Multifunctional sweat sensors with new detection methods have the potential to revolutionize non-invasive medical diagnosis and care management.
Study: flexible microfluidic nanoplasmonic sensors for refreshable and wearable recognition of biochemical sweat fingerprints. Image Credit: Billion Photos/Shutterstock.com
The creation of a flexible microfluidic nanoplasmonic detector capable of refreshable and transportable fingerprint identification using high surface-enhanced Raman scattering (SERS) activity is the subject of a recent study published in the journal npj flexible electronics.
Unlike traditional wearable SERS platforms, which are at risk of mixed effects of recent and old sweat, the microfluidic SERS device enables sweat processing at a regulated and high temporal resolution, enabling refreshable SERS evaluation.
Figure 1: Design of an epidermal sweat sensor based on a wearable SERS microfluidic platform. a stacked view showing each layer of the integrated device. b Schematic of all-in-one biofluidic flows, storage and SERS analysis based on a microfluidic plasmonic device. c Actual image of a prepared microfluidic SERS sensor attached to the skin. d System level block diagram showing the internal functional modules of the Raman portable analyzer.
Wearable sweat sensors: overview and meaning
Flexible, intelligent devices have transformed the perception, methodology and strategies of human-machine interaction and healthcare applications. Wearable sweat sensors that enable the recognition of traces at the molecular level in epidermally accessible sweat are considered critical biomedical sensing devices.
By integrating molecular identification methods, micro-nanoelectronics, interconnected hardware/software platforms and various analytical approaches, significant development can produce these wearable sweat detectors. The portable sweat detectors can now provide visual sensing access through color/absorption and fluorescent depth measurements.
Electrochemical approaches to fabricating wearable sweat sensors offer great specificity and sensitivity, with electrodes that can be built in many shapes and sizes.
However, each of these strategies has different advantages and disadvantages. As a result, continuous innovation in developing new signal reading methods is required to provide better possibilities for making wearable sweat detectors.
Figure 2. SERS detection of targeted analytes in standardized solutions. a Schematic and SERS spectra of b urea and c lactate in a label-free manner. d The mechanism of pH SERS sensing shows the probe molecule of 4-MBA labeled on the Ag surface, which can be protonated at an acidic pH and deprotonated at a basic pH. e SERS spectra and f the magnified view of the Raman peak at 1400-1425 cm−1 responded to different pH levels after normalization of the peak intensity at 1078 cm−1. g Scheme of simultaneous and multiple SERS analysis of sweat targets based on a single microfluidic chip. h Compatibility study of simultaneous SERS detection of urea and lactate. i Interference study of pH SERS sensing.
SERS Integrated Flexible Plasmonic Devices
SERS is a widely used analytical method that provides strong Raman signal amplification through targeted plasmon-enhanced stimulation and dispersion. Flexible plasmonic electronics, created by combining SERS with wearable technologies, has attracted a lot of interest in smart biomedical applications.
To date, only a few portable SERS sweat detectors have been documented in the literature. Sweat-permeable SERS substrates allow sweat to drain and fill hot spots in these non-microfluidic sweat detectors. However, these permeable SERS substrates are often physically fragile and prone to epidermal deformation upon skin contact.
Another major difficulty with the previously proposed wearable sweat SERS systems is the reading method. The traditional heavy Raman equipment limits the study of portable SERS sensors in regulated laboratory conditions, limiting their usefulness.
A new microfluidic SERS based flexible sweat sensor
SERS substrates can be spatially controlled and modified in a flexible manner using microfluidics. In addition, the dynamic transfer of sweat, enabled by microfluidics, can reduce the mixing and transfer of old and fresh sweat.
In this study, the researchers created a flexible microfluidic nanoplasmonic detector for the reusable and transportable detection of molecular sweat fingerprints.
For transportable identification of sweat biomarkers, a compact and custom-built Raman analyzer with a comfortable human-machine interface was used.
Main developments of the study
Compared to standard non-microfluidic detectors, this microfluidic biodevice allows the tunable selection of a highly active SERS substrate, yielding high temporal resolution and refreshable sweat SERS analysis.
The wearable SERS scanner can interpret sweat fingerprint data from selected biomarkers of urea, lactate and pH on a cellular scale, driving the applications of wearable SERS devices for point-of-care testing (POCT).
By using a portable Raman scanner, Raman signals can also be read anywhere, eliminating the need for expensive equipment and conventional laboratory conditions. Thus, the plasmonic microfluidic device can provide a medical research environment for customized healthcare by enabling the rapid, simple and portable screening of physiologically linked indicators in sweat.
Figure 3. On-body evaluation of the microfluidic SERS chip for personalized medicine. a Photo of a volunteer wearing a microfluidic SERS patch during continuous exercise. The inset indicates the portable Raman analyzer. b The user interface and c System level beam path diagram of the portable Raman analyzer. Discrete Raman spectra of d sweat urea, lactate, and e pH as well as f the corresponding contents calculated from the above calibration curves. The color and black data correspond to measurements performed by SERS and by commercial benchmark methods (urea test kit, lactate test kit and pH meter), respectively. g Schematic representation of the metabolic behavior of urea in the human body. Evaluation of the microfluidic SERS device in nutritional challenges by comparing the sweat urea and serum urea h to measure i without protein intake (n = 6, the dots represent raw data; the five lines from bottom to top represent minimum, bottom quartile, median, top quartile, and maximum, respectively).
Future look
Despite promising results, some issues related to the design of portable and portable SERS biofluid detectors are still prominent, such as possible peak overlap in the case of repeated SERS testing. In this context, creating a highly homogeneous substrate resistant to mechanical deformation is required for repeated SERS measurements.
The adaptability of the movable Raman analyzer to a wider range of analytes should also be explored by collecting additional experimental data.
Regardless, the researchers have made significant strides in this work in creating a reusable sweat biosignal sensor for non-invasive molecular probing. As a result, it is safe to assume that these wearable sensors will reveal innovative opportunities and open previously unrecognized frontiers in various sectors, including criminal surveillance, medical services and intelligent healthcare.
Reference
Xuecheng He. et al. (2022). Flexible microfluidic nanoplasmonic sensors for refreshable and wearable recognition of biochemical sweat fingerprints. npj flexible electronics. Available at: https://www.nature.com/articles/s41528-022-00192-6
