Multisensory Wearable Motion Analysis in Spine Biomechanics

Textile based piezoresistive transducers are an innovative category of devices that use yarns made of conductive and elastic fibers or screen printed conductive rubber coatings to sense strain. They usually satisfy wearability requirements and are used in real-time information gathering systems, being comfortable, ubiquitous and available for long term monitoring. They include knitted fiber transducers (KFTs), sewed fiber transducers (SFTs) and smeared redundant elastomer (SRETs) transducers. In the latter category, SRETs constituted by Conductive Elastomers (CEs) have been commonly employed as strain sensors networks to detect human posture and gesture. Elastic interconnection wiring is also easily realized leading to monolithic fabrication techniques which avoid the presence of metal wires and multiple solderings. Despite this, there is a strong dependence of the system performance by the body structure garment fitting. Moreover, the non-linear dynamical behaviour of SRETs requires identification algorithms, functions which relate joint angles to electrical values presented by the sensor network. The construction of these functions is quite complex and time of computation dramatically increases with the number of degrees of freedom and with the accuracy required to the system to be resolved. Recent development of CEs sensor modeling overcomed some of their main limitations and introduced new fields of operability in SRETs networks. In particular, in strain applications, a useful data processing technique is presented for treating the non-linear dynamical response, considering the different behaviour in sensor elongation and relaxation: actually, when the sensor in stretched, the breakdown of carbon black agglomerates produces an increase in resistance. Inversely, when the sensor is relaxed, the cross-link readjustment lead to different electrically conductive paths in respect to the previous states. This technique has found its implementation in multisensory systems, leading to encouraging results in biomechanical reconstruction. Furthermore, a novel approach in CEs sensing is described, relating the global curvature of a layer to its electrical resistance value variation and exploring under which conditions the resistance can be considered uncorrelated with its particular local bending profile. These devices are called Smeared Conductive Elastomer Electrogoniometers (SCEEGs) and under particular configuration they can be employed as on-body electrogoniometers. The integration of SRET arrays and SCEEGs is definitely a powerful tool for human body posture and gesture reconstruction through efficient and fast algorithms. Moreover In this study we introduce a particular realization of CE strain sensors deposed on an adhesive taping, obtaining a very low skin artifact device (VLSA). We present an electrogoniometric system in which the inextensible insulating layer has been replaced by an elastic layer, allowing the system to be employed both as strain sensor and as electrogoniometer. Finally, we present a biomechanical application in lumbar spine posture monitorization. As a matter of fact, it is known from literature that in the sagittal balance there is a strong correlation with the torso angle and geometrical parameters of lumbar vertebraes, such as the angle between subsequent upper endplates. Data obtained from piezoresistive sensors are so suitable to be used in biomechanical analysis in order to predict forces and moments acting on the functional spinal units.