E-Acrylic: Electronic-Acrylic Composites for Making Interactive Artifacts

1.1 Acrylic and E-Acrylic

In this paper, we discuss our findings on developing and designing with one such electronic composite—e-acrylic.

Acrylic is a versatile thermoplastic material that presents many useful qualities for design. It is clear and transmits light well, can be easily manufactured in different colors and surface textures, and is also easily shaped through cutting, heat forming, and chemical welding. Acrylic is used to produce many everyday products, including furniture, interior fixtures, signage, and containers.

Apart from its use in such everyday products, acrylic—particularly acrylic sheets—is a material that we are intimately familiar with as makers working at the intersection of industrial design and tangible interaction research. Acrylic sheets are a staple material in our makerspace (as well as many similar makerspaces). Acrylic sheets are easy to laser cut and engrave and the maker community has developed many ways of working with this material to build complex objects, including joints for 3D assemblies [63], thermoforming into 3D surfaces [33], and compliant structures [46]. We frequently rely on laser cut acrylic to rapidly build structures and enclosures for electronics and circuits (Figure 2).

In considering acrylic as a material with many desirable qualities for everyday objects, as well as the rich making practices we have developed surrounding acrylic sheets, we were motivated to explore how this material might be developed into a substrate for embedding electronics and circuits. Taking acrylic making practices as a point of departure, we focused on working with acrylic sheets in the laser cutter and further shaping the laser cut part into three-dimensional forms and assemblies. We explored engraving circuits onto acrylic using the laser cutting and manual processes, as well as translating physical computing structures such as double-layer circuits, sensors, and other electronic components onto laser cut acrylic.

From this exploration, we propose an approach to make acrylic-based electronic circuits that can be formed into 3D parts through thermoforming and joinery—e-acrylic. We then used our approach and built a variety of interactive products with e-acrylic to further probe its potential for tangible interaction design and physical computing. We reflected on these applications and surfaced the qualities that e-acrylic offers for physical interactive systems, as well as outlined the design space of this composite material for other designers to refer to when adopting e-acrylic in their work.

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1.2 Contributions

In line with the general contributions that electronic composites offer to HCI, we offer the following contributions in this research:

2.1 Making Interactive Artifacts with Acrylic Sheets

Acrylic sheets are a versatile material. It is easy to laser cut, shape, and assemble into 3D objects. Given its prolific use in makerspaces, HCI researchers have investigated how to support designers to build 3D objects and tangible interfaces with acrylic sheets.

Acrylic can be fabricated with the laser cutter into complex assemblies capable of a variety of structural features. For example, laserStacker [53] and laserOrigami [33] use the laser cutter to fuse or bend acrylic sheets into 3D objects. Mechanisms can also be embedded into laser cut acrylic sheets, such as articulating joints with compliant structures [29], magnets [12], or fluidic chambers [15]. Various systems have also been developed to support designers to manually assemble complex objects from flat laser cut parts [4, 32, 45, 46]. Furthermore, acrylic can be fused with other materials into multilayer composite sheets that give different structural properties when laser cut [6].

Electronic circuits can also be incorporated into acrylic sheets. For instance, MechCircuit [12] makes use of laser cut channels to fabricate circuits and magnets as electrical connectors between different acrylic parts. LaserFactory [36] makes use of a modified laser cutter to print circuits, place components, and solder electrical connections—on top of laser cutting the acrylic sheet.

In this work, we present another approach to create electronic-acrylic composites. Rather than focus on a single workflow, or material property, we adopted a broad approach towards acrylic as a material and the common making practices around it. Our investigations include different ways of shaping acrylic such as laser cutting, joinery, and thermoforming; as well as a circuit fabrication workflow that makes use of common makerspace tools. By leveraging these established practices, we aim to develop an approach to create an electronic composite that is approachable for designers and makers already familiar with acrylic.

2.2 Fabricating Electronic Composites in HCI

Electronic composites widen the material expression of physical computing systems. These composites address the tension that designers face when resolving physical materials and electronics into a cohesive texture [61], by incorporating many concerns into a single composite material. We took reference from prior work on developing electronic composites (e.g. e-textiles) to inform our research into e-acrylic.

3.1 Engraving Circuit Traces

3.1.1 Masking & Laser Engraving.

Masking is a key step in the conventional process of fabricating PCBs, whereby a photo-resist mask selectively protects the copper layer that is chemically etched away, leaving the circuit traces behind. We took inspiration from this process and adapted the masking process as a first step to define circuit traces on acrylic sheets. We start with acrylic sheets that come with a protective film which we used as a mask to apply circuits. For situations where the acrylic sheet lacks a protective film, a suitable alternative is to use masking tape to cover the surface of the material, such as “painters tape”.

We transfer the design of the circuit traces onto the acrylic sheet by laser engraving the sheet’s surface that is covered in protective film. As Figure 5A shows, laser engraving selectively removes the protective film while also engraving recessed traces into the acrylic sheet.

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3.2 Cutting Acrylic Parts

Following the laser engraving of circuit traces, we then laser cut the physical features of the acrylic part, such as its outline (Figure 5B). This step is performed right after the circuit engraving without removing the acrylic sheet from the machine to ensure accurate registration of circuit details and physical features.

3.3 Applying Conductive Paint

3.3.2 Removing the Mask.

Once the conductive paint has dried to the touch, we peel off the mask to remove excess paint—revealing the circuit (Figure 6B). Thin masks that are difficult to peel by hand can be easily removed with a plastic scraper, as the conductive traces are recessed and protected from surface abrasion.

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3.4 Integrating Electronic Components

Mounting electronic components onto electronic composite with conductive epoxy is widely used in other research [49]. In this work, due to the capability of e-acrylic in creating both flat conductive pads and vias, we can attach two main types of electronic components onto e-acrylic: surface-mount devices (SMD) and through-hole devices (THD) (Figure 7A, C).

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3.5 Assembling

We explored five methods to connect e-acrylic parts into 3D assemblies, while maintaining circuit continuity across these parts.

3.5.2 Bolts and Nuts.

Bolts and nuts are a common method to assemble laser cut acrylic pieces, and we make use of their conductivity to connect circuits across e-acrylic parts. We painted conductive ink around the hole of the bolt on one part and in the slot of the nut on the other part. When the bolt and nut are fastened, they make contact with their conductive pads to connect a circuit trace. It is important to note that each bolt and nut pair is limited to connecting one trace (Figure 8B).

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3.6 Thermoforming

Acrylic’s thermoplastic nature allows it to be reshaped when heated. Cast acrylic sheets easily deform when heated over 140°C. From our exploration, we found out that acrylic-based conductive paints also soften during heating. Circuits on e-acrylic parts can therefore bend with the heated acrylic substrate and retain its conductivity after thermoforming. We experimented with thermoforming e-acrylic using conventional heating tools such as a hot air gun, carbon fiber heater (Figure 9A), and oven (Figure 9B). In our practice, we have found that setting the heating temperature to approximately 160°C effectively softens e-acrylic with no damage to the conductive paint.

E-acrylic can be thermoformed before or after electronic components are mounted. This consideration hinges upon the ability for electronic components to withstand the heating process. In the Mesh Lamp application (Section 4.2), we demonstrate thermoforming the entire lamp after mounting the LEDs. Conversely, in the Vase application (Section 4.3), electronic components were mounted after the objects was thermoformed to safeguard delicate components, such as transistors, from potential heat-related damage.

Softening the entire part in the oven enables us to deform the whole object at once. For more precise bending, parts can be heated with a carbon fiber heater and held in place with a jig. Localized heating also minimizes heat transmission to other parts of the object. In the Game Controller application (Section 4.1), we employ a carbon fiber heater and jigs to accurately bend the object into shape while also avoiding heat to the mounted microcontroller.

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4.1 Electronic Device: Game Controller

We explored how thermoforming acrylic can be directed toward simultaneously defining the physical structure of an interactive artifact along with the necessary electronic circuits, and built a game controller formed from a single piece of e-acrylic (Figure 10). By constraining ourselves to a single piece, we also sought to challenge the capabilities of e-acrylic in terms of packing multiple electronics features onto a small physical footprint.

Taking the archetypical game controller (e.g. Playstation or Xbox controller) as a reference, we designed this controller by folding a single piece of transparent acrylic into a faceted 3D shell with two handles and four panels with touch buttons (Figure 10A). Each button is an exposed electrode, which serves as the touch point, and we routed their traces to the underside of the controller to protect them from bare-skin contact and false triggers. These touch buttons are connected to an MPR121 capacitive sensing breakout board that is read by a microcontroller (Seeeduino Xiao SAMD21) (Figure 10C). In addition, we incorporated two LEDs into the front of the controller, each pointing towards the edge of the acrylic material. These LEDs light up when a touch button is triggered, making use of acrylic’s high refractive index to illuminate the edges of the controller (Figure 10B).

The transparent acrylic visually exposes the entire controller circuit (Figure 10A, B)—even the traces running on the underside. As such, we paid special attention to the layout of these traces and designed them in neat bundles that travel along the 3D surface to different points. These circuit details contribute to the overall visual appearance and aesthetics of the tangible interface.

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4.2 Home Furnishing: Mesh Lamp

With the game controller as a point of departure, we explored other ways to build complex three-dimensional structures with e-acrylic. We took reference from flat laser cut lattice structures (kirigami) that are easily deformed into 3D volumes, such as those demonstrated by earlier work [19, 65], and experimented with embedding circuits and electronic components into these structures.

From this exploration, we designed a lighting device that incorporates 28 surface mounted LEDs (Figure 11) on a mesh-shaped e-acrylic part. We laser cut a circular fishnet-like pattern and engraved circuit traces that run from the edge of the piece to its center through the bridges between the laser cut gaps (Figure 11B). LEDs were mounted to the electrode pads using conductive epoxy. After all the electronic components were mounted, we heated up the entire piece and shaped the softened flat piece into a 3D lattice structure (Figure 11B). We used opaque white acrylic to hide the circuit traces and mounted LEDs, which emphasizes the lamp’s mesh-like structure when backlit (Figure 11A).

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4.3 Product Design: Water Level Vase

Acrylic is widely used in product design for containers and storage as it is waterproof and optically clear. E-Acrylic, including the polymer-based conductive ink that used, inherits these qualities. Taking cues from acrylic’s use in everyday products, we designed a vase that detects and indicates the water level inside. This application challenged us to make use of different acrylic crafting processes to build an artifact that is watertight, while supporting the electronic circuits on its surface.

The vase detects three different water levels that light up the corresponding LED as indicators (Figure 12). For this application, we were also inspired by circuit sculptures—three dimensional circuits with electronic components and traces arranged to be both functional and ornamental (e.g. Eirik Brandal’s artwork [1]). We made use of transparent acrylic and shaped it through thermoforming into a cone that emphasizes the embedded conductive traces through optical magnification (Figure 12A). The clear cone also offers people a clear view of the vase’s interior to check water quality or the plant’s root growth. Rather than remove excess acrylic, we tapered the form of the vase into a flat vertical surface that served as a canvas for electronic components. We sealed the base of the cone with a flat piece of acrylic and acrylic cement, following the typical process of fabricating clear acrylic containers used in the industry.

Our approach to water level detection involves a straightforward circuit that does not require a microcontroller. We used the electronic conductivity of water to act as a switch, triggering a transistor to switch on the LEDs (Figure 12C). We used small holes filled with conductive paint (i.e. vias) to connect the electrodes inside the vase to the external circuits, protecting the rest of the circuit from the water (Figure 12B).

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4.4 Interactive Toy: Pinball Machine

Joining laser cut acrylic pieces into 3D assemblies is a common method used by designers to create enclosures and other physical structures. E-Acrylic is able to connect circuits across different parts with these same joinery techniques (Section 3.5), and we explored using these joints to build an interactive device.

We designed an interactive pinball toy with embedded circuits that flow through the metal joinery holding different parts together (Figure 13). The pinball toy features a circuit comprising a battery, SMD LEDs. We also made use of compliant acrylic structures to create flaps that are also switches. The switches serve as gates that block the movement of a metal pinball (Figure 13B). When a metal ball makes contact with the gate, it closes the circuit between the conductive pads incorporated into the gate flaps, lighting up an LED. The compliant structure of the switches not only facilitates the toy’s mechanical behavior, but also serves as an electronic switch (Figure 13B). Gently shaking the toy opens the compliant gate structure, allowing the ball to pass through. Each compliant gate is an individual part assembled to the base of the pinball machine with a bolt and nut (Figure 13C). We also used the metal bolt to electrically connect the conductive contacts on the gate with the rest of the circuit on the base of the machine.

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4.5 Kitchen Appliance: Cup Warmer

We continued our exploration into acrylic compliant structures and joinery with e-acrylic, and directed these material qualities towards a different context.

We built a cup warmer that incorporates a heating element within a compliant structure (Figure 14). Placing a cup on this device pushes down the spring-like structure with the weight of the cup, which then closes the circuit by establishing an electrical connection between the traces on compliant structure and the traces at the base. This in turn activates the heating element embedded within the compliant structure that wraps around the cup. If the cup is too light, this compliant structure will stop short, deactivating the heating process as a safety measure. The compliant spiral shape enables the heating circuit to conform to the cup’s surface, ensuring more uniform heating (Figure 14C). We used brass bolts to support the appliance’s structure, as well as to electrically connect the circuits on the top and bottom acrylic plates (Figure 14B).

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4.6 Data Physicalization: Interactive Contour Map

Besides thermoforming or joinery, acrylic sheets can also be stacked to give 3D forms. Designers often use this layering structure to create landscape models or contour maps. Stacking flat layers enables rapid construction compared to 3D printing or CNC machining the same structure. For this last application, we explored creating interactive geographical maps with e-acrylic stacks.

Specifically, we combined embedded conductive traces into e-acrylic to extend the capacitive sensing capabilities of a touchscreen device [26] to create an interactive contour map of Hawai’i’s Oahu island (Figure 15). We used transparent acrylic to allow the visuals on the touchscreen to be seen through the contour map. This enabled us to map digital visuals and information directly onto the data physicalization (Figure 15B).

We laid out a series of touch points throughout the contour map, corresponding to different landmarks on the island. Touching these points on the layering structure activates the capacitive touch screen underneath, and brings up relevant information on the display (Figure 15C). For instance, touching the pattern near the harbor will trigger an introductory video about the harbor. To create these touchscreen extensions, we cut holes on each layer of acrylic to create continuous channels from top to bottom when stacked (Figure 15D). We then applied conductive paint into these channels, allowing the paint itself to establish connections between the points on the top and the screen at the bottom (Section 3.5.4 Stacking).

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5.1 Incorporating physical computing deep within acrylic making practices

Developing our approach to e-acrylic bears similarities to related work on developing electronic composites that are deeply rooted in craft practice, such as within textiles (e.g. [11]), paper (e.g.[43, 65]), and ceramics (e.g.[64]). Craft-based inquiry in HCI is characterized by “combining, aligning, and integrating analog and digital crafting techniques and processes”, “creating highly refined objects, defined by attention to detail and aesthetics”, and “creating knowledge through deep, embodied engagement” [13]. Our approach to e-acrylic builds on top of the acrylic making practice that is familiar and commonly used in industrial design maker spaces. In this research, we took cues from our own practice of using acrylic sheets paired with digital fabrication as a means of rapidly prototyping physical artifacts. We also sensitized ourselves to different ways that people have manipulated acrylic sheets to leverage its desirable physical qualities for making everyday products.

With these experiences as our basis, we developed an approach to create an electronic composite that is centered on the tools and processes that are deeply embedded within acrylic making; adapting and incorporating physical computing activities into acrylic making processes. This stands in sharp contrast to our prior experience of using acrylic sheets for interactive product design, where acrylic sheets are primarily shaped and assembled with other physical computing components such as breadboards and electronic parts.

As such, rather than a surface-level composition of laser cut acrylic shapes with interactive features supported by other parts, our research led to an approach for an electronic composite that works with the versatility and broader expressions possible with acrylic making. This is evident in the applications we explored where e-acrylic contributes in multiple ways to the three-dimensional forms and functionalities of the interactive artifacts:

The e-acrylic approach we developed is centered on acrylic making practices drawn from industrial design, and we offer additional processes aligned to this practice that imbues acrylic with electronics and interactivity—furthering the potential of this material within industrial design. On one hand, we see e-acrylic widening the rapid prototyping capabilities of acrylic for industrial design makers, particularly in developing interactive artifacts. On the other hand, we also see e-acrylic as a composite material with its own distinct materiality for designers to consider. We elaborate on the materiality of e-acrylic in the next section.

5.2 The materiality of e-acrylic artifacts

Within the practices around making interactive artifacts, acrylic sheets are commonly used as a physical structure that encloses and holds in place electronics and circuit parts—hiding most of the inner electronic workings from view and exposing an interface for people to interact with. By transplanting these computational components from distinct physical parts put together with acrylic, to features that are embedded in acrylic sheet material itself, we shift the materiality of interactive acrylic artifacts from an assemblage to a multifaceted composite material.

As we gained more experience in designing with e-acrylic, we found ourselves focusing on developing e-acrylic artifacts that exhibit a holistic use of the composite material that entangle the processes and practices around acrylic and electronics with the physical properties of the materials themselves [61]. Perhaps most strikingly, circuits that make an interactive artifact work are conventionally hidden from view—but with e-acrylic, they run on the surface of the artifact that has to also serve as a body for people to interact with. In designing the game controller (Figure 10), we were guided to consider how circuits can be routed across both faces of the bent acrylic sheet, exposing electrodes for touch points while ducking other circuit traces out of the way to prevent false positive interactions. In doing so, we began to treat the circuit traces as elements that also participate in the overall visual appearance of the artifact, and paid attention to how we designed them to not only satisfy their function as conduits of electrical signals, but also elements that contribute to the overall aesthetics of the object. Our experience with the game controller inspired the vase (Figure 12), and in this case the circuits and mounted electronic components were a prominent visual element that we intentionally drew attention to in our design. With this in mind, e-acrylic shifted our perception of circuits and electronics as designers, from things traditionally hidden from view, to things that become an integral part of the visual expression of artifacts made with the composite material.

Robles and Wiberg [61] discuss designing interactive systems as striving towards the goal of achieving a coherent appearance by considering the relationship between different elements that make up the system and the resultant “textures” that they present to people. E-acrylic—as an electronic composite—literally shortens the distance between different elements by embedding different electronic and physical features on a single surface. This in turn makes us, as designers, a lot more aware of how each element relates to each other in the interactive artifacts we were building with e-acrylic, and the overall aesthetics that it ultimately presents.

E-acrylic stands distinctly apart in the materiality it offers for building interactive artifacts when we compare it to our prior practice of using acrylic as part of a bigger assembly (Figure 2). This once again echoes the potential that electronic composites from existing and familiar materials offer for broadening the expression [55] of physical computing and tangible interactive systems.

6.1 Design Features of Acrylic Substrate

6.2 Design Features of E-Acrylic

7.1 Conductivity of E-acrylic

The resolution of e-acrylic circuits depends on the resolution of the laser engraving process. We systematically tested a range of trace widths and gaps between traces on an acrylic sheet with an Epilog Helix 30 Watt laser cutter (Figure 17). From this test, the minimum trace we were able to engrave is 0.18 mm—a comparably smaller width than laser cutter enabled traces achieved in prior work [12, 36]. It is important to note that there is a difference between the widths defined in the digital design file, and the actual engraved width. In our test, a specified width of 0.05 mm leads to an engraved trace width of 0.18 mm (Figure 17B). We attributed this difference to spot size of the laser beam (0.1016–0.1778 mm) as well as the resolution of machine movements. We also observed a limitation in the width of gaps between laser-engraved traces. Gap widths smaller than 0.30 mm damages the protective film we used as a mask for conductive ink painting. While removing material from the surface, we observed that acrylic particles collect at the edge of the engraving for thicker traces (Figure 17E). These acrylic particles reduce the actual width of the trace.

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We tested the electrical resistance of the fabricated circuits across different trace widths painted with silver conductive ink (MG Chemical 842AR). The average sheet resistance is 0.058 Ω /□ (SD=0.015) and is adequate for most interactive circuits.

7.2 Thermoformability of E-acrylic

We tested the resistance changes of e-acrylic circuits before and after thermoforming at different curvatures (0,1,2,3,4,5 mm) (Figure 18A). The results indicate that while thermoforming increases the electrical resistance of the deformed trace, all traces maintain their electrical conductivity across the bend (Figure 18C).

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We also ran tests to measure the effect of temperature on e-acrylic circuits painted with silver conductive ink (MG Chemical 842AR). We exposed e-acrylic samples to the temperature of 160 °C for 10 minutes, which completely softened a 3 mm acrylic sheet for thermoforming. After multiple heating cycles, the circuits maintain the same conductivity.

7.3 Future Considerations