The past decade has seen a growing body of research devoted to the integration of electronics with biological or other soft, stretchable, flexible systems. This field is broad, ranging from wearable bio-sensors to ultra-light, foldable plastic electronics. An important sub-field involves the development of devices or sensors to be mounted on or near the skin. Clearly, because of the soft, compliant nature of tissue and the natural bending or rotational motion associated with joints, many applications in this area will require the development of both structures and materials which can stretch, bend, fold, twist and generally deform in response to the motion of the wearer. Such behaviour is generally not compatible with traditional silicon based electronics. This has led to the development of various stretchable materials and structures which have been used to fabricate a range of other devices including transistors and sensors which can be integrated into clothing or worn on the skin.
A particularly important class of stretchable electronics encompasses wearable biosensors which can be fabricated to monitor a range of bio-signals including blood pressure, respiration rate and blood glucose levels. Particularly important are wearable strain and motion sensors. These can be used to monitor joint and muscle motion and can be used for sensing of posture, movement and even breathing. A number of applications have been suggested for such sensors: smart suits for babies, athletes or soldiers or in monitoring of patients which are elderly, suffer from chronic disease or are in rehabilitation.
Such strain and motion sensors usually work by sensing the change in resistance of a material in response to variations in its length. However, wearable strain sensors have a range of requirements that are not all fulfilled by standard strain sensing platforms. Such sensors need to be compliant so as not to limit the motion of the wearer; they need to be highly sensitive to detect small motions involved in processes such as breathing; they need to work at high strains to monitor large scale motion such as that associated with joints and finally, need to operate at high speeds/strain rates to follow fast voluntary and involuntary movement. In addition, it should be possible to produce the sensing material in various shapes, sizes and geometries, for example as fibres which could be woven into garments. It should also be cheap to produce to facilitate widespread availability. To the authors knowledge no material combines all these properties, severely limiting our ability to fabricate multifunctional wearable strain and motion sensors.
This is unfortunate as sensors made from such materials could sense not only strain but velocity, acceleration and force. Such capabilities would facilitate not only applications in human motion sensing but in a range of areas including monitoring of airbags and other inflatable devices, motion in robots or moving mechanical objects and vibration monitoring.
In order to develop materials which can perform this array of functions, many researchers have turned to materials science and specifically nanotechnology. Strain sensors have been demonstrated from a range of materials and structures including hydrogels, nano-papers, graphene woven fabrics, nanotube arrays and complex nano-engineered structures. Particularly promising have been the nano-composite strain gauges. To date, strain gauges prepared from polymer-nanoparticle composies, polymer nanotube composites, polymer-graphene composites and polymer-carbon black composites have demonstrated performance far superior than those observed for commercial metal stain gauges.
The simplest performance metric is the gauge factor, G, which describes how the relative resistance change depends on strain, ε: ΔR/R0=Gε. This is usually measured at low strain and is typically ˜2 for metal strain gauges. However, some nano-composites have demonstrated values of G as high as 30. Some results have been extremely impressive. Networks of graphene on elastomeric substrates have demonstrated high gauge factor and good dynamic performance at strains up to 8%. By fabricating arrays of platinum coated polymer nanofibers. Low-strain sensors for pressure, shear and torsion with good dynamic response up to 10 Hz have previously been fabricated. Arrays of carbon nanotubes have also been used to prepare high-strain sensors with very impressive dynamic response but relatively low gauge factor. However, no reports exist for strain sensors which combine low stiffness, high gauge factor, high-strain and fast dynamic motion sensing capabilities with the potential for simple, cheap fabrication.
It is an object of the subject application to overcome at least one of the above-mentioned problems.