Introduction: Silicone Devices

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About: A Belgium based PhD student with a profound love for making things. Making things is what makes me happy! Professional interests lie with stretchable electronics, soft robotics, exoskeletons and printed elect…

Silicone Devices deliver the early advantages of soft and stretchable electronics through a Maker-friendly approach. By following this Instructable, you will learn the basic skills necessary to create your own fully integrated soft electronic circuits. Think about Baymax! He's an excellent future vision of a soft robot which will only become reality by developing soft electronic circuits.

"Hold up Noagels... What exactly do you mean with this 'soft electronic circuits' hocus-pocus?"

Well, in short, stretchable electronics promise to naturalize the way we are surrounded by and interact with our devices. They literally are soft and 'stretchy' electronic circuits which open up new possibilities in Human-Computer Interaction and are a key driving technology behind Soft Robotics.

Silicone Devices represent a fabrication approach which is unique because it brings technology to the Maker community which used to reside at scientific research groups. Offcourse, the fabrication process demonstrated by Silicone Devices is not the only route towards stretchable and soft electronics nor is it a completely new one. Science works in incremental steps. One of our steps taken is to make the technology easy to implement and reach Makers all over the world. (This means you. Right here, right now!) Through our fabrication approach, you can create your own soft circuits. Silicone Devices supports inclusion of microcontrollers, I/O components and a power source all combined into a standalone device.

This work came together through collaboration of Raf Ramakers, Kris Luyten, Wim Deferme and Steven Nagels(that's me) at Hasselt University, Belgium. The technique presented in this instructable is published at the premier venue in human-computer interaction: Human Factors in Computing Systems (CHI 2018). This instructable aims to communicate our research results beyond the academic community. There is more in-depth information to read, if you'd like: Here is the project page of Silicone Devices, the full academic publication can be found here, and a more general background on fabrication of interconnect based stretchable electronics can be found here.

However - to make sure you don't TL;DR - let's get down to business!

What you will need:

This tutorial goes into fairly high detail! Please do not be repelled by the number of steps or lengthy descriptions. Since we're sealing our system with silicone, it will be difficult to fix errors which become apparent at the test phase. You'll therefore have to read each step carefully and get it right from the start. The whole process should not take over 2 hours if you have all tools continually at your disposal and employ casting silicone with a 15min curing time.

This tutorial uses a very basic design of a Silicone Device, consisting of 4 contact pads, 2 LEDs and 2 VIAs as a running example. The final result is shown in the photo and video on top. While this design is fairly basic, our DIY fabrication approach supports many types of SMD components and any number of layers. Hence, it our approach scales to stretchable circuits of any complexity as demonstrated by the example designs in the youtube video linked in the beginning of this instructable.

All design files (bundled as .zip) here. Handy single pdf instructions compilation here.

Step 1: Lasercut Starting + Transfer Plates

As a first step, you'll need to lasercut some rigid carrier plates to work on.

Why do you need 2 plates? Well, after creating a component layer on the smooth starting plate, we will adhere the sheet of silicone with components inside to the transfer plate, flip the stack over, take off the smooth starting plate and thereby expose the components from their backside. The transfer plate has tiny holes to allow air to escape when to a wet silicone layer in step 7.

Demands to the carrier plates:

•Need to be equal in size for proper alignment in transfer step

•Size: 280x280mm

•Material: clear acrylic (PMMA or Plexi glass)

•Mark starting plate in top left corner, transfer plate in top right

Attachments

Step 2: Prep Starting Plate for Components

We will start building our circuit on the smooth starting plate in this step. Later on however, we want to remove this plate againt. Therefore you should start by spraying a thin film of mold release spray over the entire starting plate surface. Afterwards, take a black vinyl sticker with dimensions a few centimeters under those of your starting plate. Then peel off the sticker paper and place the sticker flat on and in center of the starting plate; sticky side up. Secure the sticker in place with scotch tape (be careful not to pull to hard on the tape as this will induce wrinkles in your sticker surface). Finish off with another layer of mold release spray on top of the sticky surface. Make sure to keep the nozzle about 20 cm above the surface and spray a smooth, continuous layer. Tip: spray twice and in an overlapping grid pattern!

Prepping the starting plate:

•Cut sticker to size (approx. 2cm smaller
than plate dimensions)

•Put static charge on sticker and plate by rubbing with cotton cloth or paper towel, this will make it lie flat more evenly

•Release spray starting plate (twice and in a grid pattern)

•Scotch tape sticker to starting plate, sticky side up

•Score component placement markings with laser cutter (P = 6-7) DO NOT CUT THROUGH

•Release spray sticky sheet (twice and in a grid pattern)

Attachments

Step 3: Prep Transfer Plate for Selective Adhesion

To guarantee proper alignment during all steps subsequent to step 7, we will have our silicone create a strong bond with the transfer plate at locations outside of our soft circuit's outline. This strong bond is obtained by pre-treating the transfer plate with Bison Silicone Primer. At the end of the build process, you'll want to easily seperate your soft circuit from the build plate and thus not have bonded with it. So we need to keep the area occupied by our soft circuit clear of primer material. We do this by covering this area during spraying of the primer with a sticker cut to size. This mask is obtained by adhering a sticker (normal way, sticky side down) to the entire transfer plate's surface and subsequently laser cutting the circuit outline + 5mm margin shape out of the sticker. Excess sticker material is removed.

Keep in mind:

•Cut sticker to size (approx. plate dimensions)

•Apply sticker without introducing air bubbles

•Design should be mirrored (plate will be placed face down)

•Cut primer mask (board outlines + 5mm margin) with laser cutter (8-9W)

•Selectively remove sticker to expose underlying plexi. Leave the sticker parts which cover circuit board area.

Attachments

Step 4: Component Placement

A somewhat counter-intuitive feature is to start with the components before conductive traces. Place both resistors and led's as indicated in the image provided here.

Why do we place components first? We need our components to be nicely crosslinked with the silicone material around them. On the top and sides this is easy to accomplish. On the bottom side, however, we want to bind our silicone to the component everywhere except on the spots which will be contacted by conductive traces. One way to achieve this is by, consequetively, a) embedding and binding top side of the components in a silicone sheet, b) flipping over the stack to expose each components' contact pads, c) apply conductive traces and only thereafter d) bind the remaining exposed component bottom surface area to a second layer of casting silicone. These steps a) b) c) and d) are discussed further on in the I'ble.

General guidelines to this step:

•Place components according to circuit design on starting plate. Push component firmly through sprayed release layer into the sticker’s adhesive layer. This way it stays in place.

•Components should be SMD. Preferably 2010 size or bigger. Spacing on an IC’s neighbouring pins cannot be below 0.8mm. TQFN packages are the lower limit.

•Every placed component should have its contact pads in plane with the sticker’s adhesive layer

Step 5: Primer Application

Applying the primer is a crucial step which cannot be omitted. Without good adherence between the component and surrounding silicone, strain would create a loose fit of the silicone around each component. This loose fit would then allow liquid metal to flow across contact pads and thus introduce shorts. A thin, uniform layer of Bison Silicone Primer should completely cover all exposed parts of the component lying flat on the sticker.

For your consideration:

•Use Bison Silicone Primer and air brush (Sealey Tools AB931)

•Spray components on starting plate with a thin layer from every angle

•Let dry and immediately continue with step 6 for optimal cross-linking

Step 6: Cast/blade Coat Silicone

Next up: casting silicone around and over our components! Thickness of this layer needs to be around 300 micron more than your thickest component's thickness. For the components stated at the beginning of this I'ble, this means 1mm. To achieve this required thickness, we will use a flood bar which we sweep across the surface at exactly this height. (For the curious minds: jargon term for this is blade coating).

Casting silicone on its own is not viscous. I wouldn't keep shape after giving it a certain height. Therefore a sort of 'swimming pool' of more viscuous acrylic mastic (silicone sealant) is applied. We don't want to smear this sealant into our sample: that's why we will be coating twice and from the middle outwards.

Bullet list:

•Put acrylic mastic kit around required silicone sheet perimeter

•Mix 2 component shore 15 hardness platinum poly-addition silicone

•Pour into mastic ‘pool’, starting from the middle and on all components

•blade coate a silicone layer with height 300um > heighest component

•Wait for silicone to cure

Step 7: Adhere Transfer Plate

Hey you're doing a great job so far! Normally at this point there is a silicone, component-filled sheet smiling back at you. Components should be completely covered in silicone and have their bottom contacts lying flat onto the plexi glass carrier plate with a vinyl sticker inbetween. Let's now flip this stack and expose those contacts!

*insert misalignment warning here*

What we have at this point is a sheet of components which are placed exactly (you did do a precise job, right?) according to a digital design aligned to the top left corner of your carrier plate. We now need to place a second plate on top, adhere the silicone slab to it, flip the stack and remove the first carrier plate - all without losing this corner alignment! You will see this is easier than it sounds. Make sure you have a good vice or straight corner around against which you can push the plates into alignment.

First we need to spray our second carrier plate (the one with the air holes) on which you have already placed a vinyl sticker and cut to shape to form a primer mask. Spray in an even, continuous pattern. Afterwards, remove the primer mask sticker.

Now take your plate with the component-filled slab. Align its top left corner into your vice or straight corner. Next, mix some more silicone (about 50ml will do fine). Pour it on top of the silicone slab and spread it out to a more or less equal layer. Next, take the second carrier plate (with air holes) we just primed. Its rop right corner was marked a few steps back. Place it on top of the first plate sprayed side down and with the marked corner also downwards in alignment with the top left marking on the starting plate. Press down, squeeze out air bubbles and keep aligning the plates in between. Squeezing out more silicone through the holes makes for less air bubbles and a better bond. Coincidentally, however, this also means more difficulties for you when shifintg the plates further into alignment. So align first, then start squeezing out air.

Finally, wait for the silicone to cure.

A shortlist overview:

•Spray transfer plate with primer. Remove primer mask

•Mix 2 component shore 15 hardness platinum poly-addition silicone

•Apply an even layer on the now cured component containing silicone sheet, approx. 1mm thick

•Transfer plate, primed side down

•Align with starting plate

•Apply pressure, squeeze out air

•Double check alignment

•Wait for silicone to cure

Step 8: Remove Starting Plate

Crucial part's over. Let's now work through to the moment we can verify your alignment skills!

Take your plexi-silicone-sticker-plexi sandwich, use a cutting knife to loosen the scotch tape at the edges of your vinyl sticker. The plexi glass starting plate should come off easily now. If this is not the case, use a flat object in between the sticker and your plate or in between both plates to loosen up the stack. Be careful to not tear your silicone stack from the second plate (with holes) as this will introduce misalignments.

If the components were placed correctly - in adherence to the sticker - and the silicone process was performed careful enough to not rip components out of place; you should now have your components with their backsides nicely exposed!

Use a multimeter to measure each component's value. (resistors --> measure ohms, led's --> use diode setting to light them up). This way you can verify electrically if no thin film of sticker adhesive or casting silicone is covering the contact pads - barely visible to the naked eye.

In short:

•Loosen sticker on one side of the plexi-silicone+sticker-plexi sandwich

•Peel starting plate and sticker from the silicone embedded components

•Check components for unobstructed exposure of conductive pads

•Since we have flipped the stack, all further step need to be fulfilled with design layers mirrored (all files in this tutorial were already prepared accordingly, no further adaptations necessary)

Step 9: Stencil Mask for Top Conductive Layer

Your moment of truth! Let's check how well you did in previous steps.

Apply a new sticker to completely cover your silicone slab with exposed component contacts. Place the plate into your laser cutter while its marking is seen at the top right corner and cut the first circuit layer through the sticker.

If the stencil we cut next nicely aligns with your components you have done well in all previous steps. If otherwise.. Well damn. Problems most probably relate to your sticker not lying flat during application of silicone and/or significant misalignment of second carrier plate to the first carrier plate 2 steps back. Measure how many mm's you are off and you can correct for this through design placement in the laser cutter software.

A summary, for your convenience:

•Cut sticker to size (approx. plate dimensions)

•Apply sticker without introducing air bubbles

•Calibrate laser to precisely cut through the sticker (8-9W)

•Cut top copper circuit traces with laser cutter

•Remove sticker in areas which need to be made conductive (circuit traces, pads)

Attachments

Step 10: Top Conductive Layer

We will be working with the liquid metal in this step. Make sure your working space is completely covered (with newspaper for instance). When you spill liquid metal, it becomes a pain in the a to clean it back up. There is no real solvent for it nor does it soak into spunges or paper towel. Best is to work really really clean and just afterwards throw away the newspapers you might have made a spill on. Best wear gloves or wash your hands afterwards. There will be smears.

At this point you should have a properly defined stencil. Make sure it is sticking nicely to the silicone at the edges. We do not want any liquid metal flowing through underneath.

Now take the liquid metal and a fine brush. Apply the liquid metal to the stencil openings in short smears (pictures for reference). This should be more of a dipping action than smearing. The liquid metal has to be forced into close contact so it can adhere well. Once you have covered your stencil's pattern, take the roller and roll the surplus of liquid metal to the side. This can be recovered with a small plastic pipette.

In short:

•Make sure your sticker adheres well around edges of exposed areas

•Clean exposed silicone and component pads with isopropylalcohol

•Use a paint brush to roughly cover all exposed areas with Galinstan

•Use the roller to turn the applied galinstan to an even coating

•Recover excess galinstan back to its container

•Remove the sticker stencil carefully

•If during removal Galinstan flows to areas where it should not be, your coating was too thick. Clean surface and restart at step 9.

Step 11: Prime Component Bottoms

This step is quite self-explanatory. You have already applied primer twice before. Just do it again. Focus lies not with the silicone sheet but with the component bottom sides and especially the parts which do not have liquid metal printed on them. Let the primer dry and immediately after continue with step 12.

•Using Bison Silicone Primer and air brush (Sealey Tools AB931)

•Spray exposed component bottoms with a thin layer of primer

•Let dry and immediately after continue with step 12

Step 12: Cast/blade Coat Silicone

This one is also more of the same you did before. Most importantly here is the height on which you blade coat. Previous layer (component layer) was 1mm (recommended led was 0.7mm thick + 0.3mm as suggested before). For each circuit layer a height of 0.5mm silicone is added on top as to leave enough margin for uneven coatings with liquid metal. Height on which you blade coat here therefore becomes 1mm + 0.5mm = 1.5mm.

Detailed steps in short:

•Put acrylic mastic kit around required silicone sheet perimeter

•Mix 2 component shore 15 hardness platinum poly-addition silicone

•Pour into mastic ‘pool’, starting from the middle and on all components

•blade coat a silicone layer with height 0.5mm > current stack thickness

•Wait for silicone to cure

Step 13: Stencil Mask for Bottom Conductive Layer

And we have now fully entered the easy parts! What you find here is all repetition. Every circuit layer you apply on top is a repetition of steps performed for previous circuit layers. Here you need to create a stencil mask for circuit layer 2.

Without too much elaboration:

•Cut sticker to size (approx. plate dimensions)

•Apply sticker without introducing air bubbles

•Cut bottom copper circuit traces with laser cutter (W à calibration)

•Remove sticker in areas which need to be made conductive (circuit traces, pads)

•Make sure your sticker adheres well around edges of exposed area

•Clean exposed silicone with isopropylacohol

Attachments

Step 14: Top-bottom VIA's

Only novelty lies with the places where we need a connection between 2 subsequent circuit layers. In jargon these are called Vertical Interconnect Access or VIA for short. To create a via, you have to cut an opening in the silicone covering a previous circuit layer. When you then print new liquid metal on top for the next circuit layer, it will flow into this opening and electrically connect.

You'll first have to calibrate (refer to: calibration) the laser to precisely cut through the silicone covering layer on top of the previous circuit layer. Then just cut out the VIA's according to the file provided herewith. Remove each silicone covering layer cutout with tweezers and proceed to the next step: printing a new liquid metal circuit layer on top!

Creating VIA's, a short version:

•With the bottom conductive layer stencil mask ready

•Calibrate laser to precisely cut through the silicone layer to expose top conductive layer (12-17W)

•Cut VIA’s throughout silicone where top and bottom conductive layer need to be interconnected

•Remove cut out silicone to expose top conductive layer

Attachments

Step 15: Bottom Conductive Layer

Again, make sure your working space is covered when working with liquid metal. This will make it a lot easier to deal with spills.

Printing this layer is again a repetition of previous efforts. Make sure the stencil is sticking nicely to the silicone at the edges. We do not want any liquid metal flowing through underneath. Use the dipping action again to apply liquid metal to the stencil openings with a fine brush. Take the roller and roll the surplus of liquid metal to the side. Recover big blobs of liquid metal with a plastic pipette.

Another TL;DR version:

•Use a paint brush to roughly cover all exposed areas with Galinstan

•Use the roller to turn the applied galinstan to an even coating

•Remove the sticker stencil carefully

•If during removal Galinstan flows to areas where it should not be, your coating was too thick. Clean surface and restart at step 13.

•Use the paint brush to touch up each VIA and make sure top and bottom conductive layers connect

Step 16: Cast/blade Coat Silicone

You can start getting excited now! This is our final layer of casting silicone, which means your soft circuit is almost finished! This you have done already twice before. So I'll just keep it short and tell you what height you should aim at for blade coating. We already have a 1mm thick component layer and a 0.5mm thick first circuit layer. This circuit layer should also be 0.5mm thick. Therefore blade coat at 2mm total thickness in this step!

Fast track:

•Put acrylic mastic kit around required silicone sheet perimeter

•Mix 2 component shore 15 hardness platinum poly-addition silicone

•Pour into mastic ‘pool’, starting from the middle and on all components

•blade coate a silicone layer with height 500um > current stack thickness

•Wait for silicone to cure

Step 17: Contact Pads

While Silicone Devices can embed power (battery) and processing (microcontroller), for the simplicity of this example, we add external connectors to supply power to the LEDs. In this step we will cut through the silicone up to the contacts which we have embedded inside. Again you will need to calibrate the laser (refer to: calibration) in order to not damage underlying layers. When you have made the cuts, tear out the silicone cutouts with tweezers. Then scrape excess silicone residue of your contacts and clean with cotton swabs and apply solder to the contacts for added reliability.

Contact pads, a short story:

•Calibrate laser to cut through the silicone layer and expose copper tape contacts (20-30W)

•Cut circuit contacts with laser cutter

•Remove silicone in cutout areas

•Clean exposed copper pads with a fast drying solvent

•Apply solder to exposed pads until contacts come level with silicone. Keep resoldering while scraping excess silicone of your contacts and cleaning dirt away until your solder sticks to the pad.

Attachments

Step 18: Sample Cut Free

Time to free your soft circuit from its carrier plate! Since our transfer plate was not coated with primer underneath our soft circuit, all we have to do is cut the sides loose and we can take it off. Use the hereby attached cut file for cutting the sample. Keep repeating cuts with increasing power until the sample comes free. Z-offset of your laser should be -1 (half of the stack height). When the sample cutout is completely made through, lift a corner from one side and then cut your soft circuit free from all attachments underneath which were formed in the carrier plate air holes. Take a good look at it: your first Silicone Device! A conformable, stretchable and soft circuit!

Sample cut free in bulletpoints:

•Calibrate laser to cut through complete silicone stack (40-60W)

•Cut sample outline with laser cutter

•Lift sample from plate while manually cutting it free from silicone attachments which were formed in the transfer plate air holes

Attachments

Step 19: Admire

Now hook up your silicone device to a 5V power supply. Each connector-resistor-led-connector path has a seperate need for power. You can connect both in parallel. Just keep an eye on the polarity of your led and match your power connections accordingly. Once your soft circuit is powered, the blue led should turn on.

Give a stretch to your circuit! If you've done it right, you should easily reach 50% strain without any damage to the circuit. Main point of failure will be your contact pads as these are made of rigid foils which tear apart with high strains.

The following adjectives match your Silicone Device:

•Flexible

•Soft/stretchable

•Self-Healing

•Translucent

•Fully encapsulated

Application domains which I foresee: biomonitoring patches (on-skin), wearables, Silicone Devices embedded into textiles, electronic circuits which span mechanical joints, driving or sensing electronics for soft robots, ...

Which applications do you seem fit for these unique kinds of soft circuits? Let me know in the comments!
I cannot wait to see what you guys come up with. Let me know if your building something unique. Who knows I might be able to give you some advice!

Good luck experimenting,

Cheers,


Noagels