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. 2022 Aug 19;13(8):1343.
doi: 10.3390/mi13081343.

Variable Stiffness Conductive Composites by 4D Printing Dual Materials Alternately

Fei Long  1   2   3 , Gaojie Xu  1 , Jing Wang  4 , Yong Ren  2   3   5 , Yuchuan Cheng  1   6
Affiliations

Variable Stiffness Conductive Composites by 4D Printing Dual Materials Alternately

Fei Long et al. Micromachines (Basel). .

Abstract

Materials that can be designed with programmable properties and which change in response to external stimuli are of great importance in numerous fields of soft actuators, involving robotics, drug delivery and aerospace applications. In order to improve the interaction of human and robots, materials with variable stiffness are introduced to develop their compliance. A variable stiffness composite has been investigated in this paper, which is composed of liquid metals (LMs) and silicone elastomers. The phase changing materials (LMs) have been encapsulated into silicone elastomer by printing the dual materials alternately with three-dimensional direct ink writing. Such composites enable the control over their own stiffness between soft and rigid states through LM effective phase transition. The tested splines demonstrated that the stiffness changes approximately exceeded 1900%, and the storage modulus is 4.75 MPa and 0.2 MPa when LM is rigid and soft, respectively. In the process of heating up, the stretching strain can be enlarged by at least three times, but the load capacity is weakened. At a high temperature, the resistance of the conductive composites changes with the deformation degree, which is expected to be applied in the field of soft sensing actuators.

Keywords: 4D printing; liquid metal; phase change; thermal response; variable stiffness.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) The schematic diagram of dual phase direct write printing. (b) The design structure in a ‘sandwich’ structure.
Figure 2
Figure 2
Viscoelastic inversion measurement of (a) PDMS Sylgard 184, (b) Ecoflex 0030 and (c) PDMS SE 1700. (d) The storage modulus and (e) loss modulus versus strain for silicone elastomer. (f) The viscoelastic inversion characteristics of the combination of Ecoflex 0030 and PDMS SE 1700.
Figure 3
Figure 3
(a) Shear thinning of the selected silicone slurries (b) The fitting linear function of selected silicone slurries by Herschel and Bulkley model.
Figure 4
Figure 4
(a) The detail engineering diagram of sample splines, (b) the design structure, (c) the printed real part, (d) hollow flower structure and (e) Poisson structure.
Figure 5
Figure 5
(a) The storage modulus with LM composite with the increasing volume fraction of LM. (b) The load capacity versus the stretchable strain at low and high temperature.
Figure 6
Figure 6
(a) At high temperature (60 °C), the relative resistance changes with the stretchable strain, and (b) the stretch repeatability over time.
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