Context

The global diabetes prevalence in 2019 is estimated to be 9.3% (460 million people) and to reach 10.2% (580 million) by 2030 [1]. Regular blood glucose monitoring is an essential public health issue to manage diabetes and other glucose-related chronic illnesses. The first blood glucose meter (BGM) was marketed in the 90s [2]. BGMs provide an accurate instant glucose level. However, They do not allow monitoring of the glucose level with insulin doses in the correct quantity and time. Continuous Glucose Monitoring (CGM)  appeared in the 2000s. They do not require blood samples. CGM continuously measures the glucose concentration in interstitial fluid (ISF) [3]. CGMs deduce the blood glucose concentration from the ISF concentration, with a pharmacokinetic delay of a few minutes between the two biofluids [4]. Most CGMs have a 7 to 14 days lifetime. The three main CGMs on the market are the FreeStyle Libre™ from Abbott [5], the CGM System™ from Dexcom [6], and the Guardian™ Connect System from Medtronics [7]. Despite the significant advances of recent decades, commercialized CGMs remain painful [8]. Minimally-invasive or non-invasive technologies overcome this issue. The under-development K'Watch by PKvitality, is a minimally-invasive watch-based CGM [9]. Micro-needles measure glucose in the dermal interstitial fluid, which is shallower than the nerve endings. Announced for 2024, the K'Watch could be the world's first painless CGM. However, no completely non-invasive CGMs have been announced for commercialization to date.

Skills & Opportunities

The first skill learned through the tutorial is programming a server-client communication for Bluetooth Low Energy (BLE). BLE has firmly demonstrated its promising potential in the wearable industry [12]. It is the most incorporated technology for the wireless transmission of analyzed data, with near-field communication (NFC) [13]. A DIY data visualization display may reuse this DIY communication solution.

The second skill is electrochemical biosensing with screen-printed electrodes. This tutorial presents a non-invasive technology to measure real-time human glucose levels. It introduces molecular biosensing's basic notions, challenges, and opportunities. Building this biosensor is a first experience of electrochemical on-skin biosensing and wearable technologies.

Requirements

This project involves several prerequisites:

• Basic knowledge in Python programming,
• Basic chemical knowledge,
• A hybrid ec-Flex for open-circuit measurements (OCP) from Zimmer&Peacock (ZP) (orderable here)
• A thin Lithium-Polymer battery from ZP (orderable here)
• A first generation glucose biosensor from ZP (orderable here)
• A sweat patch collector kit from ZP (orderable here)
• A soldering station, welding fume extractor, tin coil and flux.
• A code editor such as Visual Studio Code.

Analytical chemistry notions

Zimmer&Peacock is a developer and manufacturer of electrochemical biosensors. The ec-Flex is a Bluetooth-enabled wearable biosensor platform [14]. It processes and sends the biosensor measurements. This section presents electrochemical notions to understand the functioning of ZP biosensors and the ec-Flex.

Three-electrode system

ZP glucose biosensor is a three-electrode system. It consists of a working electrode (WE), a counter electrode (CE), and a reference electrode (RE). The reference electrode compensates for the potential changes caused by large currents passing through the working and counter electrodes. The ec-Flex has a potentiostat integrated. This analytical instrument measures the working electrode's potential called Open Circuit Potential (OCP) [15].

Screen-printed electrodes

This three-electrode system is screen-printed on a substrate [16]. Screen-printed electrodes are made with thick-film deposition [17]. This process allows simple, rapid, and inexpensive production of biosensors [18]. Materials printed on ZP glucose sensor are silver/silver chloride for the reference and counter electrodes and platinum for the working electrode [19].

Enzyme-based

An enzyme is a molecule that recognizes and reacts with the target analyte [20]. In the case of glucose $(C_{6}H_{12}O_{6})$, this enzyme is glucose oxidase (GOx). GOx molecules are immobilized onto the electrochemical interface. The enzyme catalyzes glucose oxidation. Gluconolactone $(C_{6}H_{10}O_{6})$ and hydrogen peroxide ($H_{2}O_{2}$) are produced [21]. Glucose is quantified by the electrochemical measurement of hydrogen peroxide [22].

Non-enzymatic biosensors use nanomaterials to obtain stability, reproducibility, and simplicity [23].

Amperometry

ZP glucose biosensors are amperometric. The amperometric method can selectively distinguish several electroactive species in solution [24]. It involves a judicious selection of the applied potential and the electrode material. Amperometric biosensors monitor currents. Electrons are exchanged between a biological system (the sweat) and an electrode.

First-generation

This tutorial is for first-generation glucose biosensors. First-generation biosensors measure the concentration of analytes or enzymatic reaction products ($H_{2}O_{2}$ in the case of glucose). In opposition, second-generation biosensors utilize redox mediators. Third-generation biosensors measure direct electron transfer between the redox-active biomolecule and the electrode surface [18].

Tutorial

This project is divided into two parts:

1. Hardware assembly

2. Data acquisition.

1. Hardware Assembly

1.1. Battery

A welding fume extractor, a mask, and thermal protective gloves are highly recommended to weld the battery to the ec-Flex.

Zimmer & Peacock batteries use Polymer Matrix Electrolyte (PME) technology [25]. It allows the battery to be flexible and ultra-thin. It has an 8-hour lifetime, assuming a 500 ms transmission interval from the ec-Flex [26].

• Stick an electrical tape on the back of the ec-Flex to cover the exposed contacts. It avoids any short circuit from those contacts.
• Deposit some flux on the ec-Flex pads. Dispense the solder on the pads with the soldering iron. Place some flux again on the cushions.
• Sold the battery. The positive terminal of the ec-flex is the pad closest to the corner.
• Bend the battery behind the ec-Flex. Once the ec-Flex is disposed of on the skin, the battery is in contact with the skin.

The microfluidic patch ensures dynamic sweat circulation in the SPE sensing area.

• The first layer is the sweat-collecting reservoir. It firmly fixes the patch to the skin. Gently insert the SPE.
• The second layer is the micro-channels. Microchannels conduct the sweat from the collecting reservoir to the micro reservoir layer. Peel the layer and place it on top of the first layer. Remove the thin plastic protection.
• The third layer is the micro-reservoir. It controls the volume of sweat samples in the SPE sensing area. This volume is up to 15 microliters. Place the third layer on top of the second one.
• The fourth layer is the outlet layer. It assists with sweat circulation from the sensing area to the outside of the patch. Place the fourth layer on top of the third one.

• Plug the SPE biosensor into the ec-Flex. The sensitive area must be on the same side as the electronic components of the ec-Flex.

In the get_ecflex_charac.py script

This part aims to establish a Bluetooth client-server connection. The ec-Flex is the server, and the user device is the client.

• Enter the MAC address of the ec-Flex device in line 13. A packet sniffer like Bluetooth LE Explorer allows the MAC address recovery.
• Run the code. The services are printed in the terminal.
• Note down the Vendor service (service 3 - 11661) address in line 14.
• Rerun the code. The handles of Vendor service characteristics are printed in the terminal.
• Recover handles 17, 21, 24, 27, 30, 96, and 99.

In the get_ecflex_data.py script

• Complete the addresses definition block.
• Fill in the access path of the database.db and schema.sql files. Comment not to save data.
• Run the code. An ID, a timer, a temperature, and a glucose concentration value are printed in the terminal.

Conclusion

This tutorial proposes a fast-prototyping solution to build a wearable glucose biosensor. It raises analytes biosensing theoretical notions. This DIY project is a first experience with wearable biosensors for beginners and confirmed makers. However, it has some limitations:

• The battery life limits the period of sense.
• The wearer should remain within the laptop range for the BLE communication.
• The data are available only on the laptop.

The undergoing GitHub project aims to overcome this last limitation. It includes the data redirecting to a homemade smartwatch. The "How to make a smartwatch?" tutorial explains how to build this smartwatch.

The development of wearable devices providing molecular-level information is still in infancy [26]. The potential affordability and accessibility of such technologies raise interest in personalized medicine [27-28]. This tutorial aims to facilitate access to these technologies and arouse learner's interest in their medical and well-being applications. Wearable biosensing devices are the potential next frontier of wearable technologies for fitness and individual and public health monitoring [29].