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. 2019 May 20;19(10):2328.
doi: 10.3390/s19102328.

Multifunctional Freestanding Microprobes for Potential Biological Applications

Nana Yang  1 , Zhenhai Wang  2 , Jingjing Xu  3   4 , Lijiang Gui  5 , Zhiqiang Tang  6 , Yuqi Zhang  7 , Ming Yi  8 , Shuanglin Yue  9 , Shengyong Xu  10
Affiliations

Multifunctional Freestanding Microprobes for Potential Biological Applications

Nana Yang et al. Sensors (Basel). .

Abstract

Deep-level sensors for detecting the local temperatures of inner organs and tissues of an animal are rarely reported. In this paper, we present a method to fabricate multifunctional micro-probes with standard cleanroom procedures, using a piece of stainless-steel foil as the substrate. On each of the as-fabricated micro-probes, arrays of thermocouples made of Pd-Cr thin-film stripes with reliable thermal sensing functions were built, together with Pd electrode openings for detecting electrical signals. The as-fabricated sword-shaped freestanding microprobes with length up to 30 mm showed excellent mechanical strength and elastic properties when they were inserted into the brain and muscle tissues of live rats, as well as suitable electrochemical properties and, therefore, are promising for potential biological applications.

Keywords: biological application; electrode; freestanding micro-probe; stainless steel; thermocouple.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
(a) Overall design of seven 3D micro-probes on a 4 in.-diameter s.s. substrate. (b) Schematic figure of the structure of a multifunctional 3D micro-probe.
Figure 2
Figure 2
Illustration of the fabrication processes of a multifunctional 3D micro-probe. (a) Cleaned stainless-steel (s.s.) substrate. (b) Patterns of the probes with SU-8. (c) Ti–Pd patterns on the probes. (d) Cr patterns added to complete the Pd–Cr thin-film thermocouples (TFTCs) on the probes. (e) Covering with a HfO2 insulating layer, leaving testing opening windows. (f) Additional SU-8 layer to protect the TFTCs. (g) Coating of a mask layer of SPR-220 for the etching process. (h) The s.s. substrate was etched thoroughly after 65 min and cut from the substrate, resulting in a freestanding 3D probe.
Figure 3
Figure 3
AFM micrographs of the substrate surfaces. (a)–(c) Results taken from a cleaned bare SUS 304 s.s. surface. (d)–(f) Results taken from the s.s. substrate covered with a 3 μm-thick SU-8 resist layer.
Figure 4
Figure 4
Optical photographs of the devices during the fabrication process. (a) A whole 4 in. substrate with fabricated TFTC arrays. (b) A whole substrate after the etching process. (c) An enlarged local area showing a narrow bonding bridge connecting the substrate and the 3D probe. (d) A close look at a bonding bridge after the etching process was completed, showing the probes still fixed in their original positions on the substrate.
Figure 5
Figure 5
Optical images and scanning electron microscopy images of the as-fabricated device and assembled device. (a) Measurement zones on the probe before the etching process. (b) A microprobe assembled in an electrical connection socket. (c) SEM image of a measurement zone. (d) SEM image of the front part of a microprobe. (e) SEM micrograph of a probe tip. (f) SEM side-view of a probe tip.
Figure 6
Figure 6
Calibration results for the Pd–Cr TFTC on a 3D microprobe.
Figure 7
Figure 7
A typical electrochemical impedance spectroscopy of a Pd electrode.
Figure 8
Figure 8
A typical stress–strain curve under compression for an as-fabricated 20 mm-long freestanding probe (shown in the inset).
Figure 9
Figure 9
Photographs for the elasticity test of an as-fabricated microprobe with a hard rubber.
See this image and copyright information in PMC

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