Name: insertion of flexible neural probes using rigid stiffeners attached with biodissolvable adhesive
Uploaded: 2013-09-27T13:00:00
Description: Watch this Scientific Journal Video about Insertion of Flexible Neural Probes Using Rigid Stiffeners Attached with Biodissolvable Adhesive at JoVE.com

This work was supported by NIH NIDCD Y1-DC-8002-01. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

1. Assembly of Probe to Stiffener

	This section of the protocol describes fabrication of a silicon stiffener, and the assembly of a thin-film polymer probe to the stiffener. Figure 1 illustrates a typical polymer neural probe along with the proposed stiffener. The details of the stiffener design are shown in Figure 2. The novel feature of this design is the shallow &quot;wicking&quot; channel running along its length which is used to distribute liquid adhesiv...

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The method described here provides a well-controlled process to attach thin-film polymer probes to separate stiffeners with a biodissolvable adhesive. Also presented is the recommended surgical procedure to implement these removable stiffeners and a technique to validate the procedure in vitro for a given probe-stiffener configuration. Since the stiffener can be made arbitrarily rigid, the method can facilitate insertion of relatively long probes (&gt;3 mm). As such, the insertion method is expected to be an e...

Microelectrode arrays are an essential tool in neuroscience as well as emerging clinical applications such as prosthetics. In particular, penetrating micro-electrode probes enable stimulation and recording of neuronal activity through close contact with cells in the brain, spinal cord, and peripheral nerves. A major challenge for implanted neural probes is stability and longevity of the stimulation and recording functions. Modeling and experimental studies of the interaction between microelectrode probes and neural tissue have suggested that one mechanism for degradation is micro-tearing of neural tissue due to slight relative motion between the probe and tissue 1-3. One solution is to fabricate flexible probes that match more closely the bulk stiffness properties of neural tissue in order to minimize relative micromotion. As such, biocompatible thin film polymers such as polyimide and parylene have been adopted as favorable substrates for microelectrode probes 4-8.

	A tradeoff of flexible probes is that they are difficult to insert into the neural tissue. Researchers have taken various approaches to facilitate insertion of flexible probes while preserving the desirable mechanical properties. One class of designs modifies the polymer probe geometry to increase stiffness in certain sections or axes while maintaining compliance in other parts. This has been accomplished by incorporating ribs or layers of other materials 9,10. Another approach integrates a 3-D channel into the polymer probe design that is filled with biodegradable material 11. This probe can be temporarily stiffened, and after insertion the material in the channel dissolves and drains out. However, methods such as these that permanently modify the geometry of the final implanted device may compromise some of the desirable features of the flexible probe.

	One method that does not alter the final probe geometry is to encapsulate the polymer device with biodegradable material to temporarily stiffen the device 12-14. However, typical biodegradable materials have Young&#39;s moduli orders of magnitude smaller than that of silicon and would consequently require larger thickness to achieve the same stiffness. Adequately coating the probe can result in a more rounded or blunt tip, making insertion more difficult. Also, since dissolvable coatings are exposed, there is a risk of them dissolving immediately upon contact, or even close proximity, with the tissue.

	Another class of methods uses novel probe substrate materials that reduce in stiffness after being implanted into tissue. Such materials include shape memory polymers 15 and a mechanically adaptive nanocomposite 16. These materials are able to decrease in elastic modulus significantly after insertion, and can result in probes that more closely match the mechanical properties of neural tissue. However, the achievable range of stiffness is still limited, so they may not be able to provide very high stiffness equivalent to silicon or tungsten wires. Thus in the case of flexible probes that are very long (e.g. &gt;3 mm) or that have extremely low stiffness, a method of temporarily attaching a more rigid stiffener may still be required.

	Yet another promising method reported is to coat a stiffening shuttle with a permanent self-assembling monolayer (SAM) to customize the surface interaction between the shuttle and the flexible probe 17. When dry, the probe adheres to the coated shuttle electrostatically. After insertion, water migrates onto the hydrophilic surface, separating the probe from the shuttle so that the shuttle can be extracted. Shuttle extraction with reduced probe displacement was demonstrated (85 &mu;m). However, with only electrostatic interactions holding the probe to the shuttle, there is some risk of probe slippage relative to the shuttle before and during insertion.

	We have developed a method in which the flexible probe is attached to a stiffener with a temporary biodissolvable adhesive material that securely holds the probe during insertion. The probes used were made of polyimide, which has an elastic modulus on the order of 2-4 GPa. The stiffener was fabricated from silicon, with an elastic modulus of ~200 GPa. When attached, the stiffness of the silicon dominates, facilitating insertion. Once inserted into the tissue, the adhesive material dissolves and the stiffener is extracted to restore the probe to its initial flexibility. We selected polyethylene glycol (PEG) as the biodissolvable adhesive material. PEG has been used in implanted applications such as neural probes, tissue engineering, and drug delivery 11,18,19. Some evidence has suggested that PEG may attenuate neuroinflammatory response in brain tissue 18,20. Compared to other possible materials, including sucrose, poly lactic-co-glycolic acid (PLGA), and polyvinyl alcohol (PVA), PEG has a dissolution time in biological fluids that is of an appropriate scale for many implant surgeries (on the order of tens of minutes, depending on molecular weight). In addition, it is solid at room temperature and liquid at temperatures ranging from 50-65 &deg;C. This property makes it particularly suitable for our precision assembly process. Moreover, similar to the SAM described in 17, the dissolved PEG is hydrophilic, facilitating extraction of the stiffener. This advantageous approach is enabled by a novel stiffener design and methodical assembly process which ensure uniform adhesive coverage and accurate and repeatable alignment. In addition to the assembly process, we present the method of implementing the removable stiffener during surgery, as well as an in vitro procedure to evaluate extraction of the stiffener.

	The protocol presented herein assumes that the user possesses a flexible polymer microelectrode probe. The part of the protocol relating the fabrication of the stiffener and assembly of this probe to a stiffener assumes access to common tools found in a microfabrication facility. The protocol relating to insertion and extraction would likely be performed in a neuroscience-oriented laboratory.

<table border="1">
		<tbody>
		 <tr>
			<td>Name of Reagent/Material</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		 </tr>
		 <tr>
			<td>Polyethylene glycol, 10,000 g/mol</td>
			<td>Sigma Aldrich</td>
			<td>309028</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Agarose</td>
			<td>Sigma Aldrich</td>
			<td>A9539</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Flexible Sub-micron Die Bonder</td>
			<td>Finetech</td>
			<td>Fineplacer lambda</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Micromanipulator</td>
			<td>KOPF</td>
			<td>1760-61</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Digital Microscope</td>
			<td>Hirox</td>
			<td>KH-7700</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Dual Illumination Revolver Zoom Lens</td>
			<td>Hirox</td>
			<td>MXG-2500REZ</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Precision Motorized Actuator</td>
			<td>Newport</td>
			<td>LTA-HS</td>
			<td>w/ CONEX-CC controller</td>
		 </tr>
		</tbody>
 </table>

insertion of flexible neural probes using rigid stiffeners attached with biodissolvable adhesive

Microelectrode arrays for neural interface devices that are made of biocompatible thin-film polymer are expected to have extended functional lifetime because the flexible material may minimize adverse tissue response caused by micromotion. However, their flexibility prevents them from being accurately inserted into neural tissue. This article demonstrates a method to temporarily attach a flexible microelectrode probe to a rigid stiffener using biodissolvable polyethylene glycol (PEG) to facilitate precise, surgical insertion of the probe. A unique stiffener design allows for uniform distribution of the PEG adhesive along the length of the probe. Flip-chip bonding, a common tool used in microelectronics packaging, enables accurate and repeatable alignment and attachment of the probe to the stiffener. The probe and stiffener are surgically implanted together, then the PEG is allowed to dissolve so that the stiffener can be extracted leaving the probe in place. Finally, an in vitro test method is used to evaluate stiffener extraction in an agarose gel model of brain tissue. This approach to implantation has proven particularly advantageous for longer flexible probes (&gt;3 mm). It also provides a feasible method to implant dual-sided flexible probes. To date, the technique has been used to obtain various in vivo recording data from the rat cortex.

This insertion technique was used in conjunction with LLNL thin-film polyimide probes, which have passed ISO 10993 biocompatibility standards and are intended for chronic implantation. A typical thin-film polyimide probe is illustrated in Figure 1 along with a silicon stiffener that is approximately 10 mm long in the narrow region. This stiffener has one wicking channel running along its length, as shown in Figure 2. Figure 3 illustrates the mimcrofabrication process u...

Watch this Scientific Journal Video about Insertion of Flexible Neural Probes Using Rigid Stiffeners Attached with Biodissolvable Adhesive at JoVE.com

Insertion of Flexible Neural Probes Using Rigid Stiffeners Attached with Biodissolvable Adhesive

Insertion of flexible neural microelectrode probes is enabled by attaching probes to rigid stiffeners with polyethylene glycol (PEG). A unique assembly process ensures uniform and repeatable attachment. After insertion into tissue, the PEG dissolves and the stiffener is extracted. An in vitro test method evaluates the technique in agarose gel.

Microelectrode  arrays for neural interface devices that are made of biocompatible thin-film  polymer are expected to have extended functional lifetime ...

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