This is the second in a series of posts documenting a project funded by an Entrepreneur-in-Residence Award from Creativeworks London. See an earlier post for the first design iteration.

After working with the laser cutter and inkjet printer to bring some initial ideas to a physical form, I was ready to move on to the 3D printer. The Materials Processing Lab at Queen Mary has a Objet30 Prime which is printer that builds up a liquid layer of material that is then UV cured. It can print 16-28 microns per layer, depending the material being printed.

3D Printed Buttons

The biggest barrier to the 3D printer for me was learning how to create a model. I worked with AutoCAD and managed to learn enough from online tutorials to get what I wanted. I started by creating a solid cylinder, seen below in a hard white plastic, surrounded by a translucent material printed as a support structure.

The support material is washed away in a power washing station. The station has a staggering number of significant design flaws, but its little automatic windshield wiper makes it endearing.

The initial printed design was a 3D interpretation of the laser cutter layout. Eight holes were printed in a cylinder with a recessed circle so that it could hold both a PCB and contacts to reach the pins. A notch was included to allow the inserted PCB to fit in only one way. This initial print can be seen below. It is created with Tangoblack FLX973, a flexible, rubber-like material. It was printed with a matte finish, an effect caused by being printed with the support material fully encasing the object.

The first change made to the design was the position and size of the holes for the contacts. The first design iteration used sewing straight pins as contacts, which were handy for a very quick and dirty proof of concept, but certainly not ideal for anything more robust than that. After hunting through electronic component catalogues, I came across turned pin header sockets used for wire wrapping. The long legs of the sockets gave plenty of clearance to bend the legs out from under the button and reach the conductive thread of the embroidered circuit. The legs could then be hand-sewn with conductive thread.

The design change was to equally distribute the sockets around the PCB/button in a circle, as opposed to two arced groupings. This provided the maximum distance between the contacts and made it easier to create design files in multiple software packages for the different prototyping processes.

To work towards a functioning prototype, I laser cut acrylic to play the role of the PCB.

After discussing the prototype with Andrew, he pointed out that the button hole could cause too much mechanical strain on the socket legs. The stitching holding the legs in place was the only physical connections holding the button in place. To improve this, we added holes to allow the button to be stitched to the fabric directly. They were placed slightly outside the sockets so that the thread could provide a mechanical strain-relief for the socket legs.

The print below uses the gloss effect in the same rubber-like plastic as the print in the photo above. The gloss effect is achieved by not surrounding the object in the support print material, but using it only where structurally necessary.

Milled PCB

It was then time to move on to the PCB prototype. I created the design in EAGLE – with assistance from Andrew as I am far from fluent with that software tool. The PCB was then milled on the LPKF S103 PCB Mill located in the Electronics Lab at Queen Mary. Below is a PCB as it came off the milling bed on the right and after some sanding on the left.

The PCB fit cleanly into the 3D printed button with all the holes lining up as they were supposed to. However, the inserting and removing the PCB and pins into the sockets highlighted a number of weaknesses in the design.

Reflections on Iteration 2

The printed material wasn’t adhered to the sockets in any way; the sockets were held in place only by the friction of the socket housing against the printed holes. The insertion and then removal of a pin into a socket caused the socket to pull away from the button.

The legs of the sockets were bent so the button could sit against the fabric, but they weren’t formed into a shape such as a loop. This meant that the force of removing a pin from the socket could cause the leg to be slid out of the stitching holding it in place.

Lastly, the 3D print material couldn’t withstand a shearing force where the rim that holds the PCB in place meets the main body of the button. The rim quickly fell apart, though anecdotally, the glossy finish seems to hold together better than the matte.

These concerns will be addressed, and hopefully improved upon, in the next and final design iteration of the project.

I’ve started referring to electronics connected to soft sensors and hidden on a PCB in a pocket of a garment as the “pocket of shame”. By no mean do I mean any shame on the designers and engineers who were forced to place their circuitry in a pocket, but that it’s a shame we haven’t found a better solution. Beautiful, tactile, and flexible, textile electronic sensors can be woven, knit, or stitched into clothing, but the circuitry needed to process the data generated by those sensors or to send that data off to be processed on another device, is still often mounted on rigid PCBs.

This past October I started a short research project funded by the Creativeworks London Entrepreneur-in-Residence Scheme at the School of Electronic Engineering and Computer Science at Queen Mary University of London. I’m exploring how PCBs can be better integrated into garments with soft sensors, not by trying to transform the PCBs into something flexible, but by hiding them in the already “hard” parts of a garment – buttons.

I’m working with Andrew McPherson, a Senior Lecturer at QM, and Berit Greinke, a PhD student and lab technician in the Materials Processing Lab. We are using the rapid prototyping facilities available in the School to create buttons that are inspired by the chip holder/adapters shown in the photo above. The idea is to create a housing that can be sewn to a garment (with both conductive and non-conductive threads) that remains in place whilst the PCB it houses can be removed.

The Housing

The first design iteration has been focused on a SOIC package of an ATtiny and using the tools with the smallest barriers of use, whether cost, bureaucracy or my own skillset.

The housing has been developed using laser cut and etched rubber to make a recessed area for the PCB to sit in with 8 holes for pins to make contact with the PCB and fabric sensor.

The Circuit

The circuit was printed on paper using a Brother DCP 145C and silver nano particle ink.

I used clipped sewing straight pins as the contacts as they have broader flat heads that sat against the underside of the paper. They pierced the paper and then were electrically connected to the printed circuit with Bare Conductive Electric Paint.

The Fabric Connection

The remaining piece is the fabric circuit that the button interfaces with. With much training and assistance from Berit, I designed and embroidered a breakout circuit for the button to be sewn onto.

A Brother Pr1000e was used to embroider the circuit with Shieldex 110f 34dtex 2 ply yarn by Statex. It’s mostly done automatically, but some care needs to be taken to manually stop the machine’s thread cutter from trying to cut the conductive thread.

Here is the circuit when it comes off the machine, before trimming all the threads (which is something that can be programmed into the design for the machine to handle, but I hadn’t tackled that yet as this was my first time working with the software).

And here it is with the laser cut housing sewn in place. I used sewing straight pins to piece the rubber and then clipped the ends and formed them into loops.

The ATtiny was cold soldered into place on the circuit board with Bare Conductive Electric Paint and the circuit board placed in the holder.

A second piece of rubber served as a means to hold the circuit board in place.

Reflections on Iteration 1

The prototype button is able to make an electrical connection ranging from 30 to 80 Ohms of resistance from the chip leg to the sewn breakout pad. However, it is not a stable connection, but rather fragile both due to the materials and how the “sandwich” of layers meet each other.

The ability to so quickly make physical prototypes was incredibly useful, even though they are certainly not the best materials and tools to be used for the final version. Now that a general form factor has been settled on, the next iteration will utilise the 3D printer instead of laser cutting and a milled PCB instead of a printed paper circuit board.

The use of straight pins as electrical contacts were convenient as they were readily available in the lab, but won’t be used in future iterations. Another solution needs to be sourced, probably by perusing electronics components catalogues.