Year

2010

Degree Name

Doctor of Philosophy

Department

Faculty of Science, School of Chemistry

Abstract

Tissue engineering (using laboratory grown tissue to treat injury) and bionics (technologically augmenting the human body) are advancing the range of medical treatments and prostheses options for patients. New materials continue to be developed which address important tissue engineering and bionics requirements, such as improved cellular response, tissue-like softness and exibility, and bio-degradation where appropriate. These materials include biopolymers, such as chitosan and hyaluronic acid, and conducting polymers, such as polyaniline and poly(3,4 ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). In tandem with new material development, fabrication methods are needed to construct tissue scaffolds or bionic devices. Where conventional fabrication methods do not work, new methods must be developed.

Printing is a exible fabrication method, by which free-form two- and three-dimensional structures can be created. Other similar technologies, such as soft lithography, usually require a mask or template as part of the fabrication process. However, printing methods are data-driven. A print pattern can be designed using a simple image or a more sophisticated computer-aided design (CAD) program. Once a pattern is developed, the materials ink is deposited using an appropriate printing method. These methods include inkjet and syringe-based techniques such as capillary force and extrusion printing.

The viscosity of the materials ink is an important property when considering which printing method to use. Inkjet printing typically works best within a narrow range of low viscosity (10-12 mPa.s). Printing by capillary force is possible with a viscosity range including and outside (1 mPa.s) that of inkjet printing. Higher viscosity materials inks (> 1000 mPa.s) are required for extrusion printing. Other ink properties, such as the surface tension and contact angles on the substrate are important factors in the drying conditions of the printed material.

The aim of this research project was to investigate printing of electroactive polymer inks onto and within a biopolymer substrate. The printability of commercial and in-house synthesized inks was investigated. A custom printer was developed and built for the purpose of printing viscous inks. The influence of the biopolymer substrate on the drying effects of the printed materials was studied. The overall suitability of printing as a method for fabricating devices and embedding conducting polymer tracks into biopolymers was assessed.

The power law and Bingham models were applied to determine the apparent viscosities and yield stress values for the materials inks. Tracks of PEDOT/PSS solution and paste were inkjet and extrusion printed, respectively, onto the surface of dry chitosan-based films. Capillary printing was used for deposition of optically active, doped polyaniline-based inks and PEDOT/PSS on glass substrates. Encapsulated tracks were fabricated by extrusion printing PEDOT/PSS paste into chitosan solution. The tensile properties of the chitosan-based films were measured. The electrical and morphological characteristics of the tracks were compared by printing method.

Inkjet printing was found to be suitable for applications where the resolution of the printed structure is required to be 60 µm or less, and electrical resistance within the range of a few hundred kΩ/cm. Capillary printed samples exhibited resistance similar to samples created by inkjet printing. For applications where the resolution of the printed structure is not as important or the resistance must be in the range of tens of kΩ/cm, extrusion printing should be used. Both inkjet and extrusion printing onto the surface of dry chitosan-based films resulted in localized swelling due to reaction between the ink and the substrate. Encapsulated tracks are protected from direct exposure to air, exhibited the lowest resistance of all printed samples, at 2.8 ± 0.3 kΩ/cm, and did not show the swelling issue faced when printing on the surface of dry films. With the printing methods and inks investigated, it is possible to fabricate structures that could potentially be used to address the needs of future tissue engineering and bionic devices.

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