A bionic hand able to restore sense of touch

Materials World magazine
25 Mar 2019

A bionic hand can restore the sense of touch to amputees by simulating the brain’s ability to sense the position of limbs. Idha Valeur reports.

New bionic technology could help recreate the sense of touch for upper limb amputees so they would not have to rely solely on visual cues, according to researchers. The hand would be able to replicate the feel of proprioception – the brain’s ability to sense the position of limbs. This would enable patients to determine an object’s shape, size, position and consistency, even in the dark or with their eyes closed.

Working on this technology for 10 years, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Professor of Bioengineering, Silvestro Micera, said, ‘Our study shows that sensory substitution based on intraneural stimulation can deliver both position feedback and tactile feedback simultaneously and in real-time. The brain has no problem combining this information and patients can process both types in real-time with excellent results.’

Micera said the technology is composed of a robotic and sensorised hand, including implantable and biocompatible electrodes, polyimide, and an electronic controller-simulator unit. The simulator unit has customised algorithms to control the hand and to restore sensory feedback.

Micera told Materials World that the hand connects to the limb using a custom-made socket. ‘The artificial hand and the stimulator were controlled by a system controller using custom-developed software in C++. The prosthetic hand was equipped with sensors measuring the force exerted by the fingers and the aperture of the robotic hand.

‘The users’ residuum muscular signals were used to decode the intended grasp. Decoded hand motion was driven in terms of progressive position control, resulting in a gradual opening or closing of the hand.

‘The sensors embedded in the hand were used as inputs for the delivery of the afferent neural stimulation. Current-controlled stimulation was delivered through the implantable electrode, TIME [transverse intrafascicular multichannel electrode], active sites – in the median nerve and/or in the ulnar nerve – eliciting a sensory perception reliably localised within the territories of the stimulated median or ulnar sensory fascicles.

‘The stimulation was provided at fixed frequency and width of a biphasic train of pulses, whereas the current amplitude was modulated proportionally to the sensor readouts. In this way, the patient was able to control the robotic hand and to perceive sensations in real-time.’

Micera said previous efforts have used implantable peripheral interfaces to evoke sensations, while the team used TIME electrodes to restore multiple aspects of sense of touch – such as the ability to control grasping strength and single out different textures and an object’s compliance and shape.

‘To date, TIMEs may represent an interesting trade-off between other solutions, reducing their specific limitations in terms of sensation quality and stability, and allowing the restoration of features such as prosthesis grasping force control,’ Micera said.

‘Moreover, this approach can improve motor control performance in simulated ecological conditions – handling fragile objects when users can exploit both visual and tactile feedback – reducing phantom limb pain.’ The technique made it possible for two amputees to establish the size and shape of four different objects with 75.5% accuracy.

In addition to being able to recreate the feeling of position, the new bionic hand is much lighter and mimics the weight of a normal limb at approximately 1.5kg, while the 10-year-old prototype weighed as much as 15kg. The lighter one can be applied directly to the amputated limb, according to Agostino Gemelli University Polyclinic, Italy, Director of the Institute of Neurology, Professor Paolo Maria Rossini.

What’s next?

For the hand to be ready for market, the team needs to complete experiments with a fully embedded prototype. ‘This means an artificial limb having the power unit inside it, the technologies for online signal interpretation and the stimulator. We are planning to run this experiment in the next two candidates. Thereafter, the only needed step is to find a company willing to invest money in order to create a commercial type of artificial limb based on our prototype,’ Rossini said.

The researchers also believe that their technology is adaptable for use in other applications such as wrists and elbows. Even stroke patients might benefit in the future. ‘Different segments of the limbs can take advantage from our studies including individual sensorised fingers, wrists, elbows and shoulders,’ Rossini explained.

‘Microelectrodes, which are inserted in the nerves, might also be used for spinal cord partial lesions in order to bridge the site where bidirectional nervous impulses are partly blocked. Subjects who suffered from stroke might in theory receive a beneficial support.’

The paper, A closed-loop hand prosthesis with simultaneous intraneural tactile and position feedback, published in Science Robotics, can be read here: bit.ly/2ujT8TALINK