Printing technologies is an indispensable tool for most electronic/biomedical/automotive/aerospace manufacturing processes of the 21st century. They are versatile because of the availability of several degrees of freedom over its counterpart. It is widely used in the strain gauge measurements due to its superiority over rudimentary strain sensors in terms of mechanical composure, workability, low cost, accessibility and ease of assembly. Printing techniques has also enabled the use of various manufacturing styles permitting greater availability of piezo-electric resistivity range. There are various kinds of sensors in the scientific arena such as chemical, biochemical, acoustic, thermal and mechanical.
Silicon based sensors have higher dominance in the market over their metallic counterparts for the following reasons: higher piezo-resistivity, ease of manufacturing, high sensitivity and linearity. The following paper provides a brief overview of printed strain gauges or mechanical sensors providing emphasis on the advantages over conventional strain gauges. Studies show that in the year 2020, piezo-resistors, biosensors and photodetectors constitute the major market share compared to the rest of the field including capacitive, thermal, humidity and gas sensors.
Keywords: strain, gauge, piezo-resistive, mechanical
Printable electronics have surged in innovation and production in the late 21st century because of the availability of desirable substrates. Some technologies utilizing printing techniques include solar cells, electrochemical sensors, transistors and strain sensors. Strain gauges—whose principle lies on the change in physical parameters such as force, pressure or tension which are in turn recorded and deciphered as electrical impulses– are physical devices used in engineering applications. Precise measurements are now a requirement for most scientific analysis and it largely depends on rapid signal transmission for effective working. Printed strain gauges have significant benefits over conventional gauges like durability, cost-effectiveness, design, sensitivity and portability. An external application of force on an object results in the deployment of the stress-strain system where the former can be scientifically defined as the underlying deformation resisting forces and the latter, deformation magnitude because of the applied forces. This phenomenon is called the piezo-resistive effect and can be measured using the formula:
Where, rho is the resistivity, l is the length of the sensor, A is the cross-sectional area of the sensor. Strain gauges are now an industrial and academic standard for physical measurement regarding rigid body dynamics—quantities of tensile and compression forces can be measures in multi-axial conditions. Scientific strain gauges have inbuilt high resolution transducers to track minor changes in strain for various force values within the set range. The strain sensor initiates in a relaxed state where there is an initial resistivity for current flow, upon application of strain, there is an apparent change in the geometry of the sensor that results in a different value for electrical resistivity. Conventional sensors also suffer from their inability to adhere to non-standard surfaces and designs such as a parabola or irregular body—they are often too rigid for incorporation into microelectronic devices. Materials used in strain gauge manufacturing are silicon nanomembranes, silver nanoparticles, carbon nanotubes, PEDOT: PSS etc. as quotes by various scientific articles. Strain sensors have found applications in biomedical electronics, automotive, military combat, aerospace and robotic industry–often in the domain of infrastructural quality control for safety purposes. Some scientific estimates tell that printable sensor electronic industry will be worth about 8 billion US dollars in the year 2025.
Strain gauges come under the category of mechanical sensors from the sensor family consisting of acoustic, chemical, biochemical, thermal and mechanical–all of which works through their corresponding detection mechanisms. Acoustic sensors determine the propagated wave’s frequency and amplitude relative to physical characteristics such as magnetic fields, temperature, humidity etc. Thermal sensors work by the principle of quantification of heat where the thermal expansion co-efficient of varied materials are exploited and used to determine the kinetic energy dynamics. There are three kinds of strain sensors such as foil gauge, wire gauge and semiconductor gauge.
Mechanical sensors have two categories—movement bases such as displacement, velocity and force based such as pressure, acceleration, torque etc. Gauge performance depends on the following factors: sensitivity, resolution, reproducibility etc. Silicon based sensors have higher dominance in the market over their metallic counterparts for the following reasons: higher piezo-resistivity, ease of manufacturing, high sensitivity and linearity. An important parameter used in the design of mechanical sensors is the poisson’s ratio which can be defined as the ratio of the compressive strain to the extensive strain and depends in the material used. Silicon based compounds have higher gauge factor (to the order of >120 for p-type and n-type semiconductors), poisson’s ratio compared to metals giving rise to a higher piezo-resistive effect. Various printing technologies used in mechanical sensor manufacturing are screen, flexographic, inkjet, aerosol printing etc. There has also been a lot of interest in the development of low cost printing technologies for the manufacture of strain sensors. PVA based polymer composites embedded with carbon nanotubes showed superior performance such as linearity of deformation versus resistance and gauge factors. Further, it was seen to be an environmental friendly procedure. Another research showed that carbon nanotube fillers in a silicon nanocomposites could be a good fit for use in medical science and instrumentation. In another research, a trademarked technology called “INKtelligent printing” was utilized and showed good promise of sensor signal and high quality resolution. Also, some researchers have employed flexography printing technologies over paper using silver ink as well. In another study, an automatic method to measure strain was developed that involved no physical contact. Light rays were observed for displacements and it was used to suggest the surface strain on the plan of incidence. In some studies, direct screen printing is employed using CNT ink to print onto PDMS. This is a good approach for application requiring good capacitive behavior and control. The major intensive and extensive parameters involved in the process of printing are wetting phenomena of the substrate, surface tension of the interface, viscosity of the fluids, process duration, physical conditions such as temperature and pressure.
Printable strain gauges have barely begun in the progressive inventory list and there is much to come such as the inventions in the flexible semiconductor industry. Much of the research carried out in the scientific arena is towards improving the sensitivity, calibration, non-linearity, accuracy, repeatability, noise, saturation, reproducibility etc. where: sensitivity is the relationship between in the input and the output, saturation is the operation bounds, calibration is the comparison of measurable against the standard, noise is the irregular deviations in the output signal and resolution is the minimum value of the measurable quantity.
- Ameen, A. (2013). Flexible/Stretchable Strain Gauges Based on Singlecrystalline Silicon for Biomedical Applications. Retrieved from https://www.ideals.illinois.edu/bitstream/handle/2142/44793/Abid_Ameen.pdf?sequence=1This work was part of a graduat student research and covered the flexible and stretchable gauges for biomedical applications. It can be inferred that gauge factor is dependent on substrate property.
- Bessonov, A., Kirikova, M., Haque, S., Gartseev, I., & Bailey, M. J. A. (2014). Highly reproducible printable graphite strain gauges for flexible devices. Sensors and Actuators, A: Physical, 206(July), 75–80. https://doi.org/10.1016/j.sna.2013.11.034. This work mainly covered the thermal aspects of printable strain gauges.
- Enser, H., Kulha, P., Sell, J. K., Jakoby, B., Hilber, W., Strauß, B., & Schatzl-Linder, M. (2016). Printed Strain Gauges Embedded in Organic Coatings. Procedia Engineering, 168, 822–825. https://doi.org/10.1016/j.proeng.2016.11.282. This work showed the feasibility of screen printing over organic substrates for strain measurements.
- Giffney, T., Bejanin, E., Kurian, A. S., Travas-Sejdic, J., & Aw, K. (2017). Highly stretchable printed strain sensors using multi-walled carbon nanotube/silicone rubber composites. Sensors and Actuators, A: Physical, 259, 44–49. https://doi.org/10.1016/j.sna.2017.03.005
- Gonsalves, B. F., Oliveira, J., Costa, P., Correia, V., Martins, P., Botelho, G., & Lanceros-Mendez, S. (2017). Development of water-based printable piezoresistive sensors for large strain applications. Composites Part B: Engineering, 112, 344–352. https://doi.org/10.1016/j.compositesb.2016.12.047. This research showed that PVA nanocomposites are ecofriendly without any compromise on performance.
- Maiwald, M., Werner, C., Zoellmer, V., & Busse, M. (2010). INKtelligent printed strain gauges. Sensors and Actuators, A: Physical, 162(2), 198–201. https://doi.org/10.1016/j.sna.2010.02.019. In this case, the caractéristiques were found to be similar to commercial strain gauges.