Lactate, for example, is a potentially important biomarker
present in sweat, as it can indicate the onset of fatigue of an
individual and has been indicated as a marker for all-cause mortality in hospital patients. Progress has also been made toward
developing sweat-based glucose monitoring, with potential for
a combination transdermal drug delivery system for multiple
medications. Skin-based sensors can also test for the presence of
other metabolites, such as electrolytes, calcium, and heavy metal ions in sweat. Collaboration between biomedical engineers
and material scientists has led to the creation of small wearable
flexible sensors that are able to read multiple biomarkers in the
molecular composition of sweat and transmit that data to a
smart phone for real time analysis and tracking.
Both technology and manufacturing readiness level assessments for biomarker sensors are important to identify gaps
and risks for their development. Many challenges exist in the
development of flexible sensors capable of detecting biomarkers
at concentration levels expected in sweat or other bodily fluids.
Biomarkers exist in smaller concentrations in sweat and saliva
than in blood samples. The time to collect a sample of adequate
volume and concentration is highly individualized depending
on output, ambient temperatures, and other environmental factors. Providing consistent selectivity and sensitivity of readings
of a complex fluid at an affordable cost for wearable and disposable devices are significant obstacles for sensor developers and
manufacturers to overcome. Biofluid samples can be constantly
refreshed in a lab. Although advancements in microfluidics have
been promising, real world test subjects have demonstrated
them to be insufficient to replenish sweat at a sufficient rate to
maintain an equilibrium. One example is in a sweat monitoring use case where sensor selectivity, sensitivity, stability, and
reliability are critical.
Disposable biomarker sensors are typically functionalized
using chemical compounds that act as receptors for specific
biomarkers, which then turn that chemical reception into a
variable resistive load on the sensor. A number of intriguing
biomarker sensor technologies developed with the intention of
the sensors eventually being incorporated into a wearable, disposable device have been demonstrated successfully in labs, but
they will require further development to achieve desired performance metrics for demanding wearable products. Sensors must
be even more sensitive and stable with lower concentrations of
samples in the interest of achieving the application’s goals.
Sensor Demonstration Platforms
In the meantime, Molex has developed various thin and flexible
sensor demonstration platforms that can interface with a
variety of disposable sensors. The sensor platforms are typically custom configured to make a signal interpretable within
the given sensor responsiveness and application requirements.
Sensors in these demonstration platforms typically utilize a
higher concentration than will be achievable in the ultimate
wearable sensor devices to assist in showing the promise of the
technology while also acknowledging its gaps. Reception of the
biomarker compound effectively turns into identifiable load on
the sensor, which is then typically amplified and digitized and
communicated to a processor to perform logic computations
and ultimately communicate to the user via an indicator display
or other HMI (e. g. mobile app).
Commercial success of a device depends on sensor system
form factor and performance, in addition to manufacturability
and ultimately, cost. The reliability of electronics integration
is ahead of the commercialization curve relative to biomarker
sensor technologies. A range of sensor platforms can be used
to model biomarker signals on prototypes and for conducting
sub-system functional testing. Key features of a typical biomarker sensor platform would include a microprocessor, an operational amplifier and an analog-to-digital converter, indicator
LEDs or thin and flexible display, antenna, possibly additional
memory, and coin cell or thin film style batteries.
Figure 3: Flexible hybrid circuits allow the development of light-
weight, fully integrated wearable sensors. (Image Credit: Molex)
Figure 4: Flexible sensor platform integrates antenna, thin flexible
display, microcontroller, and thin flexible batteries.
(Image Credit: Molex)