A Simple and Scalable Fabrication Method for Organic Electronic Devices on Textiles

Summary

In this paper, we present a protocol to selectively deposit organic materials on textiles, which allows for the direct integration of organic electronic devices with wearables. The fabricated devices can be fully integrated in textiles, respecting their mechanical appearance and enabling sensing capabilities.

Abstract

Today, wearable electronics devices combine a large variety of functional, stretchable, and flexible technologies. However, in many cases, these devices cannot be worn under everyday conditions. Therefore, textiles are commonly considered the best substrate to accommodate electronic devices in wearable use. In this paper, we describe how to selectively pattern organic electroactive materials on textiles from a solution in an easy and scalable manner. This versatile deposition technique enables the fabrication of wearable organic electronic devices on clothes.

Introduction

The field of wearable electronics is a fast-growing market expected to be worth 50 billion euros in 2025, over three times the current market. The main challenge facing current wearable devices is that intrusive solid electronic attachments limit the usage of established devices in wearable systems. Using textiles that are already present in everyday life is a very attractive and straightforward approach to avoid this limitation. Due to its elastic capability, some parts of the clothing that we wear are naturally in tight contact with the skin. Many examples of smart clothes available on the market today are based on thin, plastic displays, keyboards, and light source devices embedded in textiles, linking electronics with humans in a fashionable way¹. In sport practice, health monitoring relies on textile electrodes, which offer comfortable alternatives to commonly used adhesive electrodes and metal wristbands. Here, conductive fibers are directly integrated with stretchy fabrics to prevent skin irritation and other discomforts during extended wear. Additionally, textiles offer a number of opportunities to integrate curvature sensors to capture motion², to integrate shear sensors for the development of functional robotic actuators³, and certainly to integrate biosensors through the detection of an analyte in sweat⁴.

Modern wearable technology relies on carbon-based semiconductor materials that deliver electronic devices with unique properties. The "soft" nature of organics offers better mechanical properties for interfacing with the human body compared to traditional solid-state electronics. This mechanical compatibility, paired with mechanically flexible substrates, enables the use of non-planar form factors in devices such as textiles. The use of organics is also relevant in life sciences due to their mixed electronic and ionic conductivity⁵. Besides, organic semiconducting and optoelectronic materials empower a large variety of functional devices with display, transistor, logic, and power capabilities^6,7,8,9. The main difficulty in the fabrication of such organic devices is the controlled deposition of functional materials on the non-planar surfaces of textiles. Conventional microfabrication techniques are primarily limited by the incompatibility of the deposition process with the structural dimensionality of textile substrates.

Here, we describe a simple and scalable fabrication protocol that allows for the selective deposition of conducting polymers on structured textiles. The presented process enables the fabrication of wearable and conformal electronic devices. The approach is based on the patterning of the commercially available conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and an elastomeric stencil material polydimethylsiloxane (PDMS) on textile. This combination allows for the efficient confinement of the aqueous PEDOT:PSS solution, as well as for the retention of the soft and stretchable properties of textiles. This simple and reliable fabrication method paves the way for the fabrication of a variety of electronic devices directly on textiles in a cost-efficient and industrially scalable manner.

Protocol

1. Patterning Conducting Polymers on Textile

1. Fix a 10 cm x 10 cm textile sheet on a planar surface for easy handling during the process. For the textile, use a 100% interlock knit polyester fabric with a thickness of 300 µm and a knit direction stretch capability up to 50%.
2. To make a mask containing the patterning design, use a 125 µm-thick polyimide film; an example of the pattern is illustrated in Figure 1.
   1. Use a laser cutter (e.g., Protolaser S, LPKF) to pattern the polyimide mask¹⁰; the pattern design of an electrode is illustrated in Figure 1.
   2. Coat the PDMS formulation (10:1 base to curing agent ratio) on top of the mask (polyimide film) using an automatic tape casting tool (K control print-coater, doctor blade) with a wet film thickness of 200 µm and at a 6 m/min coating speed. Use about 0.5 mL for a mask of 3 cm x 5 cm. Perform this process under the fume hood.
3. Gently transfer the fabric to the PDMS-coated mask. Leave for 10 min, after which the PDMS should be fully absorbed in the textile structure.
4. Cure the sample in an air-oven at 100 °C for 10 min.
5. Prepare the conducting polymer:PEDOT:PSS dispersion (80 mL), ethylene glycol (20 mL), 4-dodecylbenzenesulfonic acid (40 µL), and 3-methacryloxypropyltrimethoxysilane (1 mL) in the fume hood.
6. Brush-coat the PEDOT:PSS solution on the PDMS-free area of the textile until a homogenous penetration of the solution is obtained. Repeat this step to achieve a uniform pattern color. Apply about 1 mL/cm².
7. Cure the fabric at 110 °C for 1 h to dry the PEDOT:PSS solution. Reduce the temperature to 60 °C for textiles that are sensitive to high-temperature treatment, like nylon.

2. Organic Device Fabrication

NOTE: The protocol in Section 1 describes the selective deposition of conducting materials on textiles. The following sections will describe the additional steps needed to fabricate organic devices, like stretch sensors, OECT transistors, cutaneous electrodes, and capacitive sensors.

1. To fabricate stretch sensors, shown in Figure 3a, pattern the electrode lines on the textile, as described in Section 1, steps 1.1-1.5.
   NOTE: An example of the pattern design is shown in Figure 3a. The fabrication of such sensors does not require any additional steps.
2. To fabricate the transistor design shown in Figure 3b, pattern the transistor arrays on a nylon woven ribbon following the steps described in Section 1. Slightly modify the PDMS annealing and PEDOT:PSS curing steps to avoid the thermal degradation of nylon by curing at 60 °C for a longer time.
3. For the fabrication of cutaneous electrodes, shown in Figure 3c, deposit an ionic gel on the patterned PEDOT:PSS textiles.
   1. Prepare an ionic liquid gel mixture containing the ionic liquid, 1-ethyl-3-methylimidazolium-ethyl sulfate; the cross-linking agent, poly(ethylene glycol)diacrylate; and the photoinitiator, 2-hydroxy-2-methylpropiophenone at a (v/v) ratio of 0.6/0.35/0.05, respectively.
   2. Coat the PEDOT:PSS electrode with ionic liquid (20 µL/cm²) and add the ionic liquid gel mixture from step 2.3.1 (25 µL/cm²) by drop casting.
   3. Expose to UV light (365 nm) to initiate a crosslinking reaction for 10-15 min, until the gel solidifies. Perform this step in the fume hood. Use a UV-protective cage during UV exposure.
4. For capacitive sensor fabrication, use PEDOT:PSS textile electrodes insulated with an insulating material (Figure 3d).
   1. Insulate the keyboard-like PEDOT:PSS electrodes using the PDMS; the keyboard design can be seen in Figure 2b. Dispense the PDMS formulation on top of the fabric and remove the excess with a squeegee.
   2. Place the fabric in an oven at 100 °C for 10 min. Perform this step in the fume hood.

Results

Traditional methods for applying colors or patterns to textiles rely on removable masking layers to allow the selective deposition of dyes. In Figure 1, we show the adaptation of such an approach to the patterning of PEDOT:PSS electrodes on textiles. As a masking layer, we used hydrophobic polydimethylsiloxane, which can restrain the non-controllable diffusion of the aqueous PEDOT:PSS solution. Moreover, the softness and stretchability of knitted and woven textiles can be...

Discussion

The patterning of conducting materials is one of the first steps in the fabrication of functional electronic devices. This can become challenging, as the fabrication process needs to take into account the chemical and physical properties of such materials, and the process flow needs to consider the material cross-compatibility between the fabrication steps. In the microfabrication of organic electronic devices, these two aspects are even more significant due to the highly reactive nature of organics. Today, however, orga...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
SYLGARD 184, Silicone elastomer kit (Base and Curing agent)	Dow Corning		PDMS elastomer
The conducting polymer formulation
CleviosTM PH 1000 PEDOT:PSS	Heraeus		Conductive polymer
Ethylene glycol	Sigma-Aldrich	03750-250ML	Solvent (EG), CAS: 107-21-1
3-methacryloxypropyltrimethoxysilane	Sigma-Aldrich	M6514	Cros linker (GOPs), CAS: 2530-85-0
4-dodecylbenzenesulfonic acid	Sigma-Aldrich	44198	DBSA; CAS: 121-65-3
The ionic liquid gel
UV lamp DFE 2340	C.I.F/ ATHELEC	DP134	UV-365 nm
1-Ethyl-3-methylimidazolium ethyl sulfate	Sigma-Aldrich	51682-100G-F	Ionic Liquid (IL), CAS: 342573-75-5
Poly(ethylene glycol) diacrylate	Sigma-Aldrich	455008-100ML	Mn 700, CAS: 26570-48-9
2-Hydroxy-2-methylpropiophenon	Sigma-Aldrich	405655-50ML	Phot Initiator (PI), CAS: 7473-98-5
The textile fabric	VWR	Spec-Wipe 7 Wipers	100% interlock knit polyester fabric
The polyimide film	DuPont	HN100	Polyimide film with 125 µm thickness