PPR3: prototypes

The functionality of the S-Finger was assessed during a clinical viability study. Two partial hand amputees, both suffering of a proximal amputation of the four long fingers of the right hand, were recruited by the INAIL Prosthetic Centre – Budrio (BO). The subjects were fitted with custom research prostheses using the S-Finger. The prosthesis was controlled by means of EMG signals picked up from the skin surface of the residual limb by means of EMG electrodes, and acquired by an embedded bidirectional controller. The prosthesis is equipped with a bi-modal (both visual and vibro-tactile) sensory feedback system, which can be activated or de-activated depending on the current experimental protocol. The index finger embedded a tactile sensor which was used to provide supplementary visual (light of different colors) and vibro-tactile (vibration in the bracelet on the forearm) feedback. After a functional test, the participants completed an evaluation questionnaire in order to assess the functionality of the device and the user satisfaction. The users reported a perceived improved stability of the objects while grasping, with respect to their standard prosthesis, and particularly appreciated the sensory feedback system.

Prosthetic system based on the S-Finger. Amputees while using the prosthesis during the dexterity tests.

In upper limb amputees, a non-invasive electro-cutaneous stimulation can induce referred sensations that are spread to a wider and/or more distant area, with respect to the electrodes. Building on this, we sought to exploit this effect to provide somatotopically-matched sensory feedback to people with partial hand amputations. We investigated the possibility of inducing referred sensations in the digits by electrically activating the palmar branches of the nerves serving the hand. Six partial hand amputees were recruited in order to perform experiments to investigate the possibility to evoke referred sensations in their missing digits. Amputees experienced referred sensations in unimpaired digits, similarly to non-amputees, but we were unable to evoke referred sensations in their missing digits: none of them reported sensations that extended beyond the tip of the stump. The results of these experiments seem to go against the past hypotheses, suggesting that it is possible to provide tactile sensations referred to other parts of the hand, but still limited to the distal part of the residual limb.

A) DESC-finger prototype and its wireless charger. B-C) Residual limb of the participants involved in the home study and fitting of the prosthetic digit.

The cosmetic digital prosthesis with embedded touch feedback (i.e., DESC-finger) is able to deliver short-lasting vibro-tactile bursts when it makes and breaks contact with an object. Two partial hand amputees were provided with the DESC-finger and used it at home for two months. The effectiveness of the device was preliminarily assessed by means of a functional task (i.e., modified virtual eggs test) and an interview. Although very preliminary, the results are promising because they suggest that the device allowed the participants to improve their manual dexterity with respect to the condition when the sensory feedback was not used.

Experimental assessment. The participant performed a modified version of the virtual eggs test, using the prosthesis to transfer fragile blocks from one side of a 15cm tall wall to the other.

A prosthetic system was developed and tested with proximal partial hand amputees. The device (dubbed S-Finger) is an externally powered digit with embedded sensory feedback. The system consists of several elements:

• S-Finger: externally powered and sensorized prosthetic finger composed of 2 phalanxes (proximal and distal). The proximal phalanx is directly actuated by a brushless motor and is coupled to the distal phalanx through a four-bar linkage mechanism. The distal phalanx embeds a pressure sensor and a LED used to provide visual feedback to the user.
• Embedded bidirectional controller, able to control multiple fingers. The system is capable of acquiring two EMG signals for the control of the prosthesis.
• EMG electrodes.
• Battery charger
• Bracelet containing the battery (7.4V, lithium ion battery), the battery charger connector, the power management board and a Bluetooth module for wireless communication. An ON/OFF switch is also available, which allows to easily powering on and off the prosthesis.

The multimodal sensory system is an acquisition system equipped with pressure and either temperature or vibration sensors. The small size of the system allows polymer coating (SORTA-Clear, shore 40A, Smooth On Inc.) mimicking the shape of a fingertip. In particular, two different prototypes of this fingertip were developed:
– a first prototype equipped with a pressure sensor and a microphone: the pressure sensor provides contact information, while the microphone is able to detect objects slip occurring at the fingertip;
– a second prototype equipped with a pressure sensor (similarly to the previous one) and an infrared sensor. The latter is used for estimating the temperature of an object before touching it.
Both prototypes can be used as the distal phalanx of the S-Finger.

Pressure sensor: the control electronics acquires the pressure sensor and, based on the sensor reading, drives the vibrator embedded in the electrode. This system can be used to give object contact and release information to the user. At the same time, a LED embedded in the fingertip changes colour, providing additional visual feedback.

Temperature sensor: the prototype detects the temperature of two objects (hot and cold) at a certain distance.

The embedded control system based on surface EMG signals is a high computational capability system using decision-making algorithms for controlling robotic prostheses during activities of daily living. The implemented control strategy is a two-step procedure: in the first phase, the transient part of the sEMG signal is used to select the grasp type (i.e. the class to be detected) and preshape the prosthesis in the correct position. In the second phase, a proportional control based on the stationary part of the sEMG signal is used to control the grip force.

Embedded control system: 1) Sensors used to sample the sEMG signal; 2) Classification; 3) Execution of POWER, PINCH or LATERAL grasp movements.

Control system used to control the Azzurra robotic hand (Prensilia s.r.l.): classification of three grip classes (lateral, power, and tridigital or precision grip) and hand configuration used to release objects (relax).

Actuated finger prosthesis. It is designed to be fitted to digits amputated at the proximal phalanx level. The prototype is composed by two phalanges: proximal and distal. The phalanges are coupled together through an inverse four-bar linkage mechanism. The movement is provided by one brushless DC motor that takes place inside the proximal phalanx. The control electronics is placed inside the distal phalanx.

First prototype of actuated proximal prosthetic digit

The prosthesis is designed to be fitted to digits amputated at the proximal phalanx level. It is provided with a sensory feedback system capable of providing vibro-tactile stimulation (a vibration similar to cell-phone vibrations) in the event of contact and release of a prosthesis with an objects. The feedback paradigm was based on the Discrete Event-driven Sensory feedback Control (DESC) model which is a neuroscientific hypothesis of the mechanisms involved in human motor control. The DESC model posits that motor tasks are organized in phases delimited by sensory encoded discrete events, which are important for monitoring task progress and trigger eventual corrective actions. The DESC-finger is autonomous and can be integrated in a polymeric cosmetic layer. The system can be charged by a custom wireless charger.

Prototype of distal prosthetic digit with vibro-tactile feedback (DESC-finger): picture of the prototype with the recharger system

A series of pilot experiments on able-bodied participants (20) was performed using a custom electro-tactile feedback device. These studies evaluated the ability of the system to promote a referred sensation (a sensation perceived as coming from a skin area which is different with respect to the stimulation point) from the digits by stimulating the palm. The results are promising and seem to confirm the possibility to provide sensations perceived in as coming from the missing fingers by stimulating the hand palm or dorsum.

A healthy subject involved in the experimentation

Artificial polymeric fingerpad with integrated biaxial force sensor and custom electronics for signal conditioning. The system provides two analog outputs proportional to the normal force (e.g. grip force) and tangential force (e.g. due to slippage) at fingertip. Such sensor can be integrated on SSSA/Prensilia artificial hand (Link) and could be employed in other INAIL projects of “PPR” network.

Prototype of sensorized phalanx: biaxial force sensor

Prosthetic phalanx capable of heating the stump in partial hand amputees to relieve pain due to cold.
The system comprises:
– A heating element placed within a cosmetic digit and capable of heating the stump;
– A “heating object” that transfers the energy provided by a battery to the prosthesis by means of an inductive (wireless) coupling.

The dimension of the device allows its easy grasping and placement in a pocket.

Prototype of distal prosthetic digit with heating property: picture of the prototype with the external power supply

Platform capable of recording ultrasound (US) images of the hand extrinsic muscles (placed in the forearm) and other signals from external sensors (data glove and load cells) during motor tasks. The platform was used to record pilot data from healthy participants, which allowed to study the relationship between the movements of the hand or the forces of fingers and movements of extrinsic muscles that generated them during grasping or manipulation activities.

Picture of experimental set-up. The picture shows the scanner, the data glove worn by the subject, the US probe placed on the forearm by an ad hoc support and graphical user interface for the monitoring of data in real-time.