Wearable technology attracts significant interest in monitoring health status and the surrounding environment in real-time.
Wearable sensors, designed for recognizing various human epidermis fluids (such as glucose, lactate, pH, cholesterol) and physiological indicators (pulse rate, temperature, breath rate, respiration, alcohol, activity monitoring) have potential applications both in medical diagnostics and fitness monitoring.
The rapid emergence of a wide range of advanced nanomaterials with enhanced sensitivities, manufacturing processes, substrates, and readout circuits, open up a promising perspective to the field of wearable sensors today through cost-efficiently printing on a wide range of flexible polymeric substrates.
Printing is the most promising manufacturing approach due to the effective utilization of materials, low cost, etc. Due to their higher surface-to-volume ratio, nanoscale materials are ideal for sensing applications and are used in various sensing applications.
Sensory systems, particularly wearable biosensors, have expanded their application areas thanks to printing electronic components on flexible substrates. Chemical, physical, and optical sensors and complementary data readout and signal conditioning circuits are embedded separately or combined onto flexible substrates. Data is wirelessly transmitted to nearby computing devices or the cloud, where it is examined by medical experts who issue appropriate commands based on the health conditions.
Polymeric substrates are highly suitable for employing these sensing devices and circuits for their burgeoning properties, such as their lightweight, low cost, flexibility, bendability, foldability, stretchability, and conformability to uneven surfaces with negligible losses sensor data.
The recent developments of all-organic biocompatible or hybrid sensors on wearable substrates pave the way to realizing very effective and cheap body-worn sensing systems. In this context, printed wearable electronics have gained a lot of traction, with new strategies for conformably integrating sensing patches directly on the human body or in the form of wearable gadgets for a variety of human health-related biomarkers.
For the fabrication of sensors and electronics on non-planar substrates, printing is a rapidly growing field. It entails the deposition of functional materials from colloidal or chemical solutions at specific locations.
The number of fabrication steps is much lower than in clean-room processes using standard microfabrication technology. Printing is a bottom-up manufacturing method in which materials are layered on top of each other in successive fabrication steps. In comparison to traditional microfabrication techniques, additive manufacturing distinguishes printing as a simple and cost-effective process.
Types of printing techniques
Printing techniques are divided into two categories: contact and non-contact of the printing medium with the target substrates. The printing medium with designed structures on the surface is inked and brought into physical contact with the target substrate in the contact-based approach. Such techniques involve screen printing, gravure printing, flexographic printing, pad printing, stamp-assisted transfer printing, etc.
In non-contact-based printing, materials are ejected in micro-droplets or a continuous jet, facilitated by miniaturized printing nozzle heads. This is often referred to as digital manufacturing, where droplets/jets are ejected on-demand due to the respective actuation mechanism. Non-contact printing processes predominantly include piezoelectric inkjet printing, electrohydrodynamic (EHD) inkjet printing, slot-die, aerosol-jet printing, etc. Non-contact methods are more appealing because they are more versatile and allow for rapid changes in designed structures using computer-controlled software.
There are two types of biosensors developed fully or partially using printing technologies. They are biological fluid-based sensors, including glucose sensors, lactate sensors, pH sensors, and cholesterol, and physiological sensors that measure pulse rate, respiration, acetone for diabetes detection, alcohol level detection, temperature, motion/activity monitoring, gas presence, pressure, and strain.
The key advantages of printed wearable biosensors are as follows:
- Low cost
- Flexibility due to flexible substrate
- Light in weight and thin in size
- Wide range of inks and substrates available
- Integrability, robust and stretchable
- Complex geometries
- Possible combination with nanostructures with bio-receptors