Mechanical Development of a Scalable Structure for Adolescent Exoskeletons.

Over the past decade, many medical lower limb exoskeletons have been developed and exploited. The advantage of such a systems is to ensure the mobility of paraplegic patients, as well as their physical rehabilitation. However, existing solutions have not been widely available among the disabled population, particularly adolescents, due to the limitations of their conception caused by the rapid physical growth and morphological variation of this population.In this paper, a new scalable structure of the exoskeleton is proposed as a feasible solution to the problem of morphological changes. As this is the first time the generic term "scalability" has been used, its requirements and design methods, including the morphological changes and alignment, are presented in detail to better meet the growing needs for such a promising device. The evaluation of the proposed scalable structure shows a promising utility that is illustrated by several experimental scenarios: the load capacity of the structure, the efficiency of the fixation mechanisms, the validation of the hip alignment mechanism and finally the validation of the evolutionary structure.

Stability of Neuronal Networks with Homeostatic Regulation.

Neurons are equipped with homeostatic mechanisms that counteract long-term perturbations of their average activity and thereby keep neurons in a healthy and information-rich operating regime. While homeostasis is believed to be crucial for neural function, a systematic analysis of homeostatic control has largely been lacking. The analysis presented here analyses the necessary conditions for stable homeostatic control. We consider networks of neurons with homeostasis and show that homeostatic control that is stable for single neurons, can destabilize activity in otherwise stable recurrent networks leading to strong non-abating oscillations in the activity. This instability can be prevented by slowing down the homeostatic control. The stronger the network recurrence, the slower the homeostasis has to be. Next, we consider how non-linearities in the neural activation function affect these constraints. Finally, we consider the case that homeostatic feedback is mediated via a cascade of multiple intermediate stages. Counter-intuitively, the addition of extra stages in the homeostatic control loop further destabilizes activity in single neurons and networks. Our theoretical framework for homeostasis thus reveals previously unconsidered constraints on homeostasis in biological networks, and identifies conditions that require the slow time-constants of homeostatic regulation observed experimentally.