A self-stiffening compliant intracortical microprobe

Abstract

Utilising a flexible intracortical microprobe to record/stimulate neurons minimises the incompatibility between the implanted microprobe and the brain, reducing tissue damage due to the brain micromotion. Applying bio-dissolvable coating materials temporarily makes a flexible microprobe stiff to tolerate the penetration force during insertion. However, the inability to adjust the dissolving time after the microprobe contact with the cerebrospinal fluid may lead to inaccuracy in the microprobe positioning. Furthermore, since the dissolving process is irreversible, any subsequent positioning error cannot be corrected by re-stiffening the microprobe. The purpose of this study is to propose an intracortical microprobe that incorporates two compressible structures to make the microprobe both adaptive to the brain during operation and stiff during insertion. Applying a compressive force by an inserter compresses the two compressible structures completely, resulting in increasing the equivalent elastic modulus. Thus, instant switching between stiff and soft modes can be accomplished as many times as necessary to ensure high-accuracy positioning while causing minimal tissue damage. The equivalent elastic modulus of the microprobe during operation is ≈ 23 kPa, which is ≈ 42% less than the existing counterpart, resulting in ≈ 46% less maximum strain generated on the surrounding tissue under brain longitudinal motion. The self-stiffening microprobe and surrounding neural tissue are simulated during insertion and operation to confirm the efficiency of the design. Two-photon polymerisation technology is utilised to 3D print the proposed microprobe, which is experimentally validated and inserted into a lamb's brain without buckling.

Keywords: 3D printing; Buckling; Finite element method; Insertion; Intracortical microprobe; Self-stiffening.

Fig. 1

A side view of the proposed intracortical microprobe with two compressible structures

Fig. 2

(a) The microprobes with one and two compressible structures (CS) and (b) the axial (longitudinal) displacements of the studied microprobes’ base against the applied force in the + z direction

Fig. 3

The 3D printed microprobes with (a) one and (b) two compressible structures on the ITO glass. (c) The cross-sectional view of the fabricated microprobe

Fig. 4

The longitudinal displacements of the proposed microprobe for (a)F = 10 µN, and (b)F = 100 µN. (c) The Von Mises stress distribution at the compressible structures of the proposed microprobe under F = 50 mN

Fig. 5

The maximum principal strain generated on the brain surface when the microprobe is pushed down (a) 0.1 μm and (b) 2 μm during insertion

Fig. 6

(a) The distribution of maximum principal strain with an absolute magnitude more than 0.01 on the surrounding neural tissue of the microprobes with one (left) and two (right) compressible structures under the brain longitudinal motion, W=-30 μm. (b) The normalised maximum strain against the distance from the interface of the neural tissue and the microprobes’ tip in the z and x directions

Fig. 7

(a) The generated maximum principal strain by the proposed microprobe under the brain longitudinal motion, W=-30 μm, for different length ratios between the upper and lower cylinders, (a) 0.5, (b) 1, and (c) 2

Fig. 8

(a) The generated maximum principal strain under the brain longitudinal motion, W=-30 μm, by the microprobes with different numbers of compressible structures, (a) two, (b) three, and (c) four

Fig. 9

The insertion of the microprobe with two compressible structures into the lamb’s brain surface

Fig. 10

The applied compressive force by the Hysitron TI 950 Triboindenter against the longitudinal displacement of the stiff microprobe’s tip