*Proceeding Paper* **Miniature Flexible Reprogrammable Microcontroller Circuits for E-Textiles †**

**Tom Greig 1,\*, Kai Yang <sup>2</sup> and Russel Torah <sup>1</sup>**


**Abstract:** An e-textile system was developed, allowing USB reprogramming of miniature, flexible, integrated microcontroller circuits which allows for easier development of complex and configurable e-textile circuits. This prototype consisted of a series of five exposed pads on the edge of the PCB and a corresponding clip connector. Mounted onto the clip are a micro-USB port and necessary additional components to facilitate USB programming meaning that no additional components are required on the microcontroller board thus increasing flexibility. This system has the potential to make software development and reconfiguration of the e-textile easier while the small size and flexibility of the connector allow improved textile integration. This work provides a platform for future e-textile system development and increases the operational lifetime, thus reducing waste due to product obsolescence.

**Keywords:** e-textile; embedded microcontroller; e-waste; sustainability; flexible electronics

#### **1. Introduction**

Microcontrollers are vital components in many e-textile devices [1,2]. Their re-programmability and wide range of peripheral functions means that they can fulfil the digital processing requirements of almost any small electronic product while their small size makes them possible to include in e-textile devices.

The ability to be re-programmed is key to a microcontroller's utility, and most provide some means of uploading new programs while in situ, for example, AVRs' SPI-based "ICSP" protocol [3]. However, such systems typically only work on one brand of microcontroller and require specialised programming circuits to use. The connectors required to use these systems also occupy a large area: the pin header needed to connect the ATMEL ICE programmer to a QFN ATtiny occupies 4 times the area and 12 times the height of the chip itself, see Figure 1.

**Figure 1.** A standard 2.54 mm, 6-pin programming header (**left**) compared to the ATTiny85 microcontroller it programs (**right**).

**Citation:** Greig, T.; Yang, K.; Torah, R. Miniature Flexible Reprogrammable Microcontroller Circuits for E-Textiles. *Eng. Proc.* **2023**, *30*, 15. https://doi.org/10.3390/ engproc2023030015

Academic Editor: Steve Beeby

Published: 2 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The first problem can be solved using a USB bootloader. This is a small program which is used to load new software via a USB connection.

However, even micro-USB connectors are relatively large components in the context of e-textiles and restrict the textile integration of a microcontroller circuit. A potential solution to this is to use an edge connector. These consist of a series of exposed pads near the edge of the circuit board. The board itself is then inserted into a receptacle with contacts arranged to connect with the pads. However, existing solutions are for typically thicker (0.3 mm) flat flexible cables (FFC), therefore connections can be difficult and unreliable.

#### **2. Design**

The system developed here uses five, 1 mm pitch, pads which need only protrude a few millimetres from the body of a flexible circuit board connect to an external clip. The five connections are used for power, ground, positive and inverted USB data and a button-operated reset line which prompts the microcontroller to run its bootloader. The clip contains all the additional components needed for USB programming: the micro-USB connector, a 3.3 V voltage regulator, a reset button and several passive components. A diagram of this system is shown in Figure 2.

**Figure 2.** Design of the microcontroller and programmer. Placing the components needed for USB on the programmer clip means that only the microcontroller's IC needs to be integrated into the textile.

The contacts on both sides are tinned with solder to prevent corrosion This system was tested using an ATTiny85 microcontroller [3] with the micronucleus bootloader [4]. Both the flexible microcontroller circuit board and the programming clip's PCB were made using a standard photolithographic etching process described previously in [5].

The contacts on both sides are tinned with a thin layer of solder to prevent corrosion and to raise the contact point slightly, making the connection more reliable.

#### **3. Applications**

The test implementation was incorporated into both woven (Figure 3, top left) and stretchable, knitted fabrics (Figure 3, right) by couching the conductive thread soldered to the general purpose input/output (GPIO) pins for the controller. Another version was made by inserting the device and its connecting wires into woven pockets in a custom-made fabric (Figure 3, bottom left).

Because of its small size, the impact on the flexibility and stretchability of the fabric is minor. This is a major improvement over existing prototyping boards designed for e-textile which are much larger and used rigid PCBs (Figure 4).

**Figure 3.** Microcontroller circuit integrated into the collar of a garment (**top left**), a stretchable knitted fabric (**right**), and a custom woven textile (**bottom left**).

**Figure 4.** Existing prototyping boards designed for e-textiles, left to right: an Arduino Lilypad (diameter 50 mm [6]), an Adafruit Gemma (diameter 28 mm [7]) and this work (10 × 12.5 mm).

This programming system can easily be adapted to other microcontrollers which are reprogrammable via USB.

#### **4. Conclusions**

This work presents an easy-to-use programmer, compatible with many different types of microcontrollers. It occupies significantly less circuit board space than existing commercial equivalents, the prototype displayed here is 80% smaller than an Adafruit Gemma and requires no additional components on the microcontroller board itself.

This is important in e-textile applications where a large size or additional rigid components can compromise the textile's properties of comfort and flexibility.

The small connector size and minor impact on integration mean that, when moving beyond the prototyping stage, the connector footprint may not need to be removed, and if it is, only a small change to the circuit layout is needed. The initial demonstrators also show that this methodology of flexible microcontroller integration and flexible connection point works with both couching into existing garment structures and integration during weaving. **Author Contributions:** Conceptualization, investigation, writing—original draft preparation, T.G.; supervision, writing—review and editing, K.Y. and R.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was part-funded by EPSRC grant number EP/S001654/1, "Advanced etextiles for wearable therapeutics" and part-funded by the EU H2020 programme, grant number WEARPLEX—825339—wearplex.soton.ac.uk.

**Data Availability Statement:** No new data was created or analysed in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

## *Proceeding Paper* **Design and Test of E-Textiles for Stroke Rehabilitation †**

**Meijing Liu 1, Tyler Ward 1, Odina Keim 2, Yuanyuan Yin 2, Paul Taylor 3, John Tudor <sup>1</sup> and Kai Yang 2,\***


**Abstract:** This work presents the design and test of an e-textile based functional electrical stimulation system for post-stroke upper limb rehabilitation. The prototype was tested on five stroke survivors to assess stimulation comfort, the stimulation intensity required to achieve hand opening, and ease of use. Wrist extension was measured using two inertial measurement units. The wearable e-textile prototype achieved similar stimulation comfort compared to high-quality hydrogel electrodes with a score difference of between 0 and 1. The stimulation intensity to achieve full hand opening was the same for the hydrogel electrodes and the e-textiles for all five participants. A second design based on a knitted sleeve has been assessed in terms of usability. Additional new designs have been proposed to improve the usability.

**Keywords:** electrode; functional electrical stimulation (FES); inertial measurement unit (IMU) stroke rehabilitation; e-textiles; healthcare

#### **1. Introduction**

Stroke occurs when there is a blockage or bleeding of the blood vessels affecting the supply of blood to the brain. There are 1.3 million stroke survivors in the UK [1] and stroke costs the UK National Health Service and wider society £26 billion per annum [2]. Over half of stroke survivors have weak arm/hand movement affecting their independence and quality of life. Functional Electrical Stimulation (FES) is a technology used for stroke rehabilitation. It applies a safe electrical impulse through electrodes placed on the skin to strengthen weak muscles and improve movement functions. FES has been used to exercise muscles and assist walking for people with mobility issues since the 1960s [3]. Systematic reviews with meta-analysis have concluded that FES improves the ability to perform activities [4–6]. Existing FES products are difficult to set-up by stroke survivors without help from their carers or healthcare professionals which significantly constrains usage. Our previous work has received positive feedback regarding wearable e-textile FES for home-based stroke rehabilitation [7]. This work presents the test results of a fabric electrode based wearable FES in terms of user comfort, stimulation intensity, and functional movement (wrist extension for hand opening) on five stroke survivors (ethics approval ID: University of Southampton ERGO 70296). Ease of use has been assessed. A second knitted design has been assessed and additional new designs to improve the usability have been proposed for future study.

#### **2. Materials and Methods**

#### *2.1. Electrodes*

Wearable electrodes (5 cm × 5 cm) were fabricated by stacking in turn: a non-woven fabric, conductive wires leading to a connector, a conductive carbon film and a carbon rubber electrode layer. Encapsulating the edges holds the entire assembly together. The

**Citation:** Liu, M.; Ward, T.; Keim, O.; Yin, Y.; Taylor, P.; Tudor, J.; Yang, K. Design and Test of E-Textiles for Stroke Rehabilitation. *Eng. Proc.* **2023**, *30*, 16. https://doi.org/10.3390/ engproc2023030016

Academic Editors: Steve Beeby, Russel Torah and Theodore Hughes-Riley

Published: 6 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

electrode was attached to a fabric arm band as shown in Figure 1. The wearable electrodes were compared with commercial high quality hydrogel electrodes (PALS, Axelgaard, Fallbrook, CA, USA).

**Figure 1.** (**a**) fabric electrode arm band. (**b**) fabric electrode arm bands being worn by a stroke survivor. (**c**) hydrogel electrodes being worn by a stroke survivor.

#### *2.2. FES*

The OML Microstim 2V2 neuromuscular stimulator (Figure 2) was used in this study. Stimulation was set-up by a clinician to optimise the electrode positions and stimulation intensity. Water was sprayed on the electrodes and the skin before applying the fabric electrode to improve user comfort and stimulation effectiveness. Stimulation comfort was rated in a scale from 0 to 10 with 0 is the most comfortable and 10 being very painful. The stimulation intensity was recorded.

**Figure 2.** OML Microstim 2V2.

#### *2.3. Inertial Measurement Unit (IMU) Sensors for Wrist Bending Measurement*

The wrist angle was measured using a pair of BNO055 IMU sensor breakouts from Adafruit (Figure 3) which reports the absolute angular position of the sensor. The IMUs were attached to the user by using Velcro straps with one attached to the hand and the other to the arm. An Arduino Micro was used to gather the data from the sensors and sent it to the computer recording the data. The computer then calculated the wrist angle by taking the difference between the two absolute positions from the sensors.

**Figure 3.** BNO055 IMU sensor.

#### *2.4. Usability Test*

Participants were asked to put on and take off both the hydrogel electrodes and fabric electrode arm bands to assess their capability of using the products independently.

#### **3. Test Results and Discussion**

Five participants were recruited for the testing. Age: 56–76, Years of stroke: 3–17 years. Genders: 1 female and 4 males.

#### *3.1. Stimulation Comfort*

All participants reported the same or similar stimulation comfort with only 0 to 1 score difference between the two electrode types as shown in Table 1. One participant (P1) reported the fabric electrode was more comfortable than the hydrogel electrodes. One participant (P4) reported the same comfort scale. The other three reported the hydrogel electrodes were more comfortable than fabric electrodes. No pain sensation was reported.


**Table 1.** Stimulation comfort scale results by participant.

#### *3.2. Stimulation Intensity*

There was no difference in the stimulation level required to achieve full hand opening between the two types of electrodes (Table 2). The required stimulation levels vary from 40 mA to 50 mA. This indicates the fabric electrodes were as effective as the hydrogel electrodes in generating a functional movement.


**Table 2.** Stimulation intensity required to achieve hand opening.

#### *3.3. Wrist Bending Measurement*

Wrist movement was measured during the stimulation. All participants achieved a similar movement for the two types of electrodes. Figure 4 is a representative example of the wrist bending angle for the two electrode types.

#### *3.4. Usability*

Researchers observed that it was a challenging task for participants to peel off the hydrogel electrodes from the protective plastic film because the hydrogel electrodes are very sticky. It was even more challenging for them to take out the electrodes from the sealed bag and put them back after use. With the electrode arm band, although all participants were able to put it on and take it off independently, they found it challenging to keep the electrode in place because it moved around before it was fastened and secured in place. In

addition, it was challenging to put the Velcro hook fastener through a loop. All participants were able to spray water on the electrodes and the arm using a single hand.

**Figure 4.** Wrist bending angle.

#### *3.5. Second Design: Knitted Sleeve*

Two pairs of electrodes were printed on a knitted fabric made of wool and Lycra yarns to stimulate muscles for both hand extension and flexion (Figure 5). The electrode fabric was assembled to form a pull-on sleeve. It was noticed the electrode sleeve was difficult to put on or take off because of the strong friction between the electrodes inside the sleeve and the skin. Discussions with the stroke survivors have indicated that an open, or partially open structure, would allow the user to put on the electrode sleeve easily, then tighten the sleeve to ensure the electrodes and skin contact sufficiently.

**Figure 5.** (**a**) electrodes printed on knitted fabric. (**b**) electrode fabric with snap buttons. (**c**) electrode sleeve worn on an arm and connected to a stimulator via snap buttons.

#### **4. Conclusions and Future Work**

This work has demonstrated the stimulation comfort and functional movement delivered by a wearable e-textile activated with the OML FES stimulator Microstim 2V2. The fabric electrode achieved similar stimulation performance while providing the advantage of being suitable for wearable application and offering a long service life. All participants were able to put on and take off the fabric electrode arm bands independently, but it is time consuming and a better design is required. A new design with electrodes printed on a knitted stretchable sleeve was investigated but it was difficult to put on and take off due to the friction between the electrodes inside the sleeve and the skin. New designs, required to improve the ease of use, will be addressed in future work.

**Author Contributions:** Conceptualization, M.L., J.T. and K.Y.; methodology, M.L., T.W., O.K., P.T. and K.Y.; software, T.W.; validation, M.L., Y.Y., P.T. and K.Y.; formal analysis, T.W. and K.Y.; investigation, M.L. and K.Y.; resources, P.T. and K.Y; writing—original draft preparation, M.L. and K.Y.; review and editing, All; funding acquisition, J.T. and K.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the SIAH HEIF Research Stimulus Fund at the University of Southampton and the Medical Research Council (MRC) under grant number MR/N027841/1.

**Institutional Review Board Statement:** The study was approved by the Faculty of Arts and Humanities Ethics Committee at the University of Southampton on 21/02/2022 with a submission ID of ERGO 70296.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** No new date was created.

**Acknowledgments:** The authors want to thank the stroke survivors at Different Strokes Southampton for attending the focus group studies and prototype testing.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
