Frontiers in the Convergence of Bioscience and Information Technologies 2007

A Pilot Study of a Thermal Display Using a Miniature Tactor for Upper Extremity Prosthesis

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Keehoon Kim, J. Edward Colgate, Michael A. Peshkin Dept. of Mechanical Eng., Northwestern University, Evanston, IL 60208, USA {keehoon-kim,colgate,peshkin}@northwestern.edu

Abstract

Figure 1. Components of the tactor for the thermal display

INTRODUCTION

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We have been developing a miniature multi-function haptic device - a tactor - that generates the sense of pressure, vibration, shear force, and temperature for upper extremity amputees, especially those who have undergone targeted nerve reinnervation surgery. This paper discusses first the design of the thermal display portion of the tactor, second a pilot study of a thermal display using the tactor.

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The importance of restoring thermal sensation to upper extremity amputees is often considered minimal due to its limited functional role. In other words, thermal sensation is not critical to most activities of daily living (ADLs). However thermal sensation is important socially and emotionally. Amputees often express the desire to feel the warmth of a loved one’s hand, or the sensation of holding a warm cup of coffee. Thus, as a component of a larger effort aimed at restoring haptic sensation to those who have lost an arm or hand, we are exploring thermal display. This work is most relevant in the context of patients who have undergone “targeted reinnervation” (TRI) surgery [1–3]. TRI creates sensory spots on a patient’s skin (for instance the chest or upper arm) that are perceived as spots on the skin of the phantom hand. Thus, by placing sensors on the prosthetic hand and using these to command tactors on the reinnervated skin, it should be possible to create a realistic sense of touch. Since the tactor is attached to the body during daily life, it should be light weight and aesthetic and it should consume little power. Additionally, since the spatial resolution of the sensation created by the tactor depends on its size, it should be designed to be as compact as possible so that multiple tactors can be place in the nerve reinnervated region. The specification and the design constraints are explained in section 2.

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Recently, a number of thermal tactile displays, the vast majority based on thermoelectric (peltier) devices have been reported in the literature [4–6]. However, the devices described in the literature have limited application to our problem due to the constraints of size and power. Most design assumes that i) the peltier device has enough power to control the temperature of the contact surface and ii) the thermal systems have an infinite heat sink. However, since both the peltier device and heat sink are size-limited in our situation, we need to consider factors such as the patient’s perception of small temperature changes as well as heat flux in and out of the heat sink.

0-7695-2999-2/07 $25.00 © 2007 IEEE DOI 10.1109/FBIT.2007.88

In section 2, overall tactor is introduced briefly and the design specifications including constraints of power and size are discussed. Section 3 presents the hardware used to implement a thermal display and discusses the design of the heat spreader and the heat sink. Section 4 discusses the temperature discrimination of two nerve re-innervated amputees and a normal person. This is followed by concluding remarks in section 5.

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Table 1. Human skin characteristics for temperature sensing [7] [8] temperature (◦ C) 32 > 48 40 ∼ 48 15 ∼ 20 < 15

operation range size of thermal displayer resolution of temperature change heat flux

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Design Specifications and Constraints

15 ∼ 40◦ C 4.8mm × 11.0mm 0.3◦ C/sec 3.5 × 104 W atts

Peltier device and thermal sensor selection

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temperature on fingertips hot pain warm sensation cool cold pain

Table 2. Required specification of the thermal display

For a thermal display system, the most important elements are heat flux generators (peltier devices) and temperature sensors. As explained in section 2, there are size and power constraints that limit the selection of a peltier device. Table 3 shows commercial products that satisfy the size and power constraints. For the developed thermal display, part number TE-8-0.45-1.3 from TE technology, will be used for two main reasons: i) Ferrotec does not supply a data sheet for a heat flux function of temperature difference and current input; ii) TE-8-0.45-1.3 has smaller size and high efficiency. There are several ways to detect temperature, heat flux sensors, thermistors, and thermocouples. Though the operation range is not a major consideration, response time, sensitivity, and dimension are important factors to satisfy the performance requirements. Table 4 shows several commercial sensors and their specifications. Thermistors have good resolution less than 0.01◦ C. However slow time constants lager than 1.0sec so that it is difficult to implement closed loop control for a rapidly changing temperature profile. Heat flux sensors have the advantage of measuring the key control variable - heat flux - directly. However these devices don’t satisfy the size constraint and exhibit time constant larger than 0.6sec. 5TC-TT-J-40-36, OMEGA, has a very small size so that response time is less than 0.001sec if 0.08 diameter spherical lumped model is assumed. However, in order to calculate the temperature from the thermocouple, voltage amplifier is required. Nonetheless, this is the component that we selected.

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Fig.1 shows the configuration of a tactor head, peltier device, and a heat spreader. The tactor mechanism uses a double parallelogram linkage to keep the tactor head always parallel to the skin surface. Because the peltier device and heat spreader are mounted to the most distal link of this mechanism - which we call the “tactor head”, that link serves as a heat sink. For the peltier device, 5.2mm ×9.6mm is allowed on the tactor head. The sizing of the heat spreader will be explained in section 3. A peltier device is an efficient way to generate positive and negative heat flux from electric input. In order to size a peltier device, it is necessary to understand the heat flux requirements as well as the range of temperatures that must be displayed. Table 1 summarizes the human skin characteristics for temperature sensing. Though the objective of the thermal display is to transfer a realistic thermal sensation to a patient, it is not desirable to display painful sensation. Therefore, the operation range of temperature is chosen to be 15◦ C to 40◦ C. Ho and Jones have studied the heat flux between skin and various materials through simulations using semi-infinite body model [4, 5]. In order to differentiate a copper material1 from others under 1.4 × 104 Pa contact pressure between 24◦ C material and 32◦ C human skin, 3.05 × 104 W/m2 heat flux power is required. Table 2 shows the minimum specification for a thermal displayer.

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Design Procedure

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Size of heat spreader

The surface of heat spreader contacts the human skin and transfers the heat flux from the peltier device to the skin. It has a smooth shape for comfort, and a larger area than the peltier device in order to enhance perception. It is known, however, that temperature discrimination depends on the area of thermal stimulator [12]. Therefore, in order to determine an appropriate dimension for the heat spreader, a temperature discrimination test was performed.

This section presents the key design decisions for the thermal display portion of the tactor. We address first how we selected a peltier device and a thermal sensor, second the procedure used to size the heat spreader is described.

1 Among the materials simulated in the literature, cooper results in the highest heat flux for a given temperature differential between it and human skin.

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Table 3. Commercial Peltier Devices and Their specification product TE-8-0.45-1.3 (TE [9]) 9500/007/018M (Ferrotec [10])

maximum power consumption(W) 0.7 1.8

maximum heat flux(Watts/m2 ) 3.46 × 104 5.87 × 104

dimension 3.4 × 3.4 × 2.3mm3 4.0 × 4.0 × 2.1mm3

Table 4. Commercial Temperature Sensor and Their specification resolution 0.01◦ C 6.5µV /Btu/F t2 − Hr 0.05mV /◦ C in the operation range

time constant > 1.0sec > 0.6sec < 0.001sec for 0.08mm lumped model

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product 44033 thermistor(OMEGA [11]) HFS-3 heat flux sensor(OMEGA) 5TC-TT-J-40-36 thermocouple(OMEGA)

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ately after the heat transient time. For cooling, discrimination threshold is not much affected by either the size of the heat spreader or stabilization time. Although this experiment suggests that a larger diameter heat spreader is desirable, larger diameter also turn out to increase the power requirement for displaying normal pressure. Moreover, the improvement beyond 8.0mm are modest, thus, an 8.0mm diameter was chosen for the heat spreader. For the tactor, 5.2mm×9.6mm (equivalent to 8.0mm diameter) heat spreader has been chosen.

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Figure 2. Protocol for the experiment of temperature discrimination using different size of heat spreader to a nerve re-innervated amputee.

Temperature Discrimination Test

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Using the thermal display, the response of two TRI patients and a non-amputee to temperature transients was measured. In the experiments, subjects were asked to classify a thermal stimulus as having one of 7 levels of sensation, i.e., ‘feel nothing’, ‘warm-’,‘warm0’,‘warm+’,‘hot’,‘hot0’, and ‘hot+’. The last of these, ’hot+’, indicates that the subject feels that any further heating might cause pain. In all cases, the temperature was carefully controlled to ensure that no actual tissue damage would occur, and subjects were free to remove the display or stop the experiment at any time. Fig.3 shows the temperature profile applied to the subjects’ skin. In this experiment, the rate of change of temperature, σ and the peak temperature, ∆T , are considered simultaneously. It is known that humans cannot feel the temperature as effectively when the rate of change is fast. The experiment has been performed with two TRI subjects’ chest and a non-amputee’s chest and fingertip. Fig.4 shows the results of the experiment for two TRI subjects (TRI-1 and TRI-2) and a non-amputee’s chest and fingertip (NTRI-C and NTRI-F). As we expected, if the rate of change of temperature is higher, subjects feel the temperature change better. For ex-

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For the experiment, 3 different sizes of heat spreader were used: 5.0mm, 8.0mm, and 10.0mm diameter. The subject is an amputee who had undergone TRI on his chest. Fig.2 shows the protocol for the experiment. Initially, since the subject’s skin temperature was 32◦ C, the base temperature of the heat spreader was set to 32◦ C. From the base temperature, the temperature was increased to a desired temperature in 1 second. The stimulation temperature was maintained for given stabilization time. In this experiment, two lengths of stabilization time were applied: 4.0 second and 8.0 second. The subject was given longer than 5 seconds between each stimulation. In order to search for the threshold temperature, a 0.5◦ C resolution was used. Table5 shows the results of the experiment. For heating, larger diameter of heat spreader makes better discrimination. The stabilization time makes no difference. The subject felt the heating and cooling temperature immedi-

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Table 5. Temperature discrimination of nerve re-innervated amputee under different stabilization time and diameter of heat spreader. Heating

Cooling

4sec

8sec

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◦

12 C 8.5◦ C 7.0◦ C

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4.0 C 5.0◦ C 4.5◦ C

8sec 4.0◦ C 4.0◦ C 4.5◦ C

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12 C 8.5◦ C 7.5◦ C

4sec

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hhh hhh Stabilization time hhh hhh Diameter hh h 5mm 8mm 10mm

(d)

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Figure 4. Experimental results of heat flux transients:(a) TRI-1, (b) TRI-2, (c) a non-amputee’s chest (NTRI-C), (d) a non-amputee’s fingertip (NTRI-F)

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Figure 3. Temperature profile for the experiment of thermal sensation. f (t) = ∆T exp(−0.5(t − c)2 /σ 2

Concluding Remarks

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TRI-1’s warm sensation range is as wide as NTRI-F. TRI2’s range is as narrow as NTRI-C. Therefore, we can conclude that TRI-2’s thermal sensation is similar to a normal chest rather than a normal fingertip. We can think two possibilities to explain the difference: i) The difference depends on the location of the stimulus. ii) The TRI procedures of TRI-1 and TRI-2 are different. Since experiments have been done on a specific region which corresponds the location in Fig.5, the first possibility can be eliminated. For the second possibility, there are differences of the TRI surgeries. TRI-1 and 2 have the hybrid representation that encompasses both the intrinsic neural regeneration through the muscle. However, TRI-1 has reinnervation from neural conduits. Thus, TRI-1 does have a large region of near normal (in comparison to hand) mechanosensory thresholds whereas TRI-2s thresholds are much higher.

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We have been developing a miniature tactor to display multi-modal haptic sensations to to the skin of upper extremity amputees, especially those who have undergone targeted nerve reinnervation (TRI) surgery. This paper presents the design of the thermal display portion of the tactor. The design of the thermal display uses a heat spreader a peltier device, a thermocouple, and a heat sink subject to the limited allowed size and power consumption. The components of the thermal display were determined considering the minimum required heat flux and response time. In order to determine the heat spreader size, the temperature threshold test has been done with a TRI amputees. Using the designed temperature display, we have done temperature discrimination test to two TRI patients and a non-amputees’s fingertip and chest. The experimental results confirmed that the thermal sensation depends on the rate of temperature change as well as the peak temperature. In addition, we can conclude that the thermal sensation pattern depends on the stimulation spot. According to the tests with the TRI patients, the TRI regions of TRI-1 and TRI 2 were similar to a non-amputee’s fingertip (wide warm sensation region) and a non-amputee’s chest (narrow sensation region) even though the TRI subjects perceived the same region of the phantom hand. In this pilot study, since we have done the experiments with a few subject, it is difficult to say that the results are reliable enough. However, we have been encouraged to investigate the thermal sensation patterns for the dexterous temperature display through more temperature discrimination tests with a number of subjects (including non-amputees as well as TRI patients) for the future work.

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Figure 5. Shaded regions correspond to perceived location of stimulus on a phantom hand

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ample, in Fig.4-(a), for the same 8◦ C stimulation, TRI-1 felt different sensations, i.e., hot when σ = 1 and warm when σ = 10. From Fig.4-(c) and 4-(d), NTRI-F has wider warm sensation region than NTRI-C. In thermal display standpoint, it is easier to display abundant thermal sensation from warm to hot sensation to the fingertip compared to the chest. In other words, it is hard to display warm sensation to the chest since the thermal sensation jumps to hot pain easily. For the thermal display to the TRI subjects, we stimulated the spot that corresponds to the same perceived location at the beginning of the test, i.e., the region 1 in Fig.5. The subjects felt that the stimulation region spreads out to the region 2 when the stimulation temperature is getting higher. However, the results of the thermal display to the TRI subjects show the different thermal sensation pattern.

Acknowledgement This research was performed as part of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) Revolutionizing Prosthetics 2009

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funded by the Defense Advanced Research Projects Agency (DARPA) Contract # N66001-06-C-8005. We also wish to thank Todd A. Kuiken, Laura A. Miller, and Paul D. Marasco for their assistance and advice.

[11] Omega engineering, inc. http://www.omega.com/

[Online].

Available:

[12] D. Kenshalo, T. Decker, and A. Hamilton, “Spatial summation on the forehead, forearm, back produced by radiant and conducted heat.”

References

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[1] T. A. Kuiken, L. A. Miller, R. D. Lipschutz, K. A. Stubblefield, and G. A. Dumanian, “Prosthetic command signals following targeted hyper-reinnervation nerve transfer surgery,” in Proc. of IEEE Enginerring in Medicine and Biology 27th Annual Conference, 2005.

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[2] T. A. Kuiken, G. A. Dumanian, R. D. Lipschutz, L. A. Miller, and K. A. Stubblefield, “Targeted reinnervation for enhanced prosthetic arm function in a woman with a proximal amputation: a case study,” vol. 369, pp. 371–380, Feb. 2007.

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[3] T. A. Kuiken, L. A. Miller, R. D. Lipschutz, B. A. Lock, K. S. eld, P. D. Marasco, P. Zhou, and G. A. Dumanian, “The use of targeted muscle reinnervation for improved myoelectric prosthesis control in a bilateral shoulder disarticulation amputee.”

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[4] H.-N. Ho and L. A. Jones, “Thermal model for handobject interactions,” in Proc. Symposium on Haptic Interface for Virtual Environment and Teleoperator Systems, 2006.

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[5] ——, “Infrared thermal measurement system for evaluating model-based thermal displays,” in Proc. Symposium on Haptic Interface for Virtual Environment and Teleoperator Systems, 2007.

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[6] A. Yamamoto, B. Cros, H. Hashimoto, and T. Higuchi, “Control of thermal tactile display based on prediction of contact temperature,” in Proc. IEEE international conference on robotics and automation, 2004. [7] D. G. Caldwell, N. Tsagarakis, and A. Wardle, “Mechano thermo and proprioceptor feedback for integrated haptic feedback,” in Proc. IEEE international conference on robotics and automation, 1997. [8] I. Darian-Smith, Handbook of Physiology-The Nervous System. American Physiological Soc., 1984. [9] TE technology, inc. http://www.tetech.com/

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Available: Available:

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