A disposable, wearable, flexible, stitched textile electrochemical biosensing platform

Highlights

• A cheap and disposable wearable platform has been developed.

• This will allow the electrochemical detection of biomarkers on or lost from the body.

• The devices are exceptionally cheap, stable, reproducible, reliable, scalable and flexible.

• As proof of principle a sweat based glucose sensor has been developed.

• The sensors have a sensitivity of 126 ± 14 nA/mM of glucose and a limit of detection of 301 ± 2 pM.

Abstract

Wearable sensors are a fast growing and exciting research area, the success of smart watches are a great example of the utility and demand for wearable sensing systems. The current state of the art routinely uses expensive and bulky equipment designed for long term use. There is a need for cheap and disposable wearable sensors to make single use measurements, primarily in the area of biomarker detection. Herein we report the ability to make cheap (0.22 USD/sensor), disposable, wearable sensors by stitching conductive gold coated threads into fabrics. These threads are easily functionalised with thiolate self-assembled monolayers which can be designed for the detection of a broad range of different biomarkers. This all textile sensing platform is ideally suited to be scaled up and has the added advantage of being stretchable with insignificant effect on the electrochemistry of the devices. As a proof of principle, the devices have been functionalised with a continuous glucose sensing system which was able to detect glucose in human sweat across the clinically relevant range (0.1–0.6 mM). The sensors have a sensitivity of 126 ± 14 nA/mM of glucose and a limit of detection of 301 ± 2 nM. This makes them ideally suited for biomarker detection in point-of-care sensing applications.

Introduction

The field of wearable diagnostics has been growing in recent years (Bandodkar et al., 2019; Brothers et al., 2019; Chung et al., 2019; Gao et al., 2016; Lee et al., 2018; Matzeu et al., 2015). The ability to incorporate sensing technology into wearable devices to detect specific body movements, heart rate and body temperature has obvious benefits for the medical and sportswear industries; where products that fulfil this need have already been commercialized using conventional electronics (Germer et al., 2013; Xu et al., 2019). Notably, the success of the Apple watch and Fitbit® prove that there is a consumer demand for wearable sensing devices. To date however, most of these devices are rigid, brittle and expensive; so need to be incorporated into wearable watches or jewellery. Furthermore, they are unable to detect clinically relevant biomarkers. Whilst there have been some advances in the field of soft and flexible electronics for wearable sensing applications (Jeerapan and Poorahong, 2020), they still lack the breathability and ease of integration that a complete textile diagnostic sensing platform would offer (Keren et al., 2016; Munje et al., 2017). Textile based diagnostics have the advantage of not only being flexible, breathable and wearable; but can be easily incorporated into existing clothing manufacturing processes and can be produced on an industrial scale using existing technologies and infrastructure, which makes them cheap and scalable (Choudhary et al., 2015; Lund et al., 2018; Nilghaz et al., 2013; Oberg Mansson et al., 2020). As such, they can be used to realise disposable and single use wearable sensing modalities, which are prohibited by the high cost of smart watches and rings. Examples of disposable textile biosensing applications include, but are not limited to, testing for urinary tract infections in diapers (Reches et al., 2010) or infection in bandages (Derakhshandeh et al., 2018). Another useful application of textile sensors is in the field of glucose detection. Current technology requires diabetics to take regular blood samples to obtain blood glucose measurements using a glucometer. This is both invasive and gives a periodic sampling profile. It would be preferable to have a non-invasive continuous sensing platform capable of detecting glucose in the patients' sweat; since blood glucose levels and sweat glucose levels have been found to correlate (Gao et al., 2016; Lee et al., 2018; Moyer et al., 2012). Textiles offer the perfect platform for such devices as they can be integrated into clothing and mass manufacturing processes; and naturally come into good contact with the patients’ body and sweat. Such devices also have potential uses in the sportswear industry (Lee et al., 2017; Munje et al., 2017).

Much has been done on stress and movement sensors in the field of electrochemical textile sensors. However, non-invasive biomarker detection in excreted bodily fluids such as sweat (Katchman et al., 2018), urine (Liao et al., 2006), tears (Willcox, 2019), faeces (Pang et al., 2014), blood (Piper et al., 2018; Tsimikas et al., 2006) and milk (Alsaweed et al., 2015; Murphy et al., 2016) using electrochemical textile based biosensors is not widely reported. The current state of the art in flexible wearable sensors is primarily focussed on flexible polymer substrates with sensing elements integrated onto them (Jeerapan and Poorahong, 2020). These devices lack the breathability and ease of incorporation that an all textile sensor would offer, often needing adhesives to stick the patches onto the skin (Jeerapan and Poorahong, 2020). Alternatively screen printing electronics directly onto fabrics has been employed by Jeerapan et al., (2016), although this method would also reduce the breathability of the fabric at the sensor area. The current state of the art using conductive threads mostly involve the dip coating of threads with carbon pastes (Liu and Lillehoj, 2017) or elaborate methods of Au nanowire spin coating and electrodeposition of Au (Zhao et al., 2019); neither of which have been established as commercially viable. In the initial trails of this paper dip coated threads were tested but were found to crack and/or wick slowly towards the connection through the paste or inner cavities of a dip coated multifilament, leading to temporal instability of the signal and device failure over time.

Here we report for the first time the ability to stitch and functionalise a fully functioning electrochemical system into fabrics to create a cheap, disposable, stretchable and functional diagnostic platform, using commercially available Au plasma coated threads. In the devices reported here, the connection points of the threads can be quickly and easily separated from the electrode working area and any wicking to the connections is blocked. To validate the platform, it has been functionalised to make a label free continuous glucose sensor capable of detecting glucose in human sweat across a clinically relevant range. This involved functionalising the Au thread working electrodes with a thiolate self-assembled monolayer (SAM) compromising a glucose oxidase enzyme and a redox tagged thiol (6-Ferrocenylhexanethiol) which could be used to electrochemically regenerate the enzyme in situ and achieve a truly continuous glucose monitoring system by detecting the H₂O₂ produced by the enzymatic oxidation of glucose. Thiolate SAMs are perhaps the most widely used method of electrochemical sensor fabrication, to the best of the authors knowledge this is the first all textile device that can be functionalised with SAMs. This allows for their rapid integration with established SAM based biosensing methods to sense a broad range of biomarkers.