With versatile sensory and motor capacities, hands are the most dexterous body parts to interact with the environment. Hand amputation, mostly caused by trauma events or diseases, causes lots of inconvenience into everyday life. Commercial hand prostheses partially restore the hand motor functions, but in most cases not the sensory abilities . Academic research on tactile feedback for hand prostheses has provided various solutions. With the recent breakthroughs, invasive tactile feedback approaches to provide tactile feedback ,  would provide the best results in the long run. With less risks, noninvasive approaches  are expected to benefit hand prostheses users in the near future.
Previous research has shown that noninvasively displaying tactile information generated at the five fingertips of a hand prosthesis by a miniature tactile display device is not trivial to achieve. Except on hands and faces, the two point discrimination distance on human body surface is mostly beyond 3 cm . Therefore, small areas such as the upper arm is in principle not sufficient to noninvasively display the information of five distinct locations. Another challenge is loss of reference: subjects have frequently reported to lose tracking of tactile feedback locations . Applying tactile feedback to a larger body area, such as the back or around the waist, shows good results of perception, but the devices are bulky and awkward to use.
We propose in this study to use the remaining healthy hand as the location to apply tactile feedback for prostheses used by unilateral hand amputees. Our hypotheses about this tactile display approach are: distinguishing five tactile feedback locations would be easy to achieve with little or even no training; confusion among fingers could initially be a challenge, but will be overcome during the device usage. Although there have been tactile feedback gloves available on the market, these gloves mostly provide feedback on fingertips and/or the palm. Tactile gloves designed with this approach would interfere with the normal sensory and motor functions of the healthy hand when used as a tactile feedback device for amputees. In this study, we designed a tactile feedback glove which provides feedback on the back of fingers to reduce influences.
In what follows, our proposed tactile feedback concept and the designed glove is presented first. Then experiments designed to evaluate the effectiveness of the tactile feedback device are described. Finally, we analyze the results and discuss the implications.
2 Tactile feedback device
Although the dorsal sides of fingers are not as sensitive as the ventral, localizing on which finger a physical stimulation is applied is still quite intuitive. To deliver the tactile information of a hand prosthesis for unilateral hand amputees, a tactile feedback glove is fabricated and to be worn on the remaining healthy hand. The glove delivers tactile information generated on fingertips of a hand prosthesis to the back of corresponding healthy fingers of the remaining healthy hand. The main design objectives are: intuitive tactile information display, a compact size, light weight, and easy to wear. The tactile feedback design concept and designed glove are shown in Figure 1.
The tactile feedback glove (shown in Figure 1) mainly consists of a sport glove, five eccentric rotating mass (ERM) vibrators (iNeed Inc., HK), and control electronics. A sport glove fits tightly to the fingers, which is essential to reliably deliver vibrotactile information to the back of fingers. A sport glove is also easy to take on and off, which is an important consideration to facilitate the independent usage by hand amputees. The ERM vibrators have the advantage of low power consumption, compact sizes (diameter 10 mm, thickness 3.4 mm), low costs, and an easy control interface. These vibrators provide tactile feedback at the back of each fingers to minimize the influence to the motor and sensory functions of the healthy hand.
3 Material and methods
To preliminarily evaluate the effectiveness of the tactile feedback concept described in the previous section, we designed experiments to test the tactile feedback glove with healthy subjects. These experiments have been approved by the institutional ethics committee of the Lund University in Sweden.
3.1 Test subjects
Five healthy subjects participated in the experiments. Prior to experiments, they were informed about the procedure and possible risks of the study verbally by an investigator and by experiment information sheets. All of them have signed the participant consent forms. The subjects were 26 ± 8 years old (one female and four males). All of them are right-handed and have declared to have normal perception abilities.
3.2 Experiment setting
In preparing an experiment, a test subject sits on a chair and rests her/his right arm on a table in front of her/him. The tactile feedback glove is worn on the subject’s right hand and adjusted to fit the hand well. During an experiment, a test subject is asked to move his/her right hand around while perceiving tactile information applied on the back of fingers to mimic our intended application scenarios. Hand movements increase the difficulty to identify the tactile feedback applied by the glove. An investigator issues stimulation signals by a piece of control software running on a laptop. The software interface is shown in Figure 2. The subject reports verbally his/her judgments about the tactile feedback to the investigator. The investigator records the answers on data record forms.
3.3 Experiment procedure
The experiment consists of two sessions: the first session tests the capacity of the tactile feedback glove to display tactile information of five individual fingers; the second evaluates the device’s potential to transfer contact force strength information. We expect that the tactile feedback glove could display tactile information intuitively, therefore no training session is included. No feedback about correct answers is given to subjects during the tests.
During the first experiment session, each subject is given six groups of vibrotactile stimulations of different durations with the order of: (1) 250 ms, (2) 100 ms, (3) 80 ms, (4) 1000 ms, (5) 50 ms, and (6) 500 ms. Each group of stimulations contains 25 vibratactile feedbacks of the same strength (medium level) from the glove. Each finger is given an equal number of five tactile stimulation signals. The 25 stimulations are given in a random order. The subject should answer the perceived stimulation location as fast as possible and the total testing time of each stimulation group is recorded.
During the second session, each subject is given 15 vibrotactile stimulation of three strength levels (strong, medium, or weak) on the same finger (index). The stimulation duration is fixed to 100 ms. Each of the three stimulation levels are repeated for five times in a random order. The subject should answer the perceived stimulation level as quick as possible. The total time to finish the session is recorded.
Experiment results are presented in this section as a preliminary evaluation of the tactile feedback concept proposed in this study.
4.1 Finger detection
The error rate and adjusted response time (excluding stimulation duration) of distinguishing tactile feedback stimulations applied on the back of five fingers without training is shown in Figure 3. One can observe that in general both the error rate and response time decrease as the stimulation duration increases. With a stimulation duration of 100 ms, subjects have managed to distinguish the majority of displayed finger localization information correctly.
4.2 Force level detection
The experiment results of test subjects using the tactile feedback glove to perceive different levels of stimulation is shown in Figure 4. Because the subjects were tested without a training session in our study, they needed to learn the relative stimulation strength during experiments. It was observed that the medium feedback level was most frequently misjudged.
4.3 Learning effect
Although displaying tactile information on the back of fingers does not feel as natural as on fingertips, response time shows subjects gradually get used to this tactile information display approach. As shown in Figure 5, subjects’ response times tend to decrease as the experiments proceed. We also observed during the second experiment session that mistakes for force level detection happened mostly at the beginning. Moreover, test subjects also reported that they could perceive the information transferred by stimulations easier over time. However, the result tends to be biased by the stimulation duration: durations of 500 ms and 1000 ms also have the shortest response time.
5 Discussion and conclusion
Our preliminary experiments on five healthy subjects show that displaying tactile information on the back of fingers is feasible and intuitive. The displayed information (finger localization and contact force level) can be readily recognized even without training.
Even though it seems to be disturbing to display tactile events occurred at fingertips of the left hand at the back of the right hand, our study shows that subjects learn the tactile display quickly. The adaptability of human body surface to new tactile information has also been observed in previous studies .
The tactile feedback concept proposed in this study is to be evaluated with amputee subjects. We expect that this tactile display method will work well with unilateral hand amputees. When a hand amputee subject manipulates objects with a hand prosthesis, the tactile events are generated actively. As a result, the amputee has a prediction of the tactile feedback before it happens. Therefore, an actively generated tactile event is probably even easier to recognize than a passive one.
The authors would like to thank Mr. Adrian Stirnimann for fabricating the tactile feedback glove prototype.
Research funding: This study was Funded by Nano-Tera.ch, financed by the Swiss Confederation, and scientifically evaluated by the SNSF under the RTD WiseSkin Project (Grant No. 20NA21_143070). Conflict of interest: Authors state no conflict of interest. Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance with the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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About the article
Published Online: 2016-09-30
Published in Print: 2016-09-01
Citation Information: Current Directions in Biomedical Engineering, Volume 2, Issue 1, Pages 399–403, ISSN (Online) 2364-5504, DOI: https://doi.org/10.1515/cdbme-2016-0089.
©2016 Tao Li et al., licensee De Gruyter.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0