Design and Performance Assessment of a Solid-State Microcooler for Thermal Neuromodulation

Abstract

It is well known that neural activity can be modulated using a cooling device. The applications of this technique range from the treatment of medication-resistant cerebral diseases to brain functional mapping. Despite the potential benefits of such technique, its use has been limited due to the lack of suitable thermal modulators. This paper presents the design and validation of a solid-state cooler that was able to modulate the neural activity of rodents without the use of large and unpractical water pipes. A miniaturized thermal control solution based exclusively on solid-state devices was designed, occupying only 5 mm × 5 mm × 3 mm, and featuring the potential for wireless power and communications. The cold side of the device was cooled to 26 °C, while the hot side was kept below 43 °C. This range of temperatures is compatible with brain cooling and efficient enough for achieving some control of neural activity.

Keywords: biomedical microdevice; implantable; microdevice packaging; microsystem integration; neuronal; solid-state cooling; thermal neuromodulation; thermal simulation.

Figure 1

Developed miniaturized thermal neuromodulator. The main elements are described along with the figure. The Peltier element is used to obtain a cold volume; the heat sink, together with the thermal vias, will remove the heat away from the cold side; the control electronics is a chip used to generate the signals to operate the Peltier cooler; the output interface is set on interconnections that allows to interconnect the device with other subsystems, like a battery or an antenna. The non-highlighted aspects are non-relevant interconnections between sub-systems.

Figure 2

Simulation results with Peltier and heatsink operating in free space. The figure shows the temperature along the Peltier element, as well as the heatsink region nearby, which will be the hottest area. In addition to the color map showing the temperature distribution, also shown is the ▼-lower, and ▲-higher temperatures recorded in the structure for the selected time stamps (top—100 mA; middle—200 mA; bottom—400 mA; left—minimum temperature achieved; right—temperature stabilization).

Figure 3

Model that includes the Peltier, heatsink, and rat brain. On the right is the current curve applied to the Peltier over time. The time scale is in seconds and the current levels used were 100 mA, 200 mA, and 400 mA.

Figure 4

Simulation results with the Peltier module touching the rat brain and the heatsink radiating into air. The figure shows the temperature along the Peltier element, as well as the heatsink region nearby, which will be the hottest area. In addition to the color map showing the temperature distribution, it also shows the ▼-lower, and ▲-higher temperatures present in the structure for the selected time stamps (top—100 mA; middle—200 mA; bottom—400 mA; left—minimum temperature achieved; right—temperature stabilization).

Figure 5

Measurements results obtained on a polystyrene phantom (red line—temperature in the hot side; blue line—temperature in the cold side; green line—temperature difference between hot and cold sides), using a device based on the model of Figure 3, and shown in Figure 6.

Figure 6

Setup used to perform the experiments and microcooler prototype. Left side shows a detailed rat head opening and temperature probe placement under the Peltier element, and right side shows Peltier testing on a phantom.

Figure 7

Mean SEP (somatosensory evoked potentials) amplitude in control cooling, and recovery periods. The analysis was based on one-factor ANOVA (amplitude) for N = 10 (five rats with two electrodes per rat) for each period for each current (*: p < 0.05, **: p < 0.01, ***: p < 0.001, N: no difference).

Figure 8

Envisioned miniaturized thermal neuromodulator, and its respective model. The description of elements shown can be found in Figure 1.

Figure 9

Miniaturized thermal neuromodulator, showing the thermal vias. On the left side, the Peltier is highlighted along with the different packaging levels, and the right side shows the details of thermal vias forming the heatsink.

Figure 10

Microcooler performance assessment when operating in free space, with and without thermal vias. The figure shows the temperature along the Peltier element, as well as the heatsink region nearby, which will be the hottest region. In addition to the color map showing the temperature distribution, it is also shows the ▼-lower, and ▲-higher temperatures present in the structure for the selected time stamps.

Figure 11

Microcooler performance assessment when operating inside a volume of water. The figure shows the temperature along the Peltier element, as well as the heatsink region nearby, which will be the hottest region. In addition to the color map showing the temperature distribution, it is also shows the ▼-lower, and ▲-higher temperatures present in the structure for the selected time stamps (left—minimum temperature achieved; right—temperature values for long term operation).

Figure 12

Microcooler performance assessment when operating inside a volume of brain. The figure shows the temperature along the Peltier element, as well as the heatsink region nearby, which will be the hottest region. The inset shows the detail at the Peltier-brain interface, and has a different temperature scale to better display the temperature profile nearby the cooler. In addition to the color map showing the temperature distribution, it is also shows the ▼-lower, and ▲-higher temperatures present in the structure for the selected time stamps (left—minimum temperature achieved; right—temperature values for long term operation).