A stretchable sensor for force estimation in soft wearable robots.

Soft wearable robots to assist human movements, such as exosuits, have rapidly gained attention thanks to their compliance, low weight and accessibility. However, force measurement in exosuits still rely on load cells and rigid sensors that are not wearable or unsuitable for applications outside the lab. Soft, stretchable and lightweight sensors that become invisible when integrated in an exosuit and perfectly conform to the human body represent a promising alternative. In this work, we developed a wearable sensing system based on a soft stretchable silicone-based strain gauge to measure the forces acting in the passive elastic elements of an exosuit. To measure sensor's accuracy, two unimpaired participants walked on a treadmill at speeds between 0.9 and 2.1 $\text{m}\text{s}^{-1}$. When comparing our solution to a state-of-the-art motion capture system, we found an average root mean square error in force estimation of 12.5% and a standard deviation of 7.4%. Furthermore, we showed the portability of our sensory system by monitoring the forces exerted by the wearable robot during outdoor walking. Our study shows the potential of using stretchable sensors to monitor walking patterns in studies outside the lab and to control human-robot interaction.

Reaching higher: External scapula assistance can improve upper limb function in humans with irreversible scapula alata.

BACKGROUND: Scapular dyskinesis, i.e., the deviant mobility or function of the scapula, hampers upper limb function in daily life. A typical sign of scapular dyskinesis is a scapula alata-a protrusion of the shoulder blade during arm elevation. While some reversible causes of scapula alata can be treated with therapy, other, irreversible causes require invasive surgical interventions. When surgery is not an option, however, severe limitations arise as standard approaches for assisting the scapula in daily life do not exist. The aim of this study was to quantify functional improvements when external, i.e., non-invasive, scapula assistance is provided. METHODS: The study was designed as a randomized controlled crossover trial. Eight participants with a scapula alata due to muscular dystrophy performed arm elevations in shoulder flexion and abduction while unassisted (baseline), externally assisted by a trained therapist, and externally assisted by a novel, textile-based scapula orthosis. RESULTS: With therapist assistance, average arm elevation increased by 17.3° in flexion (p < 0.001, 95% confidence interval of the mean [Formula: see text]), and by 11.2° in abduction (p < 0.01, [Formula: see text]), constituting the potential of external scapula assistance. With orthosis assistance, average arm elevation increased by 6.2° in flexion ([Formula: see text]) and by 5.8° in abduction ([Formula: see text]). Remarkably, in three participants, the orthosis was at least as effective as the therapist. Moreover, orthosis assistance reduced average perceived exertion by 1.25 points (Borg Scale) when elevating a filled bottle during a simulated daily living task. CONCLUSION: These findings indicate a large potential for future advancements in orthotics. Already now, the textile-based scapula orthosis presented here is a feasible tool for leveraging the benefits of external scapula assistance when a therapist is unavailable, as encountered in daily life scenarios. Trial Registration ClincalTrials.gov (ID NCT04154098). Registered: November 6th 2019, https://clinicaltrials.gov/ct2/show/NCT04154098?term=scapula+orthosis&draw=2&rank=1.

Age-Dependent Asymmetry of Wrist Position Sense Is Not Influenced by Stochastic Tactile Stimulation.

Stochastic stimulation has been shown to improve movement, balance, the sense of touch, and may also improve position sense. This stimulation can be non-invasive and may be a simple technology to enhance proprioception. In this study, we investigated whether sub-threshold stochastic tactile stimulation of mechanoreceptors reduces age-related errors in wrist position estimation. Fifteen young (24.5±1.5y) and 23 elderly (71.7±7.3y) unimpaired, right-handed adults completed a wrist position gauge-matching experiment. In each trial, the participant's concealed wrist was moved to a target position between 10 and 30° of wrist flexion or extension by a robotic manipulandum. The participant then estimated the wrist's position on a virtual gauge. During half of the trials, sub-threshold stochastic tactile stimulation was applied to the wrist muscle tendon areas. Stochastic stimulation did not significantly influence wrist position sense. In the elderly group, estimation errors decreased non-significantly when stimulation was applied compared to the trials without stimulation [mean constant error reduction Δ µ ( θ c o n o f ) = 0 . 8 ° in flexion and Δ µ ( θ c o n o e ) = 0 . 7 ° in extension direction, p = 0.95]. This effect was less pronounced in the young group [ Δ µ ( θ c o n y ) = 0 . 2 ° in flexion and in extension direction, p = 0.99]. These improvements did not yield a relevant effect size (Cohen's d < 0.1). Estimation errors increased with target angle magnitude in both movement directions. In young participants, estimation errors were non-symmetric, with estimations in flexion [ µ ( θ c o n y f ) = 1 . 8 ° , σ ( θ c o n y f ) = 7 . 0 ° ] being significantly more accurate than in extension [ µ ( θ c o n y e ) = 8 . 3 ° , σ ( θ c o n y e ) = 9 . 3 ° , p < 0.01]. This asymmetry was not present in the elderly group, where estimations in flexion [ µ ( θ c o n o f ) = 7 . 5 ° , σ ( θ c o n o f ) = 9 . 8 ° ] were similar to extension [ µ ( θ c o n o e ) = 7 . 7 ° , σ ( θ c o n o e ) = 9 . 3 ° ]. Hence, young and elderly participants performed equally in extension direction, whereas wrist position sense in flexion direction deteriorated with age (p < 0.01). Though unimpaired elderly adults did not benefit from stochastic stimulation, it cannot be deduced that individuals with more severe impairments of their sensory system do not profit from this treatment. While the errors in estimating wrist position are symmetric in flexion and extension in elderly adults, young adults are more accurate when estimating wrist flexion, an effect that has not been described before.

Simplifying Exosuits: Kinematic Couplings in the Upper Extremity during Daily Living Tasks.

In the past few years, several light-weight soft wearable robots, so called exosuits, for upper extremity assistance have been developed. The design of exosuits is often based on a bio-mimetic design approach, mimicking the human biomechanics. However, in the design process, the interactions of movement directions during daily living tasks have not yet been analyzed comprehensively. Therefore, the designs of exosuits might be overly complex, as movement directions that are coupled during daily life tasks were implemented independently; or lack functionality, as relevant movement directions were disregarded. In the meta-analysis presented in this paper, the maximum angles occurring during daily living tasks in the upper extremity of unimpaired individuals were examined. To identify the kinematic couplings between joint axes, the interactions between movement directions that act against gravity were analyzed. The strongest correlations were found between rotation in the plane of elevation and humeral axial rotation ($R^{2} = 0.82$, p < 0.001), and between humeral elevation and humeral axial rotation $(R^{2}\ =\ 0.16,\ p=\ 0.001)$. Shoulder rotations and elbow flexion were not correlated. We conclude that humeral axial rotation is a relevant movement direction in the upper extremity, which, so far, has often been neglected in the design of exosuits. To simplify the design of exosuits, we propose a one degree of freedom support trajectory in which rotation in the plane of elevation (at -70° and 80°) and humeral axial rotation (at 110° and -60°) are coupled to humeral elevation (continuously from -40° to -110°).

A novel pneumatic stimulator for the investigation of noise-enhanced proprioception.

Executing coordinated movements requires that motor and sensory systems cooperate to achieve a motor goal. Impairment of either system may lead to unstable and/or inaccurate movements. In rehabilitation training, however, most approaches have focused on the motor aspects of the control loop. We are examining mechanisms that may enhance the sensory system to improve motor control. More precisely, the effects of stochastic subliminal vibratory tactile stimulation on wrist proprioception. We developed a device - based on a novel soft pneumatic actuator skin technology - to stimulate multiple sites simultaneously and independently. This device applies vibratory stimulation (amplitude < 0.50 mm, bandwidth 20-120 Hz) to the skin overlaying the tendons of a joint to target the receptors in charge of position and movement encoding. It achieves high spatial resolution (< 1 mm2), uses a soft and flexible interface, and has the potential to be used in combination with additional rehabilitation interventions. We conducted a feasibility study with 16 healthy subjects (11 younger - 6 females; 5 older - 2 females) in which a robotic manipulandum moved the subject's wrist to defined positions that had to be matched with a gauge. Comparing trials with and without stimulation we found that stochastic stimulation influenced joint position sense. The device we developed can be readily used in psycho-physical experiments, and subsequently benefit physiotherapy and rehabilitation treatments.