## 1 Introduction
Unfortunately, fabricating such arrays when scaled up to macro sizes is impractically expensive if performed using traditional methods [33; 59]. We previously reported on an alternative solution to this problem: artificial muscles that combine microfluidics, electrostatic actuation, in-built electrodes and 3D-printing [36]. Briefly, the muscle is fabricated via 3D-printing [60], where microchannels turn into conductive wiring, and the support material acts as the dielectric material [61]. Since the microchannels are initially filled with sacrificial material [59], that material has to be removed by a clearing protocol [59]. The cleared channels can then be filled with conductive \begin{table}
\begin{tabular}{|c c c c|c|} \hline
**Actuators** & **Energy-Efficient** & **Upscalable** & **Downscalable** & **Cost** \\ \hline EM motors & no & yes & no & medium \\ \hline Pneumatic & no & yes & no & low \\ Hydraulic & no & yes & no & low \\ \hline Piezolectric & yes & no & yes & high \\ \hline Thermal & no & yes & no & low \\ Electrostatic & yes & yes & yes & low \\ \hline \end{tabular}
\end{table} 
Table 1: Visual comparison of actuators. fluid to act as both flexible electrical connections and plates of parallel-plate microcapacitors. The 3D-printed resin is used both as the dielectric between the capacitor plates and the \"tendon\" to transfer force to the outside world [33; 36]. In this approach [33], a single pair of channels, forming a stack of capacitors arranged mechanically in series, can be thought of as a single artificial-muscle \"fiber\". These fibers can, in turn, be laterally arrayed to form an artificial-muscle-fiber \"bundle\". As with biological muscles, small effects on local areas can become large effects at the system level. FEA simulations using COMSOL (v.6.1). Multiphysics [36; 58; 62], a solver and simulation software package for various physics and engineering applications, have predicted up to 900 kPa saturation force density from such bundles, when the individual microcapacitors are built at the 10 \(\upmu\)m scale. An earlier work [63] and recent advancements [59; 60; 64] in 3D-printed microfluidics [65; 66] have made it possible to fabricate such devices and test them experimentally. In particular [PERSON] et al. [59], experimentally demonstrated the consistent fabrication of microfluidic channels embedded in devices that were 3D-printed. The microchannels, defined as negative features filled with support material, were embedded inside the surrounding flexible resin. Then, a novel protocol was used to remove the support material from the embedded microchannels, thereby opening them for further use [59]. Further characterization experiments have shown [60] that the results of the technique are reproducible. Hence, it is already possible to engineer 3D-printed microfluidic devices to specifications and with predictable performance. In consequence, the new 3D-printed microfluidics technique has already been successfully applied to produce depositions by self-assembly in heterogeneous systems [64], e.g., for microbial fuel cell applications. Similarly, this 3D-printed microfluidics technique now enables device fabrication and the experimental testing of artificial muscles. Hence, what is now needed is to devise practical geometries and architectures to help implement such devices. We already proposed a wave architecture for these devices, which is both scalable and practicable in principle [33]. However, the weave architecture was invented before the development of these experimental techniques [59; 60; 64]. Experimental evidence showed that clearance could reliably be achieved down to perhaps a 400 \(\upmu\)m wide channel, with partial success down to a width of 200 \(\upmu\)m. On the other hand, artificial muscles benefit from features that are as small as possible, as identified by previous theoretical calculations and simulations [33; 36]. These show that the output force density is proportional to the inverse square of the plate separation.