Chem-Phys Patch Uses Multiple Sensors on Wearable Tech

Engineers at the University of California San Diego are developing a wearable, Bluetooth-enabled sensor patch that can track electrical signals as well as biochemical signals: the Chem-Phys patch.

Nanoengineering and electrical engineering have joined forces at the Jacobs School of Engineering at UCSD. A team comprised of engineers from both disciplines are working within the school’s Center for Wearable Sensors to make strides in wearable tech.
Their most recent project presented is the Chem-Phys patch.

The Chem-Phys patch, which printed sensors and custom PCB. Image courtesy of UCSD.

The project is led by Professor Joseph Wang, Chair of Nanoengineering, and Professor Patrick Mercier, assistant professor of Electrical and Computer Engineering. The two co-direct the Center for Wearable Sensors.
Prof. Wang’s nanotech-focused team was tasked with developing the sensor and dealing with the chemistry involved in the patch. Prof. Mercier’s electrical engineering team’s responsibilities included figuring out the patch’s electronic components and data processing and transmission.

The Chem and the Phys: A Dual-Purpose Wearable Sensor

The patch, dubbed the Chem-Phys, is capable of measuring both biochemical and electrical data from its wearer.
Most sensor patches focus on recording one type of data. The Chem-Phys also tracks EKG, but it combines that tracking with the measurement of chemical signals. In particular, the patch tracks levels of lactate, a biochemical used to track physical exertion.

Image courtesy of USCD.

With this combination of measurements comes the challenge of managing two different kinds of sensors in a very small space without interfering with one another.

Chem-Phys Fabrication and Specs

The patch is printed in batches on transparent polyester substrate sheets. The sensors were then screenprinted onto the sheets via custom-designed stencils.

The Chem-Phys patch is printed in batches onto flexible polyester sheets. Images courtesy of Nature Communications.

The patch is interfaced with a custom 4-layer PCB, designed by Mercier and his team.
The board is equipped with a TI CC2541 (PDF) BLE System-on-Chip that collects and transmits the sensor data to a Bluetooth 4.0-enabled receiver. Transmission is aided by a 2.45 GHz chip antenna (2450AT42A100) (PDF) and an impedence-matched balun (2450BM15A0002) (PDF), both produced by Johanson Technology. The end result is that the patch is capable of syncing with a smartphone, laptop, or smartwatch via Bluetooth.
Also included is an ADS1293 (PDF) AFE chip, used to record the EKG readings. An accompanying LMP91000 (PDF) AFE potentiostat is used to sense lactate concentration.
For power, the patch uses a CR2032 (PDF) button cell lithium battery, which provides 3V, 220 mAh. Regulating the battery is a TPS61220 (PDF) boost converter.

Wireless readout circuit block diagram. Image courtesy of Nature Communications.

Sweat and Silicone

One of the challenges of designing the patch is that the lactate sensor needs to apply a small voltage in order to measure current across its electrodes. The lactate sensor is positioned right in the middle of the two EKG sensors, so the engineers were concerned that this voltage could disrupt EKG measurements.
When tested in the lab, the patch was applied to individuals who were asked to undergo physical exertion (in order to best ascertain whether the lactate and EKG sensors were measuring accurately). The difficulty came with the sweat resulting from that exertion. The sweat acted as a conductor of the lactate sensor’s voltage, creating a real-world scenario in which the patch’s sensors could interfere with one another.
To counter this issue, the team printed a layer of silicone rubber to repel the sweat away from the EKG sensors but not the lactate sensor.

Applications

The patch is naturally of interest to athletes the world over. The ability to track their fatigue and heart stats while exercising could open many doors when it comes to sports medicine and physical conditioning.

Image courtesy of UCSD.

However, these same measurements are also of interest to cardiologists, who could use the patch to help identify and manage risks in patients with cardiovascular disease.
In the future, the team hopes to add more chemical tracking capabilities, such as monitoring potassium or magnesium levels. If the team successfully integrates additional biochemical measuring capabilities, the Chem-Phys could become useful in the management of many more ailments.

Read the team's research paper here.