Mechanical Sensing for Lower Limb Soft Exoskeletons: Recent Progress and Challenges.

Soft exoskeletons hold promise for facilitating monitoring and assistance in case of light impairment and for prolonging independent living. In contrast to rigid material-based exoskeletons, they strongly demand for new approaches of soft sensing and actuation solutions. This chapter overviews soft exoskeletons in contrast to rigid exoskeletons and focuses on the recent advancements on the movement monitoring in lower limb soft exoskeletons. Compliant materials and soft tactile sensing approaches can be utilized to build smart sensorized garments for joint angle measurements (needed for both control and monitoring). However, currently there are still several open challenges derived from the needed close interaction between the human body and the soft exoskeleton itself, especially related to how sensing function and robustness are strongly affected by wearability, which will need to be overcome in the near future.

Biomechanics in Soft Mechanical Sensing: From Natural Case Studies to the Artificial World.

Living beings use mechanical interaction with the environment to gather essential cues for implementing necessary movements and actions. This process is mediated by biomechanics, primarily of the sensory structures, meaning that, at first, mechanical stimuli are morphologically computed. In the present paper, we select and review cases of specialized sensory organs for mechanical sensing-from both the animal and plant kingdoms-that distribute their intelligence in both structure and materials. A focus is set on biomechanical aspects, such as morphology and material characteristics of the selected sensory organs, and on how their sensing function is affected by them in natural environments. In this route, examples of artificial sensors that implement these principles are provided, and/or ways in which they can be translated artificially are suggested. Following a biomimetic approach, our aim is to make a step towards creating a toolbox with general tailoring principles, based on mechanical aspects tuned repeatedly in nature, such as orientation, shape, distribution, materials, and micromechanics. These should be used for a future methodical design of novel soft sensing systems for soft robotics.

Three-Dimensional Soft Material Micropatterning via Direct Laser Lithography of Flexible Molds.

Three-dimensionally micropatterned surfaces are attracting increasing interest in soft robotics owing to the potential of mimicking natural morphologies at the micro/nanoscale. We employ direct laser lithography to fabricate molds with complex three-dimensional (3D) micrometric features, in a positive photoresist on flexible substrates, to pattern curved macroscopic soft surfaces with shapes not achievable with standard methods (e.g., reentrant angles). We present several 3D intricate microstructures in poly(dimethylsiloxane) (PDMS) and show a soft cylinder patterned with 3D microstructures with one molding process. Finally, we deform PDMS-based 3D architectures and show soft microgripping capability, indicating the potentiality of this approach for future application in soft robotics.

A soft, stretchable and conductive biointerface for cell mechanobiology.

In mechanobiology the study of cell response to mechanical stimuli is fundamental, and the involved processes (i.e., mechanotransduction) need to be investigated by interfacing (mechanically and electrically) with the cells in dynamic and non-invasive natural-like conditions. In this work, we present a novel soft, stretchable and conductive biointerface that allows both cell mechanical stimulation and dynamic impedance recording. The biointerface stretchability and conductivity, jointly to the biocompatibility and transparency needed to perform cell culture studies, were obtained by exploiting the formation of wrinkles on the surface of a 90 nm thick conductive layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on a pre-stretched 130 µm thick poly(dimethylsiloxane) (PDMS) substrate. Cell adhesion and proliferation of SH-SY5Y human neuroblastoma cells were evaluated, and cell differentiation on the corrugated surface was assessed. We demonstrate how the biointerface remains conductive when applying uniaxial strain up to 10%, and when cell culturing is performed. Finally, a reduction of about 30% of the relative impedance variation signal was measured, with respect to the control, as a result of the mechanical stimulation of cells.

Quantification of growth and differentiation of C2C12 skeletal muscle cells on PSS-PAH-based polyelectrolyte layer-by-layer nanofilms.

Polyelectrolyte layer-by-layer (LbL) nanofilms are interesting polymeric structures, built by alternating adsorption of positively and negatively charged polyelectrolytes. They consist of multilayer sheets with nanometric overall thickness, and they can be used as supports and surface coatings for in vitro and in vivo cell and tissue growth and regeneration. The present study focuses on nanofilms based on alternated layers of poly(sodium-4-sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) fabricated using spin-assisted LbL assembly (SA-LbL). The fabrication process used to assemble polyelectrolyte nanofilms made of up to 60 bilayers is described, and the influence of different surface charges (i.e. changing the terminal layer) and of different film composition (e.g. varying PSS molecular weight) on cell behaviour is investigated. In particular, C2C12 skeletal muscle cells' viability, proliferation and differentiation on six different typologies of polyelectrolyte nanofilms are evaluated and quantified, giving a reference for skeletal muscle regeneration capabilities on such kind of structures.