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

Electronic composites incorporate computing into physical materials, expanding the materiality of interactive systems for designers. In this research, we investigated acrylic as a substrate for electronics. Acrylic is valued for its visual and structural properties and is used widely in industrial design. We propose e-acrylic, an electronic composite that incorporates electronic circuits with acrylic sheets. Our approach to making this composite is centered on acrylic making practices that industrial designers are familiar with. We outline this approach systematically, including leveraging laser cutting to embed circuits into acrylic sheets, as well as different ways to shape e-acrylic into 3D objects. With this approach, we explored using e-acrylic to design interactive artifacts. We reflect on these applications to surface a design space of tangible interactive artifacts possible with this composite. We also discuss the implications of aligning electronics to an existing making practice, and working with the holistic materiality that e-acrylic embodies.


INTRODUCTION
Composite materials are materials that combine two or more other materials in a way that ofers new physical properties or qualities that are distinct from original materials [55].Within HCI, researchers have developed composite materials that incorporate electronics and computing into physical structures; such as textiles with woven circuits and sensors [51] and paper electronics [43].Computational composites [55] considers electronics, computational behavior, and other immaterial properties [55] as materiality that can be incorporated into physical materials.Such composites expand the materiality and expressions of interactive systems.
In this research, we look into a subset of computational composites-electronic composites-where electronic parts and circuits are deeply incorporated into the structures of physical materials to imbue the chosen substrates with computational features.Within the feld of HCI, electronic composites contribute to the creation of new interaction contexts by expanding the materials used in interaction design, thereby widening the audiences that encounter physical interactive systems [5].New materials are often used as a substrate in electronic composites, and these substrates ofer relevant material qualities for various HCI contexts.For instance, fabric or silicone can be used to make soft wearable devices [35,51], while materials like paper are commonly found in learning environments, and this locality makes paper an ideal material for interactive learning systems [22].Electronic composites can also make use of materials that are easy to work with, such as materials surrounded by rich making or digital fabrication practices.This encourages the production and personalization of computational artifacts for a "market of one" [17].For example, researchers demonstrate fabricating personal electronic devices on demand by modifying a laser cutter to also print, place, and solder electronics circuits onto laser cut plastic sheets [36].
Electronic composites also contribute by expanding the communities that participate in the development of computational systems.To create electronic composites, researchers not only blend the properties of electronic and non-electronic materials, but also combine the making practices surrounding these materials.This integration of making practices and crafts can engage more diverse communities in the creation of electronics and computational systems.For instance, the incorporation of electronics into paper through painting and paper craft [44], or embedding electronics into fabric through sewing [5].Such hybrid practices also involve existing skills or traditional crafts that are conventionally disconnected from electronics or computing practices, and enable people who are already familiar with these skills and crafts to bring their values and participate in developing new interactive systems [41].Moreover, connecting existing practices to electronics and computing can also lead to new tools and workfows that enriches traditional practices (e.g.CAD software for weaving textiles [14]).

Acrylic and E-Acrylic
In this paper, we discuss our fndings 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 diferent 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 fxtures, 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 threedimensional 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 acrylicbased 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 refected on these applications and surfaced the qualities that e-acrylic ofers 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.

Contributions
In line with the general contributions that electronic composites ofer to HCI, we ofer the following contributions in this research: (1) Broadening communities for electronics practice: We developed a hybrid approach to make e-acrylic, an electronic composite based on acrylic sheet material.Our approach to e-acrylic is centered on existing acrylic-making practice and incorporates physical computing practices into these existing making workfows.E-acrylic is thus approachable to people who are already familiar with such making practices, such as industrial designers who can adopt this approach as part of their professional practice when developing new products, and the broader making community that is already working with laser cut acrylic.We detail our approach and also outline the design space of this composite material for others to consider.(2) Broadening audiences for electronic systems: We built a series of interactive artifacts with e-acrylic that showcase the new audiences that e-acrylic reaches to.On the one hand, these applications demonstrate acrylic-based electronic systems that leverage the visual, structural, and moldable qualities of acrylic as is currently applied to everyday products.On the other hand, as a composite material connected to existing design making practices, these applications demonstrate the potential of using e-acrylic as a composite material for rapidly prototyping interactive product prototypes.

RELATED WORK
We discuss the related work within HCI around working with acrylic and developing electronic composites that facilitate making tangible interactive artifacts.

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 prolifc 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 fuidic chambers [15].Various systems have also been developed to support designers to manually assemble complex objects from fat laser cut parts [4,32,45,46].Furthermore, acrylic can be fused with other materials into multilayer composite sheets that give diferent 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 diferent acrylic parts.LaserFactory [36] makes use of a modifed 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 electronicacrylic composites.Rather than focus on a single workfow, or material property, we adopted a broad approach towards acrylic as a material and the common making practices around it.Our investigations include diferent ways of shaping acrylic such as laser cutting, joinery, and thermoforming; as well as a circuit fabrication workfow 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.

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.

Leveraging the Physical
Properties of the Substrate.The substrate material plays a signifcant role in determining the fundamental qualities and overall aesthetics of the electronic composite.For instance, fabric is a typical substrate material for e-textiles.The fexibility of fabric enables e-textiles to adapt to diverse surfaces, such as garments [28,42], soft goods [28], and our bodies [39,51].Fabric can be soft and stretchable, contributing to comfortable worn experiences [31].Paper is another substrate material that enables fexible [57] and foldable [43,65] electronic composites.Notably, specialized papers ofer useful features, such as tattoo decal paper on-skin electronics [27] and a micro-capsule paper for the 2.5D conductive traces [8].Furthermore, plastics such as 3D printing flament can be used as an electronics substrate during digital fabrication to create thermoformable composites [18,20,58].Flexible flaments like TPU can be bent and stretched, making them suitable for applications requiring elasticity [48].Silicone's soft and elastic nature also attracts researchers to fabricate stretchable electronic composites [25,35,60].
Similarly, we aimed to understand acrylic's physical properties, and how they contribute to the materiality and functionality of e-acrylic.

Making
Contexts.Electronic composites comprise diferent materials and components, each with its own contexts of making, crafting, and fabrication.For instance, fabric is a traditional material with a rich legacy of associated textile crafts.Researchers leveraged these traditional practives in e-textile fabrication, such as weaving [11,51], stitching [23,56], and embroidery [2,3,34].Increasingly, researchers have also turned to newer digital fabrication processes to construct electronic composites, such as inkjet printing [7,37,66], laser cutting [19,21,62,65], and 3D printing [47,52,57].Additionally, researchers demonstrate how existing design modeling activities contribute to making electronic composites, such as sandblasting [64], drawing [30], and spraying [59].By incorporating various making practices into electronic composite fabrication, researchers can not only tap into the rich legacy of making and crafts associated with these materials but also enable makers and designers from various backgrounds to engage in physical computing.
In this research, we focus our development of e-acrylic on the tools and processes commonly found in design makerspaces.We hope that this will open up the use of e-acrylic to people already familiar with processes such as laser cutting and acrylic bending.

Application Contexts.
Materials like fabric, paper, and plastics, have applied to many applications.These existing applications can infuence and inspire how we apply electronic composites based on these common materials.For instance, e-textiles inherited the clothing and soft goods contexts that textiles used for wearable devices [39,51] and soft tangible interfaces [23,40,42,50].Similarly, paper has already been used in books, packaging, and decorative crafts.Integrating electronics into paper can enhance interactivity in such traditional paper artifacts, including pop-up books [43], kirigami [65], and paper sculptures [38].Other examples include using silicone to fabricate highly stretchable interface [24,60].In our work, we considered the existing contexts that acrylic is applied to while developing applications on top of e-acrylic.

FABRICATING E-ACRYLIC COMPOSITES
E-Acrylic is a composite material that integrates electronic components, such as sensors, actuators, microcontrollers, and conductive materials, into acrylic sheets.We incorporated diferent physical computing practices into existing acrylic making practices such as laser engraving and cutting, thermoforming, as well as acrylic joinery-and propose a coherent workfow that is easily adopted in industrial design makerspaces.
In the process of developing our e-acrylic approach, we frst sensitized ourselves to the material subject and its associated making methods with a series of self-directed acrylic manipulation workshops.
A B Throughout this exploration, we gained a deeper understanding and appreciation of the material itself, the crafting and digital fabrication techniques, and the constraints of making artifacts with acrylic.The approach we propose in this paper stems from these initial explorations, and we detail the various steps within this approach in this section.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 frst step to defne circuit traces on acrylic sheets.We start with acrylic sheets that come with a protective flm which we used as a mask to apply circuits.For situations where the acrylic sheet lacks a protective flm, 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 flm.As Figure 5A shows, laser engraving selectively removes the protective flm while also engraving recessed traces into the acrylic sheet.3.1.2Making Double-layer Circuits.Details can be accurately laser engraved on both sides of an acrylic sheet.This process makes use of fxtures or jigs to precisely fip and register the position of the acrylic sheet between laser engraving operations.We extend this process to make double-layered circuits on e-acrylic.
First, the top layer circuit is laser engraved onto the masked acrylic sheet (Figure 5A).Small holes are cut into the sheet to connect top and bottom traces, taking cues from vias used in PCBs.Then, the acrylic sheet is fipped (Figure 5C) and the bottom layer circuit is engraved (Figure 5D).We typically use a jig in the form of a mirrored outline to ensure accurate alignment of the fipped part.

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.

Applying Conductive Paint
3.3.1 Painting Conductive Paint.We remove the part from the laser cutter and manually paint conductive ink into the exposed circuits traces on both sides of the acrylic sheet.The process of manual painting with a brush (Figure 6A) is straightforward and also ofers the painter a level of control to apply paint at regions of interestavoiding excessive conductive ink in other related processes, like air-brushing or spray-painting.
In this research, we used an acrylic-based silver conductive paint (MG Chemicals 842AR) that cures at room temperature diluted with a thinner (MG Chemicals 4351) for easier application.The thinner can not only slow the paint drying, but also reduce the viscosity of the paint and enable it to fow more easily into small traces and holes.

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

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 fat conductive pads and vias, we can attach two main types of electronic components onto e-acrylic: surface-mount devices (SMD) and throughhole devices (THD) (Figure 7A, C).

Assembling
We explored fve methods to connect e-acrylic parts into 3D assemblies, while maintaining circuit continuity across these parts.
3.5.1But Joint.Acrylic can be joined with acrylic adhesive (e.g.acrylic cement) via a butt joint.We investigate two methods to connect the circuit traces across the joint.First, we can brush conductive paint directly across the joint to connect circuit traces.
Conductive paint is viscous and dries rapidly to solidify the electrical connection across the joint.Second, we can use conductive tape to connect the circuit traces on diferent acrylic pieces.To do so, we designed a wider pad at the end of the circuit traces to accommodate the conductive tape (Figure 8A).
3.5.2Bolts 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).

Slot-together.
Slot-together structures perpendicularly intersect two acrylic sheets.To connect the circuit traces with this joint, we apply the conductive paint onto the inner surface of the slots that come into contact when the pieces are joined (Figure 8C).

Stacking.
Acrylic pieces can be stacked to build 3D structures.We make use of through holes cut into the acrylic sheet and flled with conductive paint as channels to connect circuit traces across diferent layers (Figure 8D).

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 fber heater (Figure 9A), and oven (Figure 9B).In our practice, we have found that setting the heating temperature to approximately 160°C efectively 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 fber 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 fber heater and jigs to accurately bend the object into shape while also avoiding heat to the mounted microcontroller.

EXPLORING E-ACRYLIC APPLICATIONS
To probe the possibilities of e-acrylic, we adopted the approach we developed as practicing industrial designers and explored a range of interactive applications that might be built from this electronic composite.In this exploration, we aimed to develop artifacts that  made use of the wide variety of processes found within acrylic making practice, in conjunction with the electronics-related processes we proposed in the earlier section.We also developed artifacts that probed diferent real world contexts that e-acrylic might be applied to.

Electronic Device: Game Controller
We explored how thermoforming acrylic can be directed toward simultaneously defning 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 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 diferent points.These circuit details contribute to the overall visual appearance and aesthetics of the tangible interface.protect them from bare-skin contact and false triggers.These touch

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 fat 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 fshnet-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 fat 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).

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 diferent acrylic crafting processes to build an artifact that is watertight, while supporting the electronic circuits on its surface.
The vase detects three diferent 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 magnifcation (Figure 12A).The 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 fat vertical surface that served as a canvas for electronic components.We sealed the base of the cone with a fat 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 flled 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).

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 diferent 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 fow through the metal joinery holding diferent 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 faps 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 faps, 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.

Kitchen Appliance: Cup Warmer
We continued our exploration into acrylic compliant structures and joinery with e-acrylic, and directed these material qualities towards a diferent 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).a cup with water will press down the compliant structure and connect the circuits on top piece and buton piece through brass; turn on the heating; remove the cup will recover the compliant structure, and disconnect the circuits to turn of heating.C: The compliant spiral shape with the circuit is generating heat and warping the cup's surface.

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 fat 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.Specifcally, we combined embedded conductive traces into eacrylic 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 diferent 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 3.5.4Stacking).

IMPLICATIONS OF E-ACRYLIC FOR BUILDING PHYSICAL INTERACTIVE ARTIFACTS
In previous sections, we outlined the approach we developed for creating e-acrylic composites.We also adopted this composite material and explored interactive artifacts that we might build with it as industrial designers.With this approach and artifacts, we refected on our frst person experience in working with e-acrylic and considered the implications that it has for how designers build physical interactive artifacts.

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 refned objects, defned 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 diferent 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: (1) The pinball artifact (Figure 13) draws from acrylic joinery, and we demonstrate how diferent hardware fasteners can be used to construct complex physical structures that also maintain the electronic connections across these parts and joints.
(2) The data physicalization application (Figure 15) draws from using acrylic stacks to make contour and site maps, and we demonstrate how conductive traces can weave through the diferent layers through the stack to route interactive features.
(3) The game controller (Figure 10) and lamp (Figure 11) draws from thermoforming acrylic, and we demonstrate how to simultaneously shape 3D acrylic surfaces and the circuits embedded in them.(4) The cup warmer (Figure 14) draws from designing compliant structures from relatively brittle acrylic, and we demonstrate how to direct such compliant structures for both physical and electronic interactivity.
The e-acrylic approach we developed is centered on acrylic making practices drawn from industrial design, and we ofer 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.

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 diferent 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 diferent elements by embedding diferent 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 ofers 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 ofer for broadening the expression [55] of physical computing and tangible interactive systems.

DESIGN SPACE OF E-ACRYLIC
In Section 4, we explored several applications that demonstrate the possible interactive artifacts that can be built with e-acrylic.However, these outcomes are by no means exhaustive.Like earlier work in e-textiles (e.g.Weaving a second skin [51]) or electronic paper crafts (e.g.Electronic Popables [43]), we envision that e-acrylic can be a broader practice adopted by industrial designers in constructing physical artifacts.Therefore, we refected on our work in e-acrylic thus far and propose a design space that systematically accounts for the possible design features [16] of e-acrylic.In this design space, we organize the details into properties that are inherited from the acrylic substrate and the emerging set of physical computing features.Figure 16 frames the distinct design features of e-acrylic that we made use of in the diferent applications we explored.

Design Features of Acrylic Substrate
6.1.1 3D Structures.Laser-cut acrylic parts can be assembled into 3D structures with specialized joints, such as fnger joints and slot inserts, into 3D assemblies such as stands and enclosures.External hardware fxtures such as bolts and nuts can also be used to secure parts together.Acrylic parts can also be stacked into terraced 3D shapes.This is commonly used to represent contour maps for geographical sites.Besides that, acrylic sheets can be bent with localized heating, or stretched and deformed into non-planar shapes by heating the whole sheet.
Designers can utilize joints and fxtures to rapidly prototype 3D structures on various scales, as demonstrated by the box-like pinball machine (Figure 13).Stacking acrylic enables designers to create visual depth and layering in their designs, as exemplifed in the data physicalization application (Figure 15).Additionally, when organic shapes or ergonomic considerations are necessary, designers can opt for thermoforming to shape acrylic into a 3D surface, as showcased in the game controller (Figure 10) and mesh lamp (Figure 11).

Material
Properties.The waterproof features properties of acrylic enable designers to create interactive applications with circuits that mediate direct contact with water or other liquids, as we demonstrated in the water level vase application.Designers can also make use of the acrylic compliant structures to embed haptic feedback and fexible mechanisms into acrylic artifacts, or to construct hinges for conformal shape design, such as the haptic spring gate in pinball machine application (Figure 13).

Visusal Appearance.
The waterproof features properties of acrylic enable designers to create interactive applications with circuits that mediate direct contact with water or other liquids, as we demonstrated in the water level vase application.Designers can also make use of the acrylic compliant structures to embed haptic feedback and fexible mechanisms into acrylic artifacts, or to construct hinges for conformal shape design, such as the haptic spring gate in pinball machine application (Figure 13).
Beyond a color/material/fnishing choice, transparent acrylic enables three dimensional artifacts that are see-through.For instance, users can observe the underlying visuals on the display below the contour map (Figure 15), blending physical information with digital content.On the other hand, the availability of opaque sheets enables designers to build artifacts that can enclose and conceal specifc elements.

Design
Features of E-Acrylic 6.2.1 Circuit Features.E-acrylic composites take on the following features ofered by the combination of conductive inks embedded on traces on acrylic sheets: (1) Conformal: The embedded conductive traces also conform to the three-dimensional deformations of a thermoformed acrylic surface.(2) Double-layer: Circuits can be fabricated on both faces of an acrylic sheet and connected with holes flled with conductive paint (vias).( 3) Visible: E-Acrylic circuits are visible as they sit on the surface of the composite material.Circuits therefore play a signifcant role in the visual appearance of an artifact made with e-acrylic composites, in contrast to, e.g., a woven etextile circuit where conductive yarns are hidden by other non-conductive yarns.In addition, clear e-acrylic composites expose circuits on both faces as a layered pattern, while opaque e-acrylic composites enable designers to route the circuit to selectively reveal parts of it on the visible face (Figure 16L).( 4) Conductive joints: Circuits across diferent e-acrylic parts can be connected with conductive joints, such as by painting the edges of an acrylic slot-insert joint, or by using metal hardware fxtures.
As discussed in Section 5.2, these qualities of embedded e-acrylic circuits present a unique materiality that weaves electronic functionality and visual aesthetics (in contrast to using acrylic simply as a physical structure) for designers to work with.6.2.2 Functional Electronic Traces.Embedded conductive traces can be shaped into sensors and transducers.In this work, we explored fabricating the following functional electrodes onto e-acrylic.
(1) Capacitive touch/proximity sensor: E-Acrylic electrodes can detect touch or bare skin proximity via capacitive sensing.
We used an MPR121 breakout board to detect multiple capacitive touch sensors.(2) Resistive bend sensor: Thin e-acrylic structures are compliant and can be used for bend sensing [21,54,65].When the e-acrylic is bent, the silver particles inside the conductive paint will pack closer or further apart based on the bending direction, resulting in an overall decrease or increase in electrical resistance.We use a voltage divider circuit to measure the resistance change to detect the bending direction and magnitude.(3) Resistive water level sensor: Due to the waterproof property of acrylic, e-acrylic can be used for water level or moisture sensing.By measuring the resistance of the water between two electrodes of e-acrylic, we can detect the water level as the resistance will decrease with more water between two electrodes.(4) Touchscreen extension: E-Acrylic can be used to extend the touch sensing capabilities of a touchscreen device [26,64].We can fabricate touchscreen extension circuits with vias that connect through multiple-layer acrylic or 3D conformal circuits that come into contact with the touchscreen.(5) Switch: E-Acrylic can be fabricated into switches that de tect the connection and disconnection of the circuit traces.For example, the connection between two electrodes can be mediated by other conductive materials such as a metal ball bearing, or another e-acrylic part.Switches can also detect damage or cracks-breaking conductive traces when e-acrylic is exposed to high-impact force.(6) Heating element: E-Acrylic traces can convert electric current to heat via resistive heating.

3D Geometry Circuit
These electrodes can be designed to achieve interactivity and electronic functionality that work alongside the physical properties ofered by laser cut acrylic sheets.For example, heating elements that conform to the surface of a vessel with the help of compliant structures (cup warmer, Figure 14B), or water level sensors embedded on the inner wall of a vase (Figure 12B).

6.2.3
Mounts for External Electronic Components.As described in Section 3.4, external SMD or THD components can be mounted onto e-acrylic circuits.These components add a whole range of electronic features to e-acrylic, including input (e.g.buttons, sensors), output (e.g.LEDs, motors), and logic (e.g.transistors, IC chips).Designers can further employ the broad range of of-the-shelf electronic components to supplement the functional electronic traces that e-acrylic afords.As we demonstrated, we can mount microcontrollers or transistors to control the artifact's behavior (controller, vase), and LEDs to provide illumination (lamp, controller).

TESTING THE LIMITATIONS OF E-ACRYLIC
In the previous section, we outlined the design space of e-acrylic for other designers to refer to.In this section, we discuss some constraints and limitations that designers need to consider when working with this composite material.

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 laser cutter (Figure 17).From this test, the minimum trace we were able to engrave is 0.18 -a comparably smaller width than laser cutter enabled traces achieved in prior work [12,36].It is important to note that there is a diference between the widths defned in the digital design fle, and the actual engraved width.In our test, a specifed width of 0.05 leads to an engraved trace width of 0.18 (Figure 17B).We attributed this diference to spot size of the laser beam (0.1016-0.1778 ) 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 damages the protective flm 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.We tested the electrical resistance of the fabricated circuits across diferent 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.

Thermoformability of E-acrylic
We tested the resistance changes of e-acrylic circuits before and after thermoforming at diferent curvatures (0,1,2,3,4,5 ) (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).We also ran tests to measure the efect of temperature on eacrylic 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 acrylic sheet for thermoforming.After multiple heating cycles, the circuits maintain the same conductivity.A wider selection of conductive paint could enable e-acrylic circuits to take on diferent electrical properties at diferent regions, enhancing the capabilities of this composite material.For instance, using a more resistive ink for resistive sensors, and a more conductive ink for the rest of the circuit.
7.3.2Sofware tools.Our digital design practice with e-acrylic made use of common digital drawing tools familiar to designers, such as Adobe Illustrator.This approach eliminates the need for expertise in conventional PCB design software.A notable advantage is the fexibility it ofers in designing the visual appearance of circuits.However, there are inherent drawbacks to this process.These design tools are unable to check for circuit integrity, such as short circuits or interference.Furthermore, the manual drawing of each circuit trace can be labor intensive.We plan to develop a circuit editor for e-acrylic to address these challenges and support multiple fabrication-aware features that designers have to consider when working with this material.

OUTLOOK
Acrylic is traditionally valued in the design profession for its visual properties, and is often used as part of the color/material/fnishing considerations of a product or interior space.With the democratization of digital fabrication machines, in particular the laser cutter, we see the trajectory of the acrylic shift from a fnishing material to a versatile construction material used by professionals and hobbyists alike to realize their physical prototypes.
Through our process of developing e-acrylic, we deepened our understanding of acrylic making practices, and introduced electronic features into this versatile material through the processes that are already found in the craft.Through this investigation, we seek to ofer not only the design space that e-acrylic facilitates for building interactive artifacts, but also an electronic composite that is centered on making practices that are familiar to industrial designers and makers at large who are already using laser cutters and acrylic sheets.This projects adds to the growing conversation within HCI about learning from craft and making practices in mutually benefcial ways [10,64], including deeply appreciating and learning from the technical knowledge that is embedded within crafts and making practices that holds value for HCI [9].
Moving forward, we will continue to explore e-acrylic in our own practice as industrial and interaction designers, and further develop the diferent tools and methods for shaping this electronic composite.We also plan to run workshops with professional designers and design students to study the extensibility and relevance [67] of e-acrylic for designing and building interactive systems.

Figure 2 :
Figure 2: A typical use case of acrylic involves enclosing circuits.

Figure 3 :
Figure 3: Acrylic manipulation workshops: A: Workshop participant manipulating the acrylic.B: Some results from the workshop with distinct making craf and purpose.

Figure 4 :
Figure 4: The making process of e-acrylic 3.1.1Masking & Laser Engraving.

Figure 5 :
Figure 5: Laser engraving process of e-acrylic: A: Laser engraving circuit traces onto the acrylic sheet with surface detail showing a recessed and textured surface.B: Laser cuting physical features without moving the acrylic sheet.C: Flipping the laser cut part in preparation for double-layer circuits.D: Laser engraving circuit traces on the botom layer of the flipped laser cut part.

Figure 6 :
Figure 6: Applying conductive paint: A: Painting conductive paint into traces and vias with a brush.B: Peeling of the mask with excessive conductive paint on it.

Figure 7 :
Figure 7: Integrating two types of electronic components: A, B: SMD components can be atached on the surface by injecting conductive epoxy between the component's leads and the conductive trace.C, D: THD components can be inserted into conductive holes and atached by injecting conductive epoxy into the connection point.

Figure 8 :Figure 9 :
Figure 8: Five methods to connect circuits across common laser cut acrylic assembly structures: A: Brushing conductive paint across the but joint; Using conductive tape to connect circuits between acrylic parts.B: Leveraging the conductivity of bolts and nuts to connect circuits across acrylic parts joined with a "T-slot".C: Applying conductive paint onto the inner surface of slots that come into contact when joined.D: Applying conductive paint into vias that connect the circuit traces across the multiple staking layers.

Figure 10 :
Figure 10: Game controller: A: A hand controller with twelve touch butons.B: The transparent acrylic difuses the light from LEDs, and lights up the controller and circuit patern.C: Individual pieces of transparent acrylic containing circuits and mounted electronic components.D: The process of bending the acrylic pieces to create the ergonomic shape of the controller.

Figure 11 :
Figure 11: Mesh lamp: A: The side view of a mesh lamp with organic curves and the shadow on the wall during use.B: The fishnet-like patern with mounted SMD LEDs before and afer the thermoforming process.

Figure 12 :
Figure 12: Water level vase: A: An acrylic vase that can detect the water level and indicate the water-level information through LEDs.B: The electrodes wrap around the vase's body.The conductive paint in the vias connects the internal vase electrode to external circuits, efectively sealing vias.C: The through-hole electronic components of the vase circuits and the base-bias circuit diagram.

Figure 13 :
Figure 13: Pinball machine: A: A pinball machine that can detect the pass-through of the ball and light up the LEDs.B: The details of the gate flaps, including the compliant structure of e-acrylic and the circuit joinery using bolts and nuts.C: The 2D e-acrylic parts of the pinball machine to be assembled into the 3D structure.

Figure 14 :
Figure 14: Cup warmer: A: A acrylic cup warmer with compliant structure in use.B: Placing a cup with water will press down the compliant structure and connect the circuits on toppiece and buton piece through brass; turn on the heating; remove the cup will recover the compliant structure, and disconnect the circuits to turn of heating.C: The compliant spiral shape with the circuit is generating heat and warping the cup's surface.

Figure 15 :
Figure 15: Interactive contour map: A: A 3D contour map made from stacked transparent acrylic.B: The contour map enhances the digital information with physical structure.C: Touching the touch points on the map to interact with the touchscreen underneath.D: The via connects the touch point on the top layer and the circuit patern on the botom layer to extend the touchscreen.

Figure 16 :
Figure 16: Design space for Making Interactive Artifacts With E-Acrylic

Figure 17 :
Figure 17: Evaluating a laser-engraved sheet with micro lens photos.B: The actual width of the specified 0.05 -wide patern is around 0.18 .C: The actual width of the defined 0.2 -wide patern is around 0.33 .D: The actual remaining mask width of the originally designed 0.5mm gap is 0.34 .E: Condensed acrylic particles on the edge of the engraved trace.

Figure 18 :
Figure 18: Thermoforming evaluation and results: A: The thermoforming evaluation setup, including the carbon-fiber heater and 3D printed ABS modules with diferent curvature corners.B: A close-up view of the conductive paint traces afer bending outward with a 0 radius.There are some crackles on the surface of the conductive traces, while the conductivity remains.C: The performance of conductive traces induced by the radius of curvature (0, 1, 2, 3, 4, 5 mm).

7. 3
Future Considerations7.3.1 Conductive Material Choices.Although we only demonstrate one type of conductive paint in this work, we have begun to explore other conductive paints that can potentially work with e-acrylic, such as silicone-based conductive paint.