐tunable soft actuators by employing variable stiffness materials and structures, the reported stiffness‐tunable actuators generally suffer from limitations including slow responses, small deformations, and difficulties in fabrication with microfeatures. This work presents a paradigm to design and manufacture fast‐response, stiffness‐tunable (FRST) soft actuators via hybrid multimaterial 3D printing. The integration of a shape memory polymer layer into the fully printed actuator body enhances its stiffness by up to 120 times without sacrificing flexibility and adaptivity. The printed Joule‐heating circuit and fluidic cooling microchannel enable fast heating and cooling rates and allow the FRST actuator to complete a softening–stiffening cycle within 32 s. Numerical simulations are used to optimize the load capacity and thermal rates. The high load capacity and shape adaptivity of the FRST actuator are finally demonstrated by a robotic gripper with three FRST actuators that can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.
Four-dimensional
(4D) printing that enables 3D printed structures
to change configurations over time has gained great attention because
of its exciting potential in various applications. Among all the 4D
printing materials, shape memory polymers (SMPs) possess higher stiffness
and faster response rate and therefore are considered as one of most
promising materials for 4D printing. However, most of the SMP-based
4D printing materials are (meth)acrylate thermosets which have permanently
cross-linked covalent networks and cannot be repaired if any damage
occurs. To address the unrepairable nature of SMP-based 4D printing
materials, this paper reports a double-network self-healing SMP (SH-SMP)
system for high-resolution self-healing 4D printing. In the SH-SMP
system, the semicrystalline linear polymer polycaprolactone (PCL)
is incorporated into a methacrylate-based SMP system which has good
compatibility with the digital light processing-based 3D printing
technology and can be used to fabricate complex 4D printing structures
with high resolution (up to 30 μm). The PCL linear polymer imparts
the self-healing ability to the 4D printing structures, and the mechanical
properties of a damaged structure can be recovered to more than 90%
after adding more than 20 wt % of PCL into the SH-SMP system. We investigated
the effects of PCL concentration on the thermomechanical behavior,
viscosity, and the self-healing capability of the SH-SMP system and
performed the computational fluid dynamics simulations to study the
effect of SH-SMP solution’s viscosity on the 3D printing process.
Finally, we demonstrated the self-healing 4D printing application
examples to show the merits of the SH-SMP system.
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