Wearable Sensors for Chemical & Biological Detection

One of the strengths of the Pacific Northwest National Laboratory (PNNL) is the ability to conduct comprehensive technology foraging and objective assessments of various technology areas. This article highlights leading research by others in the area of chemical and biological (chem/bio) detection that could be further developed into robust, highly integrated wearables to aid preparedness, response, and recovery.

The current wearables market is approximately $5 billion and projected to grow to over $15 billion in a couple years, and to $50 billion by 2023. Fitness and sports wearables that monitor various physiological and biomechanical parameters comprise the bulk of units currently sold. Health care wearables offer improved monitoring of at-risk patients with inherent overall medical cost savings and are rapidly growing in capability and utility. Wearables can also monitor farm animals, high-value animals (e.g., zoo animals, racehorses), and even pets.

Although most consumer products measure only a subset of the following parameters, more specialized wearables, including those used in health care (which tend to be much larger), may include:

• Heart rate
• Skin temperature (core body temperature is still under development)
• Breathing rate
• Ultraviolet (UV) light exposure
• Blood pressure
• Electrocardiogram (ECG) – heart
• Electroencephalogram (EEG) – brain
• Electromyography (EMG) – muscle
• Acoustic (coughing, wheezing, heart sounds)
• Blood oxygenation

Additionally, using advanced algorithms, “composite” sensor measurements can provide estimates of activity/mobility/falls (using three-axis accelerometers), distance traveled, calories burned, sleep quality, stress/exertion/fatigue, among many others.

Although the most common wearable format is a smartwatch, other forms are being developed including glasses, chest straps, skin “tattoo” sensors, jewelry, earrings, clothing/textiles, and even implantables (e.g., for glucose monitoring). The wearables discussed above are mostly “inward looking” sensors (i.e., self-monitoring). Not traditionally considered a “wearable,” dosimeters or utility-belt-worn devices (“outward looking” or environmental sensors) can offer valuable information for improved safety and health, particularly for emergency responders. For example, people with asthma or other respiratory ailments can currently wear small, real-time respirable particulate monitors. Miniaturized analytical instruments and dosimeters are also available for measuring various chemical species and biological agents.

Essential Developments to Enable Chem/Bio Wearables

The above parameters and composite measurements for inward-looking wearables are based on physiological and biomechanical measurements and often suffer inaccuracies. Eight enabling technology areas summarized below are essential to developing future advanced wearables.

• Miniaturization – Miniaturized instruments (e.g., Raman, Fourier transform infrared spectroscopy [FTIR], mass spectrometry), dosimeters, airborne particulate/biological monitors, and microfabrication/microfluidic platforms allow multiple complex measurements and operations to be conducted in very small form factors, including polymerase chain reaction.
• Biomarkers of disease/exposure – Many prodromal indicators of exposure to harmful chemical or biological agents are known, but these are often associated with several possible causative agents or conditions. Biomarker suites are likely to improve the identification of specific causative agents, and this is yet another challenging but necessary area requiring ongoing research.
• Nanomaterials – Nanomaterials allow faster, more rapid and sensitive detection in very small sizes.
• Sensors – Implantable sensors are the ultimate in ease-of-use, but they have limited ability to measure chem/bio species and have challenges associated with long-term accuracy. Stable and reversible chem/bio transducers are a particularly challenging area requiring further development.
• Robust/flexible electrical systems – Numerous researchers have demonstrated various approaches to designing and producing stretchable tattoo sensors, including important advances by Joseph Wang (University of California, San Diego), John Rogers (University of Illinois, Urbana-Champaign), and others. Wang recently developed wearable glasses that incorporate sensors in the nose pads to measure sweat electrolytes and metabolites. Rogers’ “Biostamp” includes an impressive array of transistors, diodes, capacitors, inductors, oscillators, temperature sensors, strain gauges, light emitting diodes, together with an inductive coil and antenna that can serve as a platform for various sensors.
• Transdermal biological fluid extraction – Current research is rapidly advancing the suite of chem/bio parameters measurable in sweat, interstitial fluid, and blood using sweat inducer/collectors and microneedle arrays. Jason Heikenfeld and others at the University of Cincinnati Novel Devices Lab recently reviewed the field of wearable sweat sensors.
• Microscale power/storage – This continues to be a challenge for long-term wearables operation, but incremental progress continues, including the use of energy harvesters from movement, light, and heat.
• Communications – Data transmission, storage, management, analytics, security, and use of the cloud present an area where continued essential developments will enable effective chem/bio wearables.

Notable Chem/Bio Wearables

Hyunjae Lee et al. at the Korea Center for Nanoparticle Research demonstrated a highly integrated wearable system on diabetic mice that includes a graphene-based electrochemical device for glucose monitoring and a thermally activated polymeric microneedle array for sampling interstitial fluid and administering drugs. Dongyang Cai et al. at the State Key Laboratory in Beijing, China, developed an integrated microfluidic device that uses dielectrophoresis to extract up to 20 different pathogens from blood followed by 4-channel polymerase chain reaction for identification in nanoliter volumes. Wei Gao et al. at the Department of Electrical Engineering and Computer Science at the University of California Berkeley demonstrated an unprecedented degree of integration in a multiplexed sweat sensor that incorporates complex signal conditioning on a flexible printed circuit board combined with a skin-interfaced flexible sensor array for monitoring hydration status in real-time. The sensors measure metabolites (glucose and lactate) and electrolytes (sodium and potassium ions) and use a skin temperature sensor to improve sensor accuracy. Results are wirelessly transmitted to a smartphone. The wrist-worn system was shown to enable the monitoring of hydration status on humans engaged in prolonged indoor and outdoor physical activities.

Future Outlook

Wearable devices show great promise for improving the health, safety, and effectiveness of emergency responders, but they require ongoing research in numerous areas. Key enabling technology areas include: (a) biomarker panels of disease or exposure; (b) stable/reversible chem/bio target receptors; and (c) nanomaterials for faster, smaller, and more sensitive detection. As with the evolution of the cellphone, wearables are expected to improve in capability, usability, and affordability as enabling technology allows higher degrees of integration in ever-smaller form factors.

About Pacific Northwest National Laboratory (PNNL): Interdisciplinary teams at PNNL address many of the United States’ most pressing issues in energy, the environment, and national security through advances in basic and applied science. Founded in 1965, PNNL has a team of 4,400 staff and an annual budget of nearly $1 billion. It is managed by Battelle for the U.S. Department of Energy’s Office of Science. As the single largest supporter of basic physical science research in the United States, the Office of Science is working to address some of the most pressing challenges of the time.