Page 7 - Read Online

Brasier et al. Soft Sci 2024;4:6 https://dx.doi.org/10.20517/ss.2023.39 Page 5 of 10

Table 1. Typical specifications of wearable sweat rate sensors
Device Materials and design of µ-fluidics Sweat rate measurement Ref.
1 A coiled tubing kept in position by a round plastic frame Sensor: water-responsive chromogenic reagent [36]
Read-out: external camera for time-stamped picture
Pros/Cons: simple implementation/not thin film
so rather bulky, the read-out relies on the post-processing of
the picture taken from an external camera (semi-quantitative)
2 A bottom PDMS layer (thickness, 500 μm) embossed with Sensor: water-responsive chromogenic reagent [35]
appropriate relief geometry (uniform depth, 300 μm) and with a Read-out: picture taken from an external camera launched by
top-capping layer of PDMS that serves as a seal (thickness, 200 near field communication chip for time
μm)
Pros/Cons: simple design/ the read-out relies on the post-
processing of the picture taken from an external camera (semi-
quantitative)
3 Two main parts: (i) a microfluidic; and (ii) an electrical sensing Sensor: capacitive (analog/continuous). The sweat rate [34,
component. The microfluidic channel is prepared with PDMS sensor contains two parallel Cr/Au spirals that are aligned 38]
and is covalently bonded to PET containing sensing electrodes with the microfluidic channel. Sweat rate is quantified by the
layer, via O plasma etching and silanization. In some designs, change of impedance
2
the collection well is filled with a patterned SU8 filler coated Read-out: external PCB for signal processing and
with a thin saturated hydrogel layer that contacts skin for sweat communication via bluetooth
uptake
Pros/Cons: quantitative read-out/the read-out signal depends
also on the changing ionic concentration of the sweat
4 Design similar to the one of device 3 but with the impedimetric Sensor: capacitive (digital/discrete). The multi electrodes [41]
sweat rate sensor formed by two electrodes with interdigitated design results in discrete/digital changes of the impedance
fingers over which the serpentine channel repeatedly passes that enable time-volume synchronization independently form
the ionic concentration
Read-out: external PCB for signal processing and
communication via bluetooth
Pros/Cons: quantitative read-out not depending on the ionic
sweat concentration/ rather complex implementation
(multiple electrodes)
5 The device is formed by three main layers: (i) an adhesive layer Sensor: the sensing mechanism relies on the measurement of [40]
to strengthen the contact with the skin; (ii) a PDMS layer with the resistance between metal pads patterned onto the wall of
microfluidics and electrodes for sensing; (iii) flexible PCB to the microfluidic channels. Two separate channels allow to
connect electronics and communication chips solved the interdependence of the resistance on rate and
electrical conductivity of the sweat. AC modulation is
implemented to avoid the formation of electronic double layer
that may foul the reading
Read-out: flexible PCB for signal processing and
communication via NFC
Pros/Cons: quantitative read-out not depending on the ionic
sweat concentration/rather complex implementation (two
separate channels)
6 Design consists of (i) an adhesive layer; (ii) a PDMS Sensor: the device implements a flowmeter by reading the [42]
microchannel; (iii) a PDMS PCB that connects two thermistors temperature difference between two thermistors. A heater
and a heater; and (iv) finally a PDMS cover. The design is positioned midway between the thermistors set their
simpler than the one of device 3 and 5 since it is rely on the temperature which is identical before the flow of the sweat and
direct measurement of the speed of the sweat flow rather than that is different after. The difference of temperature yields a
of the volume. The modular assembly facilitate the re-use of the change of the resistance in the two thermistors
PCB with with the disposable layers connected via magnets to Read-out: flexible PCB for signal processing and
the flexible PCB
communication via BLE
Pros/Cons: elegant solution implementing a micro-thin film
flow-meter/sensitive to stretching because the distance
between the two thermistors changes, the heater could be
power hungry if not properly biased

AC: Alternate current; BLE: bluetooth low energy; PCB: printed circuit board; PDMS: polydimethylsiloxane; PET: polyethylene terephthalate.

analysis . Currently, we register a few commercial devices that aim to track sweat rates and composition.
[50]
One is KuduSmart (https://kudusmart.com/), and the other one is the GX patch (https://www.gatorade.
com/equipment/gx-sweat-patch/gx-sweat-patch).