Insertion shuttle with carboxyl terminated self-assembled monolayer coatings for implanting flexible polymer neural probes in the brain

Abstract

Penetrating microscale microelectrodes made from flexible polymers tend to bend or deflect and may fail to reach their target location. The development of flexible neural probes requires methods for reliable and controlled insertion into the brain. Previous approaches for implanting flexible probes into the cortex required modifications that negate the flexibility, limit the functionality, or restrict the design of the probe. This study investigated the use of an electronegative self-assembled monolayer (SAM) as a coating on a stiff insertion shuttle to carry a polymer probe into the cerebral cortex, and then the detachment of the shuttle from the probe by altering the shuttle's hydrophobicity. Polydimethylsiloxane (PDMS) and polyimide probes were inserted into an agarose in vitro brain model using silicon insertion shuttles. The silicon shuttles were coated with a carboxyl terminal SAM. The precision of insertion using the shuttle was measured by the percentage displacement of the probe upon shuttle removal after the probe was fully inserted. The average relative displacement of polyimide probes inserted with SAM-coated shuttles was (1.0+/-0.66)% of the total insertion depth compared to (26.5+/-3.7)% for uncoated silicon shuttles. The average relative displacement of PDMS probes was (2.1+/-1.1)% of the insertion depth compared to 100% (complete removal) for uncoated silicon shuttles. SAM-coated shuttles were further validated through their use to reliably insert PDMS probes in the cerebral cortex of rodents. This study found that SAM-coated silicon shuttles are a viable method for accurately and precisely inserting flexible neural probes in the brain.

Fig. 1

IR spectroscopy. Gray: control Si wafer coated with 100 Å Ti and 1000 Å Au. Black: Si wafer coated with 100 Å Ti, 1000 Å Au, and 11-mercaptoundecanoic acid. Three peaks are seen at wavenumbers 2919 cm⁻¹, 2851 cm⁻¹, 1714 cm⁻¹. Peaks indicate that 11-mercaptoundecanoic acid was present on the wafer and the shuttle.

Fig. 2

(A) The flexible probe and SAM-coated insertion shuttle were inserted together with forceps. (B) The polymer probe was then peeled from the top of the insertion shuttle to the base of the tissue surface. (C) A drop of ACSF was used to help separate the polymer from the insertion shuttle. (D) The insertion shuttle was then slowly explanted using forceps.

Fig. 3

Polymer probes prepared on SAM-coated insertion shuttle. (A) 196 µm wide polyimide probe. (B) 200 µm wide PDMS probe. (C) Tip of polyimide probe on the shuttle. (D) Tip of clear PDMS probe on the shuttle, the clear PDMS probe can be identified by its white outline.

Fig. 4

Polymer probes were inserted ~8.5 mm deep and imaged though 0.5% agarose gel. Left, after implantation, before shuttle explantation. Right, after shuttle explantation. (A–C) 196 µm wide polyimide probes. (D and E) 200 µm wide PDMS probes. (A and D) Probes implanted with the SAM-coated insertion shuttle. (B and E) Probes implanted with the SAM-coated insertion shuttle imaged from the side demonstrating no measurable deflection occurs during shuttle insertion or explantation. (C) Polyimide probe implanted with a non-coated insertion shuttle. (E) Blue dye was used to help visualize the PDMS probe. The PDMS probe can be seen as a white track in the blue dye. PDMS probes inserted with non-coated insertion shuttle resulted in explantation of the PDMS probe (not shown).

Fig. 5

A craniotomy was made over the motor cortex. Dura was cut and folded back. A clear PDMS probe was inserted into the rat motor cortex using the insertion shuttle. Left; PDMS probe was released from the shuttle and then the shuttle was removed leaving the PDMS probe only. Arrow indicates the point of insertion. Right; shading highlights the location of the clear PDMS probe.