Environmentally-controlled Microtensile Testing of Mechanically-adaptive Polymer Nanocomposites for ex vivo Characterization

Summary

A method is discussed by which the in vivo mechanical behavior of stimuli-responsive materials is monitored as a function of time. Samples are tested ex vivo using a microtensile tester with environmental controls to simulate the physiological environment. This work further promotes understanding the in vivo behavior of our material.

Abstract

Implantable microdevices are gaining significant attention for several biomedical applications^1-4. Such devices have been made from a range of materials, each offering its own advantages and shortcomings^5,6. Most prominently, due to the microscale device dimensions, a high modulus is required to facilitate implantation into living tissue. Conversely, the stiffness of the device should match the surrounding tissue to minimize induced local strain^7-9. Therefore, we recently developed a new class of bio-inspired materials to meet these requirements by responding to environmental stimuli with a change in mechanical properties^10-14. Specifically, our poly(vinyl acetate)-based nanocomposite (PVAc-NC) displays a reduction in stiffness when exposed to water and elevated temperatures (e.g. body temperature). Unfortunately, few methods exist to quantify the stiffness of materials in vivo¹⁵, and mechanical testing outside of the physiological environment often requires large samples inappropriate for implantation. Further, stimuli-responsive materials may quickly recover their initial stiffness after explantation. Therefore, we have developed a method by which the mechanical properties of implanted microsamples can be measured ex vivo, with simulated physiological conditions maintained using moisture and temperature control^13,16,17.

To this end, a custom microtensile tester was designed to accommodate microscale samples^13,17 with widely-varying Young's moduli (range of 10 MPa to 5 GPa). As our interests are in the application of PVAc-NC as a biologically-adaptable neural probe substrate, a tool capable of mechanical characterization of samples at the microscale was necessary. This tool was adapted to provide humidity and temperature control, which minimized sample drying and cooling¹⁷. As a result, the mechanical characteristics of the explanted sample closely reflect those of the sample just prior to explantation.

The overall goal of this method is to quantitatively assess the in vivo mechanical properties, specifically the Young's modulus, of stimuli-responsive, mechanically-adaptive polymer-based materials. This is accomplished by first establishing the environmental conditions that will minimize a change in sample mechanical properties after explantation without contributing to a reduction in stiffness independent of that resulting from implantation. Samples are then prepared for implantation, handling, and testing (Figure 1A). Each sample is implanted into the cerebral cortex of rats, which is represented here as an explanted rat brain, for a specified duration (Figure 1B). At this point, the sample is explanted and immediately loaded into the microtensile tester, and then subjected to tensile testing (Figure 1C). Subsequent data analysis provides insight into the mechanical behavior of these innovative materials in the environment of the cerebral cortex.

Protocol

1. Sample Preparation

1. Prepare PVAc-NC film of thickness in the range of 25-100 μm using a solution casting and compression technique^10-12.
2. Adhere film to a silicon wafer by heating on a hot plate for two min at 70 °C (above the glass transition temperature) to promote intimate contact between the film and the wafer. This step ensures that the prepared film remains flat and fixed to the Si wafer, which is necessary for planar micromachining processes.
3. Pattern th.......

Discussion

The advancement of implantable biomedical microelectromechanical systems (bioMEMS) for interacting with biological systems is motivating the development of new materials with highly-tailored properties. Some of these materials are designed to exhibit a change in material properties in response to a stimulus found in the physiological environment. One recently-developed class of materials responds to the presence of hydrogen bond-forming liquids (e.g. water) and elevated temperatures to reduce the Young's modulus.......

Materials

Name	Company	Catalog Number	Comments
Name of Reagent/Material	Company	Catalogue Number	Comments
Silicon wafer	University Wafer		Mechanical grade
Extruded acrylic sheet	Professional Plastics	SACR 062EF	Thickness 0.062"
Razor blade	McMaster-Carr	3962A3
Tweezers	McMaster-Carr	8384A47	#5 tip
Super Glue Gel	Loctite	130380
Air Brush	Snap-on Industrial	BF175TA
Air Compressor	Paasche	B002YKN8YO	D500
Thermocouple	Omega	HH12A
Hot plate	Cimarec	SP131325Q
CO2 direct-write laser	VersaLaser	3.5
Dessicator	Fisher Scientific	08-595
Lamp			custom-built
Microtensile tester			custom-built