Highly conductive 3D metal-rubber composites for stretchable electronic applications

Since the turn of the millennium, the field of stretchable electronics has grown tremendously with the rapid adoption of digital technologies and increasing demand for ubiquitous electronics. To meet the demand, electronic devices are beginning to shift from rigid structures to stretchable formats to allow electronic devices to conform better to the human body,¹ with various applications in epidermal electronic devices,² biomedical engineering,^3,4 healthcare monitoring,^5–8 soft robotics,^9–12 electronic skins,^13–15 and human-machine interfaces.¹⁶ The development in stretchable electronics was further propelled by the swift development of low-dimensional nanomaterials and material processing techniques during the past decades. Compared with the conventional rigid printed circuit boards, the stretchable counterparts demonstrate the ability to bend, twist, and stretch by having soft elastomeric materials as their substrates.

Electrical conductors and conductive interconnects are crucial to the development of stretchable electronic circuits. Several excellent strategies have been investigated for creating stretchable conductors.^17–21 Composites made using nanomaterials show excellent potential as stretchable conductors because they provide both mechanical stretchability and electrical conductivity. Many research efforts have shown that nanomaterials such as carbon nanotubes (CNTs)^22–24 and silver nanowires (AgNWs)^25–27 can be dispersed into or coated onto elastomeric substrate as stretchable conductors. They can also be printed onto polymer substrate as stretchable interconnects.²⁸ Stable integrity and conductivity performance can be achieved by introducing different structural designs such as network,²⁹ wavy,^23,30 buckled,^31–33 kirigami,³⁴ and origami.³⁵ However, challenges remain in developing stretchable conductors: (i) conductivity tends to decrease with increased strain and (ii) relatively low conductivities are exhibited (10³ S cm⁻¹), compared with the conventional rigid metal interconnects (10⁵ S cm⁻¹).

In this work, we propose another strategy of fabricating highly conductive and stretchable metal interconnects. In order to achieve high conductivity with high strain-invariance, we developed and investigated 3D micro-interconnects in helical form factors. In this technique, we embed a helical-shaped metal wire in stretchable polymer substrates illustrated as in Fig. 1(a). This method aims to address the two main challenges described above in meeting the requirements of high conductivity and stretchability in stretchable interconnects.

The fabrication process of the stretchable interconnects is shown in Fig. 1(b). They were fabricated by using off-the-shelf enameled copper wire. We measured the average wire conductivity to be 5.9 × 10⁵ S cm⁻¹ (s.d. = 0.8%) (supplementary material, Fig. S2 and Table S1), which is similar to the value quoted in the data sheet. This value is two orders of magnitude higher than metallic nanowires such as AgNWs. We first formed the wire into a 3D helical structure, abbreviated as “Helix-Cu.” The method used to form the Helix-Cu is facile and can be automated, akin to spring winding, i.e., a method used to wrap a wire at a certain speed around a moving rod.³⁶ We fixed the dimensions of the Helix-Cu in this study at 10 mm in length, 0.85 (±0.01) mm in diameter, with 10 numbers of active coils and 1 mm pitch.

Next, we encapsulated the Helix-Cu into silicone-based elastomers to enhance its elasticity, i.e., ability to return to its original shape. Without the silicone elastomer, the Helix-Cu is unable to stretch elastically. However, due to the significant differences in moduli between the Helix-Cu and the elastomer, interfaces between the Helix-Cu and silicone substrate tend to slip and delaminate while stretching. In order to strengthen their bonding strength, the Helix-Cu was surface treated before encapsulating it in silicone. We chose a silicone adhesive (Sil-Poxy™), a single component adhesive explicitly made for bonding between silicones and between silicone to another substrates including polymers, ceramics, and fabrics for the surface treatment. Because the enameled copper wire used in this study has a polyurethane insulation layer, the silicone adhesive provides relatively flexible and robust bonding between the copper wire and the silicone substrate. To ensure a uniform coating onto the copper wire’s surface, dichloromethane (DCM) was used to dilute the silicone adhesive before coating. The Helix-Cu was dipped into a 5 wt. % of silicone adhesive (5 g of the silicone adhesive in a total volume of 100 ml solution) briefly. After the DCM evaporated, the silicone adhesive was coated onto the surface of interconnect uniformly. The coated wire was then placed into a mold (12 mm in length, 8 mm in width, and 3 mm in thickness) that was pre-treated with silane (Sigmacote^®) for an effortless removal of the polymer substrate from the mold later on. Uncured silicone (Ecoflex™ mixed in 1A:1B by weight) was poured into the mold and subsequently cured for 4 h in room temperature. An image of the fabricated elastic helical copper interconnect, abbreviated as “eHelix-Cu,” after removing from the mold is shown in Fig. 1(b).

A sample image of the Helix-Cu is shown in Fig. 2(a) (left). Based on its geometrical specifications, if it is fully stretched until perfectly straight, the length of the straight wire is 2.7 times longer than the unstretched wire. In other words, the maximum geometrical strain of the designed Helix-Cu is 170%. However, Helix-Cu will not return to its original helical shape after stretching to 170% strain. This can also be supported by simulation (Abaqus) of the stretching performance of the Helix-Cu which is shown in Fig. 2(a) (right). We generated a finite element analysis (FEA) model using the geometrical specifications mentioned above with the following material constants [density 8.9 g/cm³, Young’s modulus 117 GPa, Poisson’s ratio 0.34, and ultimate tensile strength (UTS) 148 MPa, according to the datasheet]. It was found that when the Helix-Cu is stretched beyond 15%, it was not able to return to its original shape. As the strain increases beyond 15%, every element of the Helix-Cu enters its plastic deformation range and cannot completely recover its original shape.

To enhance the stretchability of the Helix-Cu, we fabricated eHelix-Cu by embedding the winded wire into a stretchable polymer substrate shown in Fig. 2(b) (left). A model of the eHelix-Cu was built up in simulation and shown in Fig. 2(b) (right). We used the Mooney-Rivlin model, one of the available hyperelastic models, to describe the material behavior of silicone Ecoflex. Two parameters in the Mooney-Rivlin model: C₀₁ = 10 410.8 Pa, C₁₀ = 21 362 Pa, were used.³⁷  Figure 2(b) shows that the model of eHelix-Cu can be stretched to 100% strain and then recover to its original shape after releasing from the support of the encapsulating polymer substrate. Our experimental result is in good agreement with the simulation results, where the eHelix-Cu can sustain much larger strains than Helix-Cu. eHelix-Cu can be stretched and return entirely within a 100% strain range.

To study the mechanical performance of the Helix-Cu embedded in elastomers of different modulus, we used Ecoflex 00-10, Ecoflex 00-30, and Ecoflex 00-50. These materials vary in their Young’s modulus and elongation, as shown in Table I. eHelix-Cu samples using different embedding substrate materials were fabricated with the same method described earlier. For the uniaxial tensile tests, the sample was clamped and then stretched by using a linear translation stage (Newmark) at 5% strain rate and 0.5 mm s⁻¹ speed. The maximum geometrical strain limit for the Helix-Cu used in this study was calculated to be around 170%. Hence, we analyzed the tensile performance within 170% strain due to the geometrical limitations.

We found that when embedded in different elastomer substrates, eHelix-Cu could extend and return with different values of maximum strain (Fig. 3). As shown in Fig. 3(b–iii), when using the softest Ecoflex 00-10, eHelix-Cu could be stretched to within 80% strain and completely recovered without structural deformation. Beyond 80% strain, out-of-plane deformation occurred in the middle part of helix interconnects, and the whole interconnect remained wrinkled. For Ecoflex 00-30, which is slightly stiffer than Ecoflex 00-10, the maximum strain limit of the eHelix-Cu without structural deformation increased to 140% strain, as shown in Fig. 3(b–ii).

On the other hand, when embedded in the stiffest material among Ecoflex series (Ecoflex 00-50), there was no structural deformation in eHelix-Cu even after being stretched and released from its maximum geometrical strain limit, as can be seen from Fig. 3(i). Therefore, different substrate materials with various Young’s moduli affect the stretching performance of the eHelix-Cu. Figure 3(a) shows the stress-strain curve of each Ecoflex series materials for their respective maximum strain during stretching and releasing without plastic deformation. At the same strain ratio, Ecoflex 00-50 experienced larger stress than the other materials, which also meant that it can provide more force to pull the eHelix-Cu interconnects back to their original position after stretching. Since the interfacial bonding between the Helix-Cu’s surface and polymer substrate is strong enough to prevent delamination, Ecoflex 00-50 as a polymer substrate can support the eHelix-Cu function as stretchable interconnects within its maximum geometrical strain limit of 170%. Table I summarizes the effect of increasing the Young’s modulus of the substrate materials.

Moreover, we also compared stress versus strain relationship of a single Ecoflex series and eHelix-Cu interconnects embedded in Ecoflex series (supplementary material, Fig. S1). We found that the mechanical properties are quite similar. During the strain release phase, the stress-strain curve of the eHelix-Cu decreases more than just the Ecoflex elastomer itself. This can be attributed to greater energy loss caused by possible Helix-Cu plastic deformations and friction with Ecoflex during releasing.

To analyze the durability of the eHelix-Cu (embedded in Ecoflex 00-50), cyclic tests with different strain limits: 50%, 100%, and 150%, were performed [Fig. 4(d)]. For 50% and 100% strain limits, the samples can be stretched to 1000 cycles without damage. Pictures in the figure show that the samples remain intact after 1000 cycles of stretching. For 150% strain, the micro-interconnects broke within 100 cycles of stretching and releasing. Thus, within 100% strain, the eHelix-Cu interconnects have outstanding durability as they can be stretched over 1000 cycles. We also tested stress versus strain curves of eHelix-Cu interconnects embedded in Ecoflex 00-50 with 100% stretching strain at 1st, 10th, 100th, and 1000th cycles (supplementary material, Fig. S5). It shows that the mechanical performance of eHelix-Cu is very stable within 1000 cycles of stretching.

Moreover, we connected a light emitting diode (LED) to our eHelix-Cu interconnects to illustrate an example of using the eHelix-Cu as interconnects for stretchable electronics. The I-V characteristic curves under different strains were similar in a stretch cycle (to 150% strain) [Fig. 4(d)]. Pictures of the sample being stretched are shown in Fig. 4(e) at different strains of 0%, 50%, 100%, and 150%. The intensity of the LED did not change because the conductivity of our eHelix-Cu remained constant with strains. This demonstrates that our eHelix-Cu interconnects have outstanding stability under different mechanical strains. In the future, various structural designs of Helix-Cu may be incorporated by tuning parameters of the helical structure such as length, pitch, and diameter to increase the strain performance in the future.

In conclusion, we introduced a method to enable a highly conductive and reversibly stretchable 3D helix metal-interconnects that can be embedded in an elastic substrate for stretchable electronic applications (eHelix-Cu). Remarkably, the electrical conductivity remains invariant with stretching up to 170%. The eHelix-Cu interconnects can also withstand 100% strains over more than 1000 cycles of stretch. The electrical conductivities of the eHelix-Cu interconnects were two orders of magnitude higher than the prior studies [Fig. 4(b)]. With the outstanding conductivity and stretchability, the eHelix-Cu has great potential to be utilized for stretchable electronic applications, especially as conductive interconnections between devices in stretchable electronic applications.