Fabrication

This section breaks down the necessary steps to mold and assemble the EGaIn sensor described previously. The sensor consists of two halves that are molded separately and then bonded together. The molding and bonding processes are assisted by an oven and spin-coater to speed up the curing and bonding procedures. EGaIn is then injected into the channels inside the sensor and electrical wires are attached. An overview of this process is given below.

Overview of steps to be taken

(A) Velcro is laser cut into desired shape.

(B) Velcro is placed in the bottom-half mold and encapsulated in stiff silicone rubber.

(D) After curing the bottom-half mold, a small amount of soft silicone rubber is spun on to it to act as an adhesive layer for lamination.

(E) The top-half sensor is demolded and laminated to the bottom-half.

(F) Liquid metal alloy is injected into the microchannels with one needle while a second is used to evacuate the entrapped air.

(G) To complete the sensor, wires are inserted into the soft elastomer and encapsulated with rigid epoxy for strain-relief.

Diagram of full sensor

Overview of Mold

Top mold (mold, acryllic plate and transparency shown)

Top mold: This half of the mold has the negative impression of channels that will contain EGaIn. Transparency and acryllic plate used to close the mold are shown in the bottom picture.

Bottom mold:

The first row on the image to the right shows the bottom half of the sensor mold. The stiff ends of the sensor (elastomer covering velcro) are made in this half of the mold. The rest of the bottom half of the sensor is also made in the same mold (so there is no need to demold the stiff ends). The circle in the center is for the spin coater to grip. Part of the end of the mold is raised and creates a slot that the velcro pieces are placed in. These cover part of the velcro, preserving some functional uncoated velcro area.
The middle row is a top plate which has one flat side and one side with a raised area that’s used to keep elastomer out of the center when molding the stiff ends.
The third row shows the transparency.

When molding the soft elastomer, use the transparency and the flat side of the top plate.

When molding the stiff elastomer, use the raised side of the acrylic plate.

Bottom mold (mold, top plate and transparency shown)

Bill of Materials

This section will give a list of items that are used in this project with selected links to suppliers. You can download a more detailed Bill of Materials sheet here.

Note: Many of the items listed are just examples and you can use your own discretion to substitute parts which are easier or cheaper to obtain.

Mold Components


Top Mold	Acryllic plate and Transparency film

Bottom mold (mold, top plate and transparency shown)

Click here to download the .STL files needed to 3D print the molds for this sensor.

Click here to download the .DXF files needed to laser cut the transparencies and plates.

If you would like to modify the molds click here to download the SolidWorks part files.

Polymer Materials

Soft silicone rubber (EcoFlex 0030, Shore OO-30 hardness, E = 30 kPa)

(Supplier link)

Stiff silicone rubber (SORTA Clear 40, Shore A-40 hardness, E = 1.3 MPa)

(Supplier link)

Mold Release (Ease Release 200 works well)

(Supplier link)

Other Materials


Velcro material, Loop 3008 type* (Suggested suppliers)	Eutectic indium gallium alloy (EGaIn). Need ~100 uL per sensor though it depends on channel geometry. (Supplier link)	Silpoxy silicone adhesive (Supplier link)

Transparency film (Supplier link)	Acrylic (¹/₄" thick) (Supplier link)

*Type selected because it is very thin (low profile)

Tools & Hardware


Tongue Depressors	Tweezers	Weights

2 small syringes (1.5-3 mL) (Supplier link)	2 needles (27 gauge) (Supplier link)	Electrical wire** (Supplier link)

Wire Strippers

**Smaller diameter is better. Here we use 36 AWG copper wire.

Equipment


Vacuum chamber (Supplier link)	Laser cutter (Supplier link)	Mass scale (Supplier link)

Lab oven (Supplier link)	Centrifugal mixer (Supplier link)	Mixing cups

3D Printer*** (Supplier link)	Spin-coater (Supplier link)	Multimeter (Supplier link)

Microscope (optional) (Supplier link)

***Minimum need 10x better than dimensions of your structure. i.e. object spatial res. On micron level, but smallest features are 100 microns. Same for FDM. Aim for few hundred microns wide/tall

	SRT_EGaIn3DPrinterFiles.zip	1.24 MB
	SRT_EGaInLaserCutterfiles.zip	9 KB
	SRT_EGaInMoldModel.zip	2.1 MB
	EGaIn Detailed BOM.xlsx	13 KB

Step 1: Prepare Velcro Fasteners

The first step involves laser cutting the shown pattern from Velcro sheets. Laser cut the Velcro with the fuzzy loop side facing upwards, not the hook side. The .DXF files used to create this pattern can be found here.

Note: Make sure to cut 2 pieces for each sensor you want to make.

Links to Velcro suppliers can be found in the Bill of Materials section.
A velcro strip that has been laser cut is shown below. Three velcro end pieces have been removed and two are shown above the strip.

Step 2: Mold Stiff Ends

The next step is to make the stiff end pieces of the sensor. To begin, inside a fume hood, spray the center-displacement piece with mold release. Make sure to spray the side with the raised ridge. Sorta-clear is very stiff, so without mold release the 2 mold pieces will be glued together
Mix a small amount of Sorta-clear elastomer in a cup. Not much is needed, only roughly 10 grams.
Refer to the Bill of Materials section for vendor information.

Pour stiff rubber

	Insert velcro pieces into mold through the slots on the ends with the fuzzy size face down.
	With a tongue depressor, apply elastomer beneath the exposed velcro. Make sure the fuzz is thoroughly coated.
	Align the velcro using the pegs, then apply elastomer to the top of the velcro as well. Excess elastomer should be left in the end circular areas of the mold.
	The image on the left shows a mold with fully coated velcro end pieces. Note how there is some velcro sticking out of the ends of the mold.

Degas elastomer

Place the mold with velcro and elastomer into a vacuum chamber and degas (set the vacuum chamber to -90 kPa). Release and restore the vacuum several times to help pop many of the bubbles. Then leave the mold in the machine for a few more minutes until bubbles start dying down.

Remove the mold from vacuum chamber and inspect for bubbles, particularly ones near the interface between the velcro and elastomer, as these will turn into failure points.
To remove any remaining bubbles, dip tweezers into the elastomer and slowly “push” the bubbles towards the edges and “lift” them out of the elastomer.

Seal and press mold

	Put on the top piece of the bottom mold, raised side down. Press it down gently so the pegs align, then apply gentle finger pressure from the center moving outwards. This process will keep the elastomer out of the middle portion of the mold, which is where the softer elastomer will go later.
	Place the molds in a 60 ^oC oven and put weights on top. Remove from the oven after 15 minutes.

Check for defects

	After taking the mold out of the oven, peel off any excess elastomer from around the edges. Pry off the top using a tool to gain leverage, such as a screwdriver. The pieces should be non-tacky but brittle (it will take a day to reach full strength).
	There should be no gaps or bubbles and the interface/edge of the stiff elastomer should be clearly defined. Peel excess elastomer off the pegs to make next step easier.

Step 3: Mold Top and Bottom Layers

The next step is to fill in the center of the sensor with a softer elastomer to connect the two stiff ends that were just made. The top half of the sensor will also be made (this half is entirely made of the softer elastomer).
Have the top mold ready, alongside bottom mold with stiff pieces in it.
Mix a small amount of Ecoflex 0030 in a cup and prepare to pour (need ~20 g).
Refer to the Bill of Materials section for vendor information for EcoFlex.

Pour EcoFlex and Degas

	Carefully pour the Ecoflex that was just mixed onto both the top and bottom molds. Don't worry if there is excess Ecoflex in the mold.
	Degas like what was just done in the previous step to remove bubbles.

Add Transparency

Line one end of the transparency film up with the pegs and slowly lower it from one end to the other, making sure not to trap any bubbles under the film.


	Once the transparency reaches the other end, let go of the transparency and use tweezers to gently align it with the second set of pegs. At the same time, make sure to push any remaining air gaps/bubbles out of the end. Note: There is a transparency for both the top and bottom mold and they both need to be placed down at this time.

Cover Mold


Now cover the molds with their respective top plate. For the top mold (shown above on the left) this will be the acrylic plate. For the bottom mold (shown above on the right) this will be the reverse side of the plate used in Step 2. Place both molds in the 60 ^oC oven and add weights on top. Let cure for 10 minutes before removing from oven.

Demolding

	Take the acrylic and transparency off the top mold and peel away any excess material around the edges and the alignment pegs.
	For the bottom mold, remove the rigid top but keep the transparency on for now. This will be removed at the last minute before bonding to keep the surface as clean as possible. Make sure to peel off excess material around the alignment pegs however.

Step 4: Bond Top and Bottom Layers

The next step involves using a thin layer of Ecoflex 0030 to bond the top and bottom halves of the sensor. If the layer is too thick, it will plug the channels. To ensure a thin, uniform layer a spincoater is used.
The spincoater should be set to 2000 RPM for 60 Seconds.
As done in the previous step, mix a small amount of EcoFlex 0030 and degas it.
Refer to the Bill of Materials section for vendor information for EcoFlex.

Add Adhesive Layer

	Place the bottom mold into the spincoater and spin briefly to test that the vacuum works. The spincoater chuck should be completely covered by the mold to ensure a good seal. Peel off the transparency at this time and remove any excess elastomer around the edges.
	Pour the EcoFlex onto the mold, making sure to wet as much of the surface as possible.
	Run the spincoater program for the full 60 seconds at 2000 RPM.
	In the meantime, demold the top half of the sensor (the one with the channel grooves) from its mold.

Set New Layer in Oven

	Remove the bottom mold from the spincoater, and place it in the 60 ^oC oven for 40 seconds.

Align Layers

	Next the two layers need to be aligned correctly. Holding the bottom half steady, the top half will be lowered gently onto it. The side that contains the channels will be face down.

	Similar to how the transparencies were applied, slowly lower top piece from one end to the other, being careful to keep alignment and to avoid trapping air bubbles. Use tweezers to guide the top half. Use the tweezers to push out any bubbles that do appear as well.
	Try to avoid “scooting” or going backwards if possible.
	Once you reach the end, use tweezers to align the top piece to the pegs again. Cure in the oven for 5-10 minutes.

Step 5: Inject EGaIn

Setup

The next step involves injecting the EGaIn into the hollow channels of the sensor. In order to do this, you will need 2 syringes with 27-gauge needles.

Before getting started, make sure both syringes have their plungers depressed all the way.
In one syringe, suck up 0.5 mL of EGaIn from a vial, then depress the plunger slightly to ensure there are no air bubbles inside the needle.
Though not required, doing the injection under a magnifying scope will make it easier.

Injecting

	Approaching at a low angle, insert the needles into the rounded channel ends of the soft sensor. This low angle reduces the risk of puncturing the bottom of the sensor and creating a leak. You should be able to see the needle exit the rubber and enter the empty channel.

Slowly inject the EGaIn with one syringe while slowly pulling the plunger out of other syringe to evacuate the trapped air. If doing this simultaneously is too difficult, alternating the injection with the vacuum is fine. You should be able to see the EGaIn travel through the channels. Before the EGaIn reaches the end, remove the vacuum needle. Inject a little excess so that some EGaIn leaks out the other side – and if there are any trapped air bubbles, keep injecting until all are removed. Make sure there are no air bubbles. Suck up any excess EGaIn that leaked and wipe/dab off the exit using a tissue wetted with IPA.

Step 6: Plunge Wires

Prepare and Insert Wires

Cut a length of thin electrical wire (i.e. 36 AWG). The length of wire needed depends on your application, here we use 15-20 cm.
Strip the wire ends to expose 2-3 mm which will be inserted into the soft sensor.
Grab the wire end with tweezers and push it into the soft sensor, following the path left by the syringe.
Push it in until the wire comes in contact with the round EGaIn channel end.
Repeat this process for the other channel end.

Test and Seal

	Touch a multimeter to both loose wire ends to make sure that the connection is intact.
	Add Silpoxy to the area where the wire is inserted into the sensor, covering any exposed metal and making sure to coat a few mm of the wire as well. This seals the EGaIn in the sensor and prevents the wires from slipping out.

Step 7: Demold

Peel off the completed sensor, pulling out the rigid ends first.

Wait 1 day for rubbers to strengthen before using the sensors.