**3. Sensory feedback by electrical stimulation**

### **3.1. Purpose**

(a)Naive (b)Neuron transplantation (c)Surgical control

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

**Figure 3.** Controlled reproduction of ankle motion and measurements of dorsiflexed angles. The ankle motions were restored with electrical stimulation of the peroneal nerve in neural transplantation group (A) as compared to surgical

The transplanted rat embryonic motoneurons could survive in the rat peripheral nerve, and transplanted motoneurons formed functional motor units. Transplantation of motoneurons into peripheral nerve provided the reproduction of ankle dorsiflexion. The transplantation of embryonic neurons into peripheral nerve could restore ankle motions in conjunction with electrical stimulation, even though no neural connection between central nervous system and

Althoughallratswere able toflex their ankle,the smallnumberof axonswaspresentinthe tibial nerves of cell transplantation group as compared with naive group. Endoneurial environment in peripheral nerve may not provide suitable condition for motoneurons. Transplanting

**Figure 2.** Myelinated axons in tibial nerve were assessed with toluidine blue stain.

control group (B).

Biomedical Engineering

18

**2.4. Discussion**

muscle were present.

The purpose of this section is to show the possibility of sensory feedback in nueroprosthetic devices. If we have a way to send the signals from the assembled sensors to sensory nervous

system, leading neuroprosthetic devices with two‐way communication will be achievable. In this Section, we show a preliminary study on the possibility of sensory feedback via axial fibers of peripheral sensory neurons. We have tested several patterns of electrical signals and the placements of surface electrodes for carrying sensory information to brain via axial fibers of neurons. In a preliminary experiment with one channel electrical stimulation, the amplitude modulation of the stimulation voltage achieves only 2 bit resolution. Then, we increase the channels and take a method of frequency modulation of the stimulation electrical signal.

The applied voltages were tuned for each subject in similar way for the good identifiabil‐ ity. The electrical stimulation of surface electrodes with about 2000 Hz gives some feeling of vibration around at the electrodes without any pain. The stimulation frequency of the channel (C, D) was set at five frequencies: 2000 Hz, 2001 Hz, 2002 Hz, 2004 Hz and 2010 Hz. The change of stimulation frequency on the channel (C, D) causes different sensa‐ tions to the subjects. More specifically, the subjects sense the frequency difference be‐ tween the two channels in addition to the base frequency 2000 Hz. In other words, the subjects sense the interference potential of the pair electrical potentials. For an example, 1 Hz sensation will be felt by the subject if the frequencies of 2000 Hz and 2001 Hz are selected. We applied electrical stimulations of 2000 Hz to a subject by using the pair of the electrodes (A, B), and set the same and different frequencies for the pair of the electro‐ des (C, D) for the discrimination experiments. The phase shift was zero degree when the both frequency were set at 2000 Hz. After enough trials for each combination of the stimulation frequencies, each subject carried out 40 trials of the discrimination. In the trials, all combinations of frequencies were presented in random order. Five subjects were asked

Neural Interfaces: Bilateral Communication Between Peripheral Nerves and Electrical Control Devices 21

The average rate of the correct answers over 40 tests per subject was 90%. All errors were occurred in the cases of nearest frequencies difference of the two pairs electrodes. This results show that the difference of the two channel stimulation frequencies is key point for the identifiability of stimulations. The bigger difference in stimulation frequencies, the higherrate

The simple amplitude modulation of one channel electrical stimulation achieves only 2 bits resolution for transferring sensory information to brain. On the other hand, frequency modulation with interference potential of the pair electrical potentials has achieved a high resolution which corresponds to the number of combination of given different stimulation frequencies. For an example, we can obtain 3.5 bits (1 digit in decadal system) resolution if we use five different frequencies. We can increase number of the channels; thus, interference potential of the pair electrical signals with more than two pairs of electrodes holds forth the possibility that we can achieve a high resolution for sensory feedback via axial fibers of

Further research on multi‐channel electrodes, specially more than three channels, will be required to clarify the effect of multiple interference of stimulation frequencies on the sensory resolution of transferring the information. The frequency range which is most appropriate for transferring sensory information should be also clarified. It is common that electrical stimulations with a certain frequency band involve a pain of the subject. Appropriate frequency bands should be identified not only to achieve a high resolution,

to identify the combinations of the electrical stimulations.

of the correct answers will be achieved.

**3.3. Results**

**3.4. Discussion**

peripheral sensory neurons.

but to avoid subjectʹs pain.

### **3.2. Methods**

One pair of surface electrodes, one anode and one cathode, is usually used for one channel electrical stimulation. The two pairs of surface electrodes in our experiments are shown in Figure 4a. The placements of the electrodes were well controlled for each experiment to certify the repeatability based on the markers on skin shown in Figure 4b. We set the two pairs of electrodes (A, B) and (C, D), and made the electrical current flow from A to B or from C to D in each pair. The duty ratio of stimulation voltages was 5%, and the frequency was set at 2000 Hz for the channel (A, B). The duty ratio 5% was defined because we found it looks good for all five subjects to identify the stimulation frequency in several trials with one channel stimulation.

**Figure 4.** The electrodes placement

The applied voltages were tuned for each subject in similar way for the good identifiabil‐ ity. The electrical stimulation of surface electrodes with about 2000 Hz gives some feeling of vibration around at the electrodes without any pain. The stimulation frequency of the channel (C, D) was set at five frequencies: 2000 Hz, 2001 Hz, 2002 Hz, 2004 Hz and 2010 Hz. The change of stimulation frequency on the channel (C, D) causes different sensa‐ tions to the subjects. More specifically, the subjects sense the frequency difference be‐ tween the two channels in addition to the base frequency 2000 Hz. In other words, the subjects sense the interference potential of the pair electrical potentials. For an example, 1 Hz sensation will be felt by the subject if the frequencies of 2000 Hz and 2001 Hz are selected. We applied electrical stimulations of 2000 Hz to a subject by using the pair of the electrodes (A, B), and set the same and different frequencies for the pair of the electro‐ des (C, D) for the discrimination experiments. The phase shift was zero degree when the both frequency were set at 2000 Hz. After enough trials for each combination of the stimulation frequencies, each subject carried out 40 trials of the discrimination. In the trials, all combinations of frequencies were presented in random order. Five subjects were asked to identify the combinations of the electrical stimulations.

#### **3.3. Results**

system, leading neuroprosthetic devices with two‐way communication will be achievable. In this Section, we show a preliminary study on the possibility of sensory feedback via axial fibers of peripheral sensory neurons. We have tested several patterns of electrical signals and the placements of surface electrodes for carrying sensory information to brain via axial fibers of neurons. In a preliminary experiment with one channel electrical stimulation, the amplitude modulation of the stimulation voltage achieves only 2 bit resolution. Then, we increase the channels and take a method of frequency modulation of the stimulation electrical signal.

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

One pair of surface electrodes, one anode and one cathode, is usually used for one channel electrical stimulation. The two pairs of surface electrodes in our experiments are shown in Figure 4a. The placements of the electrodes were well controlled for each experiment to certify the repeatability based on the markers on skin shown in Figure 4b. We set the two pairs of electrodes (A, B) and (C, D), and made the electrical current flow from A to B or from C to D in each pair. The duty ratio of stimulation voltages was 5%, and the frequency was set at 2000 Hz for the channel (A, B). The duty ratio 5% was defined because we found it looks good for all five subjects to identify the stimulation frequency in several trials with one channel

**3.2. Methods**

Biomedical Engineering

20

stimulation.

**Figure 4.** The electrodes placement

The average rate of the correct answers over 40 tests per subject was 90%. All errors were occurred in the cases of nearest frequencies difference of the two pairs electrodes. This results show that the difference of the two channel stimulation frequencies is key point for the identifiability of stimulations. The bigger difference in stimulation frequencies, the higherrate of the correct answers will be achieved.

#### **3.4. Discussion**

The simple amplitude modulation of one channel electrical stimulation achieves only 2 bits resolution for transferring sensory information to brain. On the other hand, frequency modulation with interference potential of the pair electrical potentials has achieved a high resolution which corresponds to the number of combination of given different stimulation frequencies. For an example, we can obtain 3.5 bits (1 digit in decadal system) resolution if we use five different frequencies. We can increase number of the channels; thus, interference potential of the pair electrical signals with more than two pairs of electrodes holds forth the possibility that we can achieve a high resolution for sensory feedback via axial fibers of peripheral sensory neurons.

Further research on multi‐channel electrodes, specially more than three channels, will be required to clarify the effect of multiple interference of stimulation frequencies on the sensory resolution of transferring the information. The frequency range which is most appropriate for transferring sensory information should be also clarified. It is common that electrical stimulations with a certain frequency band involve a pain of the subject. Appropriate frequency bands should be identified not only to achieve a high resolution, but to avoid subjectʹs pain.

segments. It is possible to extend this type of neuroprosthetic system with portable sensors

Neural Interfaces: Bilateral Communication Between Peripheral Nerves and Electrical Control Devices 23

The purpose of this section is to conduct walking simulation which will play an importantrole of our succedent neuroprosthetic system. The basic concept of the neuroprosthetic system is shown in Figure 5. The role is to check whether the designed artificial CPG controller works adequately or not, and whether the sensing system is able to provide necessary information forthe controllerʹs adapting to the environmental changes. Thus, the walking simulation given in this section means the preliminary study of motion planning and control for our succedent

The neuro‐musculo‐skeletal model of rat is composed of a rigid link system, joint torque generators, neural controllers of CPGs with the upper level triggers. The inertial properties of the entire body are represented by a three dimensional 17‐rigid‐link system with 16 joints. All the joints are one degree‐of‐freedom, and the axes of the joints are perpendicular to sagittal plane. The schematic views in sagittal and horizontal planes are given in Figure 6. The dappled

A viscoelastic passive moment induced by soft tissues and a passive moment of nonlinear elasticity induced by tendons and bones assumed to act on each joint. The characteristic is

*k k*

parameters are constants representing the joint characteristics. These parameters were determined from the estimations for human joints [23]. The inertia properties of each body segment, such as the mass and the moment of inertia, were estimated from the size of anato‐

(a)view in sagittal plane (b)view in horizontal plane

x

50mm

 

(1)

*k c*

*<sup>i</sup>* is the *i*‐th joint angular velocity variable, and another

x

z

y

1 23 4 5 <sup>6</sup> , exp exp *J JJ J J J J i i i ii i i i i i i i i ii passive k k k*

 

.

mized each body segment with the average density of rat body [24].

y

z

110mm

and other electrical devices to more practical one for disabled persons.

neuroprosthetic system.

given by

60mm

**4.2. Neuro‐musuculo‐skeletal model**

 

**Figure 6.** Assumed skeletal system of rat.

where *θ<sup>i</sup>* is the *i*‐th joint angle variable, *θ*

circles indicate the center of mass of each segment.
