Two-photon polymerization-based 4D printing and its applications

Two-photon polymerization(TPP)is a cutting-edge micro/nanoscale three-dimensional(3D)printing technology based on the principle of two-photon absorption.TPP surpasses the diffraction limit in achieving feature sizes and excels in fabricating intricate 3D micro/nanostructures with exceptional resolution.The concept of 4D entails the fabrication of structures utilizing smart materials capable of undergoing shape,property,or functional changes in response to external stimuli over time.The integration of TPP and 4D printing introduces the possibility of producing responsive structures with micro/nanoscale accuracy,thereby enhancing the capabilities and potential applications of both technologies.This paper comprehensively reviews TPP-based 4D printing technology and its diverse applications.First,the working principles of TPP and its recent advancements are introduced.Second,the optional4D printing materials suitable for fabrication with TPP are discussed.Finally,this review paper highlights several noteworthy applications of TPP-based 4D printing,including domains such as biomedical microrobots,bioinspired microactuators,autonomous mobile microrobots,transformable devices and robots,as well as anti-counterfeiting microdevices.In conclusion,this paper provides valuable insights into the current status and future prospects of TPP-based4D printing technology,thereby serving as a guide for researchers and practitioners.

Hydrogel-elastomer-based stretchable strain sensor fabricated by a simple projection lithography method

Stretchable strain sensor detects a wide range of strain variation and is therefore a key component in various applications.Unlike traditional ones made of elastomers doped with conductive components or fabricated with liquid conductors,ionically conductive hydrogel-based strain sensors remain conductive under large deformations and are biocompatible.However,dehydration is a challenging issue for the latter.Researchers have developed hydrogel-elastomer-based strain sensors where an elastomer matrix encapsulates a hydrogel circuit to prevent its dehydration.However,the reported multistep approaches are generally time-consuming.Our group recently reported a multimaterial 3D printing approach that enables fast fabrication of such sensors,yet requires a self-built digital-light-processing-based multimaterial 3D printer.Here,we report a simple projection lithography method to fabricate hydrogel-elastomer-based stretchable strain sensors within 5 minutes.This method only requires a UV projector/lamp with photomasks;the chemicals are commercially available;the protocols for preparing the polymer precursors are friendly to users without chemistry background.Moreover,the manufacturing flexibility allows users to readily pattern the sensor circuit and attach the sensor to a 3D printed soft pneumatic actuator to enable strain sensing on the latter.The proposed approach paves a simple and versatile way to fabricate hydrogel-elastomer-based stretchable strain sensors and flexible electronic devices.

A high-fidelity human cervical muscle finite element model for motion and injury studies

Active muscle response is a key factor in the motion and injury of the human head and neck.Due to the limitations of experimentation and the shortcomings of previous finite element models,the influence of material parameters of cervical muscle on motions of the head and neck during a car crash have not been comprehensively investigated.In the present work,a model of the cervical muscle in a 50th-percentile adult male was constructed.The muscles were modelled using solid finite elements,with a nonlinear-elastic and viscoelastic material and a Hill material modelling the passive and active parts of each muscle,respectively.The head dynamic responses of the model were validated using results obtained from volunteer sled tests.The influence of the material parameters of a muscle on head and neck motions were determined.Our key finding was that the greater the stiffness and the contraction strength of the neck muscles,the smaller the rotation angle of the head and the neck,and,hence,the lower the risk of head and neck injury to occupants in a car crash.

Stiffness-Tunable Origami Structures via Multimaterial Three-Dimensional Printing

Origami structure has been employed in many engineering applications.However,there is currently no strategy that can systematically achieve stiffness-tunable origami(STO)structures through proper geometric design.Here,we report a strategy for designing and fabricating STO structures based on thick-panel origami using multimaterial 3D printing.By adjusting the soft hinge position,we tune the geometric parameterψto program the stiffness and strength of origami structures.We develop origami structures with graded stiffness and strength by stacking Kresling origami structures with differentψ.The printed structures show great cyclic characteristics and deformation ability.After optimizing combinations of structures with differentψ,the multi-layer Kresling STO structures can effectively reduce the peak impact,showing a good energy absorption effect.The proposed approach can be implemented in various origami patterns to design and tune the mechanical properties of origami structures for many potential applications.