**1. Introduction**

In contrast to the uniaxial or torsional displacements of traditional actuators, soft elastomers can be programmed to undergo changes throughout the whole structure with the significant advantages in low density, high mechanical flexibility and multidimensional movement. As an aspect that has received a lot of attention surrounding soft actuators recently, tunable stiffness refers to the ability of materials or systems to transform between a compliant and rigid load-bearing state after applying external stimulus. Until now, such controllable stiffness materials have been applied in a wide range of fields, such as more efficient catheters and endoscopes to perform non-invasive procedures [1] in medicine, adaptive wings that improve aircraft performance [2] in aeronautics and building materials that lower the damages from wind and earthquakes [3] in architecture. To avoid time and material consuming with the tedious fabrication process, a suitable approach with additive manufacturing (AM) is required for effective fabrication of 3D structure with complex topographical feature. Taking a particular instance, various 3D printing techniques have

**Citation:** Long, F.; Xu, G.; Wang, J.; Ren, Y.; Cheng, Y. Variable Stiffness Conductive Composites by 4D Printing Dual Materials Alternately. *Micromachines* **2022**, *13*, 1343. https://doi.org/10.3390/mi13081343

Academic Editor: Pingan Zhu

Received: 25 July 2022 Accepted: 11 August 2022 Published: 19 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

been applied for designing versatile microfluidic systems [4] to detect different analytes [5] and different clinically relevant diseases [6].

The additive manufacturing technique, also known as three-dimensional printing (3D printing), can enable one-step patterning of multi-materials, such as plastics [7], metals [8], ceramics [9] and woods [10]. Traditional processing techniques such as milling, molding and engraving result in high ambient temperatures, which have been a hindrance to pattern metal and polymeric materials directly. The conventional 3D printing objects are considered as statistic, whose dynamic evolutions were restricted in potential applications. Recently, the significant efforts on 3D printing have yielded four-dimensional (4D) printing structures that the structure or properties change over time; in other words, it is an updated version of 3D printing technologies with stimuli-materials involving heat [11], light [12], magnetic field [13], electrical field [14], etc. Until now, 4D printing has attached worldwide attention from micro- to macroscale as a result of various functional applications in soft robotics, flexible electronics, and biomedicine [15].

The materials with significant variable stiffness can be primarily classified into three groups that are shape memory polymers (SMP), smart materials and low melting point materials. Shape memory materials can experience the irreversible transitions from metastable to globally stable states [16] or response to environmental conditions to undergo reversible changes (such as LCEs [17]). The phase transition of major shape memory materials operates through temperature change, in detail that SMPs must be heated indirectly using external heaters [18]. For smart materials (for example, electrorheological fluids (ERF) [19] and magnetorheological fluids (MRF)) [20], the principle of phase transitions depends on the order of the inner nanometer or micrometer to change own viscosity. Low melting point materials represented by metal alloys [21] can be heated directly due to the thermal conductivity, and its heating rate is almost 100 times of SMPs with 18 W/mK [22]. Since both smart fluids and low melting point materials will exist as flowing state in a compliant state, they must be encapsulated to perform stiffness-changing functions without the loss of fluids.

As one of the low melting point materials, liquid metals (LMs) exhibit the properties of traditional rigid metal in a solid state, while it can be dispensed, stretched and deformed easily after melting. With the inherent soft state, LMs are suitable for applications in the devices where the desired materials need to endure varying degrees of stress, such as soft actuators and soft robotics. In addition, the features in high electrical and thermal conductivity can be applied in cases where design stretchable circuits and strain sensors [23], such as heat dissipation [24]. However, the surface tension of LMs is high with approximately 700 mN/m [25], that is poor wettability with solid substrates, resulting in the leakage or spillage problems when utilizing devices [26]. Cheng et al. [27] reported that the oxidized LMs have ability to reduce surface tension and improve wettability, but its lower thermal conductivity with around 1 Wm−<sup>1</sup> K <sup>−</sup><sup>1</sup> will deteriorate severely. Until now, to balance wettability and thermal performance, Cu particles have been mixed by various shapes and sizes [26]. Due to LMs fluidity in the liquid state and inherent brittleness in the stiff state, it is difficult to pattern in the solid form for further processing [28]. It is possible to introduce direct ink printing (DIW) that is a common approach in the AM field in spite of existing challenges to print LM directly [29], so the precise control of distance between nozzle and surface enables the direct printing of non-spherical shapes by shearing metal from the nozzle. As the printed structures maintain a liquid state at room temperature, further encapsulations are required for most applications.

In this paper, a controllable stiffness composite has been proposed, which consisted of LM as functional materials and silicone elastomer as the encapsulation layer, respectively. Here, to explore the absolute stiffness and hardness, three types of silicone have been selected, including Ecoflex00-30, PDMS Sylgard 184 and SE 1700. Additionally, different mixing ratios of silicone have been adjusted to satisfy the necessary rheological conditions for DIW. Through printing dual materials alternately, simple splines, hollow flower patterns and Poisson structures can be fabricated. The stiffness change of LM composites can be

controlled by the thermal response of LMs, which results in large changes in stiffness after LM melting completely, while the silicone guarantees that the melted LMs retain the pre-molten shape. The DIW of LMs has broken through the difficulty of filling approach and can achieve a larger volume content of LM composites that were promoted a wider range of stiffness change. At a temperature above the melting point, the addition of LM will greatly increase the tensile deformation capacity, but the load-carrying ability will be weakened to a certain extent. Owing to the electrical conductivity of LMs, their resistance can change during stretching and the recyclability has certain advantages compared to previous studies, which is expected to be applied into soft sensing actuators.

### **2. Experimental**

### *2.1. Dual Phase Direct Write Printing*

A challenge for LM elastomers is that filling LM into the cured silicone mold is difficult at the room temperature due to the high surface tension of LM. A multi-material alternate printing approach, previously used for water-soluble support in 3D bioprinting systems has been proposed to stabilize the surface contact between materials. Bodaghi et al. [30] has fabricated dual-material lattice-based meta-structures by fused deposition modelling technology. In this study, the progress of dual phase direct write printing (Figure 1a) has been divided into three layers as a 'sandwich' structure, shown as in Figure 1b. In the DIW printing process, the relevant typical parameters can refer to the 3D printing of stretchable elastomers by Zhou et al. [31]. With a suitable pressure P = 0.1 MPa, the slurries are extruded from the narrow nozzle with a diameter D of 450 mm at a speed c which is pressure dependent. Since the extruded slurries will experience die-swelling [32], the diameter of the filament can be defined as αD, in which α is die-swelling ratio. Meanwhile, the deposited slurries are extruded from the moving nozzle at a speed V of 8 mm/s and a height H of 0.05 mm which is the distance between the printer layer and the nozzle. As the surface tension between the LM in solid state and the silicone elastomer in semi-solid state is relatively small [33,34], cooling LMs to solid can alleviate such tough problems. In detail, after completing the first layer, the carrier platform needs to be cooled down to −10 ◦C for printing the second layer, which ensures that LMs can be cured quickly after printing to contact with the first layer in the solid state. Finally, the third layer is printed to encapsulate the LMs. This one-step additive manufacturing technology can complete complex structures in a short time, effectively reducing preparation time and saving costs. The process of extruding the slurries from the narrow nozzle can be regarded as preparing the tiny droplets under the microfluidic control, so the rheological parameters are extremely significant. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 4 of 12

**Figure 1.** (**a**) The schematic diagram of dual phase direct write printing. (**b**) The design structure in a 'sandwich' structure. detail settings have been shown in the ESI. **Figure 1.** (**a**) The schematic diagram of dual phase direct write printing. (**b**) The design structure in a 'sandwich' structure.

The liquid metal applied in this research is composed of 75.5% gallium (Ga) and 24.5% indium (In), purchased from Jintai Alloy Corporation, Guangdong, China. The se-

The experiments on the melting point of LMs were used a DCS 214 (NETZSCH, Selb, German) with high purity alumina ceramic crucible that can withstand 100 °C and bear the corrosion of Ga. The measurement temperature range was from −40 °C to 40 °C, and

The experiments on rheology were conducted by Discovery HR-20 (TA, New Castle, DE, USA), equipped with a 20 mm parallel plate geometry. To minimize the effect by measuring, all samples were pre-sheared and tested for three times. Additionally, the spe-

Mechanical experiments were conducted using Dynamic Mechanical Analyzer DMA Q800 (TA, New Castle, DE, USA) and universal testing machine UTM Roell Z030 (Zwick, Ulm, German). Both instruments have been equipped with the heating function, and the

of high surface tension with around 0.624 N/m and low viscosity with 0.0024 Pas; the above data were provided by supplier, which are consistent basically with previous study by Koster [35]. Here, there are three types of silicon elastomers selecting in this study, containing EcoflexTM 0030 (Smooth-On, Macungie, PA, USA), PDMS 184 (DOWSILTM, Sylgard, Midland, MI, USA) and PDMS 1700 (DOWSILTM, SE, Midland, MI, USA). Ecoflex 0030 was prepared in a 1:1 base to curing agent weight ratios, while two kinds of PDMS were mixed at 1:10. In order to obtain the printable slurries, Ecofelx 0030 and SE1700 would be mixed at 1:1 in weight, while Sylgard 184 and SE 1700 should be mixed at 1:2. The printable substrates have been mixed with a planetary mixer (VM300SA3, Mi-

antangshinuo Corporation, Jiangsu, China).

*2.2. Materials*

*2.3. Measurements*

2.3.1. Thermal Characterization

the heating rate was 10 K/min.

2.3.2. Rheological Characterization

2.3.3. Mechanical Characterization

cific steps have been discussed in further detail in the ESI.

### *2.2. Materials*

The liquid metal applied in this research is composed of 75.5% gallium (Ga) and 24.5% indium (In), purchased from Jintai Alloy Corporation, Guangdong, China. The selected LMs start to melt from 20 ◦C to 40 ◦C approximately, which has been verified by DSC measurements, as shown in Figure S1. At room temperature, LMs remain liquid state of high surface tension with around 0.624 N/m and low viscosity with 0.0024 Pas; the above data were provided by supplier, which are consistent basically with previous study by Koster [35]. Here, there are three types of silicon elastomers selecting in this study, containing EcoflexTM 0030 (Smooth-On, Macungie, PA, USA), PDMS 184 (DOWSILTM, Sylgard, Midland, MI, USA) and PDMS 1700 (DOWSILTM, SE, Midland, MI, USA). Ecoflex 0030 was prepared in a 1:1 base to curing agent weight ratios, while two kinds of PDMS were mixed at 1:10. In order to obtain the printable slurries, Ecofelx 0030 and SE1700 would be mixed at 1:1 in weight, while Sylgard 184 and SE 1700 should be mixed at 1:2. The printable substrates have been mixed with a planetary mixer (VM300SA3, Miantangshinuo Corporation, Jiangsu, China).
