Background 

Soft electronics have received intense interest for years and are proposed as the next generation of electronic devices for wearables, robotics, and more. While soft electronics capable of flexing and stretching hold the promise of matching the complexity of modern rigid devices, such advanced systems are still not widely available. To achieve efficient and multifunctional soft electronics, techniques to take individual circuit planes and stack them into multilayer structures is critical. For example, electronic chips used in consumer electronics are typically multilayered structures, from simple devices to complex systems. Stacking layers helps minimize the electrode length, decreases the complexity of the electronics, and increases the energy efficiency, among other advantages. Multilayer electronics require a vertical connection between these different layers, and this has been achieved using a feature called a via.  

However, although vias have been made in soft electronic devices, simply adapting conventional via techniques has several challenges. This includes laborious manual processing to create each individual via and stress concentrations which can lead to failure between the soft polymeric substrates and the electrical interconnect materials. For these reasons, high-performance multilayered structures remain a significant bottleneck in soft electronics. To address this challenge, we introduce a new way of fabricating vias, which we call Liquid Metal Stratification for Three-dimensional Assembly of electrical InteRconnects, LM-STAIR (Figure 1).  

Figure 1a. 3D Schematics of the LED circuit using the LM-STAIR technique (right) and micro tomography image of the soft via region (left). b. LED circuit that includes a via junction and crossing top and bottom layer planar interconnects created through the LM-STAIR technique. 

Liquid metal microdroplets and photocuring behavior 

The LM-STAIR technique combines liquid metal microdroplet stratification with programmed photocuring. Liquid metal (LM), an alloy of gallium and indium, remains liquid at room temperature and is key to the creation of our soft electronic devices with high electrical conductivity and flexibility. We mixed LM with photoresin, a polymer cured by exposure to light, typically used in resin 3D printing and other electronic processes. The LM and photoresin mixture is blended using a planetary mixer, during which shear stress breaks LM into microdroplets. These microdroplets are denser than the low viscosity photoresin, which enables the droplets to settle to the bottom of a photoresin mixture. This will become something we take advantage of to assemble LM droplets in 3D. 

Another key element of LM-STAIR is photocuring, and we take advantage of a photocuring defect called undercutting. When photopatterning, light is projected through a photomask, so that patterns can be defined within a plane. But this photocuring is influenced by light exposure. If light exposure is not ideal, instead of a perfect rectangular cross section with purely vertical sides defined by the mask, the resin can cure into an inverted trapezoidal shape with angled edges (Figure 2). This defect is known as undercut behavior. This inverted trapezoidal shape resembles a strair-like structure, which is an ideal via geometry to connect one layer to another. We then use the settlement behavior for the LM droplets, but now the LM droplets can be directed to stratify across the thickness in precise locations, enabling the formation of 3D LM structures which can be electrically conductive. With these two key elements in place, let’s look at the LM-STAIR process. 

Pour, Shine, Flip 

We begin by pouring the liquid phase photoresin/LM microdroplet solution onto a substrate. A stencil mask defines where the material is deposited in plane. Next, the solution is covered and a shadow mask is applied.  Then, we shine a UV light on the solution to cure the exposed portions of the shadow mask. After curing, the entire composite is flipped for directed stratification. During this process, LM microdroplets in the uncured solution stratify under gravity, and those near the edges of the shadow mask stratify on the undercut slope, forming a continuous connection between the top and bottom layers. This stratification completes in less than a minute, and since it occurs throughout the plane, the fabrication time remains constant regardless of the number of vias. Once stratification is complete, the composite is post-cured to fix the LM microdroplets in place. 

Multilayer circuits with 3D interconnects using LM-STAIR 

To enable more complex multilayered designs, we can also independently control the circuit layout within multiple layers and control connectivity across these layers. This is achieved by modifying the curing direction and sequence. This allows us to discretize a single layer of LM microdroplets into multiple individual layers. This can create planar interconnects that can cross in different layers/planes without being electrically connected, except at specified locations where the LM-STAIR method places soft vias.  

We can also perform these processes sequentially to create an arbitrary number of soft circuit layers. This allows for multilayers with different types of in-plane and thru-plane connectivity, where we created 6 soft circuit layers with multiple via connections within a single device structure.    

To demonstrate the LM-STAIR technique for creating devices, we fabricated a thin, soft circuit that has different functionalities on different layers. On one layer the device had 9 Hall effect magnetic sensors, and on the other layer 9 LED indicators. These layers were connected with 21 soft vias so that the sensors would sense a magnetic field and then illuminate the LEDs on the other layer (Figure 3). These layers were seamlessly connected with planar interconnects and vias, which were rapidly created through the LM-STAIR technique.  

Figure 3 a. Circuit diagram of a magnetic field indicating circuit. Hall effect sensors are installed at the bottom layer and LED indicators are installed at the top layer. b. Side view of the thin and soft magnetic field indicating circuit. c. Image sequence of the magnetic field sensing circuit in operation.  

Future 

We believe that the LM-STAIR technique offers versatility that is applicable to various situations. The LM-STAIR technique can be applied to a range of photoresins, which we demonstrate in our study. This demonstrates that the stratification-based fabrication method in fluidic environments is a generalizable approach to create 3D structures of electrically conductive materials in a rapid and scaleable fashion. 

In addition, LM exhibits a stable electrical response under physical deformation and high resilience in harsh environments. This characteristic is beneficial for electronic applications that require mechanical stability in extreme conditions. 

Although this study establishes the LM-STAIR technique, there is still potential to explore advancements such as mask-free curing systems, nanosized LM droplets, and volumetric photoresin curing. We hope that the LM-STAIR technique will offer new perspectives for fabricating high-performance electronics and provide greater design flexibility for complex soft electronic circuits. 

For more details, check out our paper “Soft electronic vias and interconnects through rapid three-dimensional assembly of liquid metal microdroplets” in Nature Electronics.