Stretchable electronic strips for electronic textiles enabled by 3D helical structure

The development of stretchable electronic devices is a critical area of research for wearable electronics, particularly electronic textiles (e-textiles), where electronic devices embedded in clothing need to stretch and bend with the body. While stretchable electronics technologies exist, none have been widely adopted. This work presents a novel and potentially transformative approach to stretchable electronics using a ubiquitous structure: the helix. A strip of flexible circuitry (‘e-strip’) is twisted to form a helical ribbon, transforming it from flexible to stretchable. A stretchable core—in this case rubber cord—supports the structure, preventing damage from buckling. Existing helical electronics have only extended to stretchable interconnects between circuit modules, and individual components such as printed helical transistors. Fully stretchable circuits have, until now, only been produced in planar form: flat circuits, either using curved geometry to enable them to stretch, or using inherently stretchable elastomer substrates. Helical e-strips can bend along multiple axes, and repeatedly stretch between 30 and 50%, depending on core material and diameter. LED and temperature sensing helical e-strips are demonstrated, along with design rules for helical e-strip fabrication. Widely available materials and standard fabrication processes were prioritized to maximize scalability and accessibility.

2 Helical e-strip fabrication Four categories of helical e-strip were constructed, as detailed in Supplementary Table 1.Fabrication of helical e-strips was performed using a variation on a standard PCB fabrication process, as follows: 1.The circuit layout was created in Autodesk EAGLE and edited in Adobe Illustrator to create outlines for the planar e-strip.
2. Using a vinyl cutter (Model GX-24, Roland DGG, Hamamatsu, Japan), the planar e-strip outlines were cut from copper-clad polyimide, mounted on a transfer tape backing to preserve alignment.Excess material was peeled off, as shown in Supplementary Figure 2A.
3. A mask was fabricated to allow the circuit to be etched on the planar e-strip outline.
a.For helical interconnect and 4 mm diameter LED e-strips, a dry film photolithography process was used.This involved: i. Using a vinyl cutter to create a negative UV mask from black heat transfer vinyl.Heat transfer vinyl is a polymer film with a heat-activated adhesive backing, designed for making graphics on clothing.However, as it comes on a clear backing film, it can also be used to make a mask for photolithography.Black Cricut Sportflex heat transfer vinyl was cut to create masks, as shown in Supplementary Figure 2-B.This method was found to be preferable over the more standard method of printing masks on transparent film using an inkjet printer, as the printer used was not able to print the mask so that it was completely opaque.
ii. Applying photosensitive dry film to the planar e-strip outlines using a laminator iii.Placing the UV mask on top of the planar e-strip outlines and taping it in place iv.Exposing the photosensitive film to 30 s of UV light to crosslink and harden exposed areas of the film, using a UV exposure box v. Removing the UV mask, and using a potassium carbonate developer solution to wash away unexposed dry film.The developer solution was constantly agitated by an orbital shaker to aid the developing process.
b.For temperature sensing e-strips and 2 mm diameter LED-strips, the dry film process couldn't be fine-tuned sufficiently to achieve the resolution for these fine pitch circuits.
To overcome this issue, an alternative method was developed.Adhesive vinyl, which is similar to heat transfer vinyl, except that its adhesive doesn't need to be heat activated, was applied on top of the planar e-strip outlines before vinyl cutting.Adjusting the cutting force of the vinyl cutter allowed the adhesive vinyl to be cut to form a mask for etching.Excess vinyl was removed with tweezers.This is shown in Supplementary Figure 2-C.
4. Etching in a sodium persulphate solution in a bubble etch tank removed excess copper.
5. Removal of the dry film mask was performed by soaking in acetone, and the vinyl mask was removed using tweezers.
6. Solder paste was dispensed onto solder pads, and SMD components were placed using a pick and place machine.
7. Components were soldered using a hotplate, with the exception of pin header connecters, which were hand soldered using a soldering iron.
8. Encapsulation was applied and exposed to UV light to cure in accordance with manufacturers' recommendations.
9. The planar e-strip was bonded to the core using cyanoacrylate adhesive.First, one end of the planar strip was bonded to the core.Then, using a printed guide to maintain correct helix angle, and silicone paper to prevent any accidental bonding between the helical e-strip and the surface on which it was assembled, the planar e-strip was gradually wrapped around the core and bonded.This process is shown in Supplementary Figure 2-D.
10. Heat shrink was applied to the interface between connector and the rest of the e-strip, and activated by a hot air gun to make it conform tightly to the e-strip.Small amounts of cyanoacrylate were applied between heat shrink and e-strip using a syringe, to make sure it stayed in place during tensile testing.3 Evaluation of materials for the helical e-strip core The material used as the helical e-strip core needed to satisfy the following requirements: 1. Stretch: the core must be highly stretchable, so that the finished helical e-strip can stretch.
2. Recovery: it must have good recovery properties, returning to its initial length after stretching.
3. Compressibility: it must be compressible, to allow components facing the interior of the helical e-strip to compress into its surface, maintaining a smooth helical shape, and cushioning the components to provide support.
Rubber foam cord was identified as a good candidate, as it satisfies all of the above requirements.As there are several varieties of rubber foam cord, three of the most widely available options were selected for evaluation: neoprene, silicone, and ethylene propylene diene monomer (EPDM).These were subjected to mechanical tests to assess their suitability as helical e-strip core material.

Tensile test
A tensile test was performed, based on ISO 20932-1:2020+A1:2021 1 .A length of rubber foam cord was clamped in a Shimadzu AG-X Universal Testing Machine, such that the length between clamps was 10mm.The cord was then stretched to 50% extension 5 times, at a rate of 500 mm / minute.The resulting load was measured by the machine throughout the test.The average force at maximum extension was then calculated, and used to compare the cords.3 mm and 5 mm diameter cords of each material were tested.After each test, the sample was removed from the testing machine, and its length was measured at 30 seconds, and again after 30 minutes, to assess recovery after stretching.
Tensile test results are shown in Supplementary Figure 2. Thicker cords of all materials required more force to stretch to 50%, as is expected, because thinner pieces of the same material will be inherently more stretchable.For 5 mm cords, EPDM was the most stretchable, though for 3 mm cords, EPDM and neoprene showed comparable stretch.However, recovery results shown in Supplementary Table 1 show that 3mm neoprene was significantly more deformed than the EPDM and silicone, when measured 30 seconds after the tensile test.All cords, of all diameters, did return to their original length by 30 minutes after the test.But as helical e-strips are designed to be embedded in stretch textiles, quick recovery is important.If a helical e-strip is embedded in a sports garment, the embedded electronics need to conform to the body, and not become temporarily deformed when the athlete kicks or throws a ball, or jumps, for example.
Supplementary Figure 3. tensile testing of rubber foam cords: EPDM, neoprene and silicone rubber foam cords, of both 3 mm and 5 mm diameter, were stretched to 50% elongation, 5 times.The average force at maximum elongation is shown here, averaged over 5 cycles.

Compression test
A compression test based on ISO 7743:2011 2 was carried out.Samples of each cord were placed between circular metal compression plates and compressed by 25% at a rate of 1 mm / minute.This was performed 4 times.Both 3 mm and 5 mm diameter samples were tested for each of the three rubber types.The results are shown in Supplementary Figure 3. Silicone cord of both diameters was the least compressible, and 3 mm EPDM cord was most compressible, with 5 mm EPDM and both diameters of neoprene in between.
Supplementary Figure 4. Compression testing.3 mm and 5 mm diameter EPDM, neoprene and silicone rubber cords were compressed up to 25% at a rate of 1 mm / minute.

Material choice
Other factors were also taken into consideration.The cost of cords was compared, with silicone foam cord being twice as expensive as EPDM and neoprene.All materials are available in medical grade, making them all suitable for medical e-textiles.EPDM and neoprene have high compatibility with

Supplementary Figure 2 .
Fabrication of the helical e-strip: A) Excess material is removed from planar e-strip outlines after the outlines are cut with a vinyl cutter; B) UV mask for photolithography fabricated from heat transfer vinyl, after vinyl cutting the circuit pattern (top) and after removing the excess vinyl with tweezers (bottom); C) Adhesive vinyl mask applied to temperature sensing e-strip outlines; D) Forming the helical structure: the planar e-strip is placed on silicone paper with a printed grid underneath, then cyanoacrylate adhesive is applied, and the core is adhered to the end of the e-strip.The planar e-strip is then rolled along the core, adding further adhesive (not pictured) to form the helical structure.

Table 1 :
Categories of helical e-strips fabricated in this work, with their materials and characteristics

Table 1 .
Recovery properties of rubber foam cords: Cord length was measured 30 seconds after tensile testing, and % deformation calculated relative to cord's initial length.Cords were also measured 30 minutes after testing, to check whether they had returned to their original length.