Polydimethylsiloxane (PDMS) is a silicon based organic polymer that is inert and non-toxic. The mechanical, chemical, and optical properties of PDMS make it a material used in a range of medical related applications, such as artificial organs, prostheses, catheters, contact lenses, and drug delivery systems. Non medical applications include microfluidic devices, microreactors, lab on chip diagnostics, soft-lithography, membranes, electrical insulators, water repellents, anti foaming agents, adhesives, protective coatings, sealants, and a wide variety of other uses.
PDMS is commercially available from several vendors as a two part kit consisting of an elastomer base and a cross linking agent, both in liquid form. A range of kits are available in which elastomer base molecular weight and/or branching are varied. Polymerization is initiated upon mixing the elastomer base with the cross-linking agent, producing an optically clear rubbery solid PDMS elastomer with hydrophobic surface characteristics. The hydrophobic nature of PDMS is often an undesirable property for various applications stated previously. In particular, microfluidic devices may require hydrophilic surfaces to permit polar liquids to pass through. Biomedical devices, such as contact lenses, that are easily wetted improve user comfort. A variety of strategies have been developed to render the PDMS surface hydrophilic, which include exposure to oxygen plasma, ozone, corona discharge, and ultraviolet light. In addition hydrophilic surface modification has been achieved through physical adsorption of charged surfactants, polyelectrolyte multilayers, and entangling amphiphilic co-polymers using a swelling-deswelling method in organic solvent. Covalent modification of the PDMS surface requires activation of the surface, generally through an oxidative process followed by deposition of the reactive molecule from solvent or chemical vapor deposition. Some of the most widely used methods for production of hydrophilic PDMS are described briefly below. Cost effective methods to render PDMS hydrophilic that do not compromise mechanical, optical, or gas permeability properties are of the essence.
It is well established in the literature that exposing PDMS to various energy sources can alter its surface properties. Energy sources such as oxygen plasma, ultraviolet light, and corona discharges have been used to create hydrophilic PDMS surface by oxidation. Oxygen plasma and ultraviolet light have been the most widely used methods in modifying PDMS surfaces. However, these methods generate an unstable and brittle hydrophilic glass like silicate surface layer that compromises elasticity and is unstable over time, allowing the PDMS surface to recover its hydrophobic nature.
Chemical grafting of hydrophilic molecules to the surface of PDMS is stable but is difficult to achieve because PDMS is chemically inert. Thus the first step is to render the surface reactive through exposure to an oxygen plasma or other energy source as discussed previously, resulting in a glass-like silicate layer with chemically reactive groups (e.g. hydroxyl groups: —OH) on the surface. Additional surface modification is achieved via chemical coupling of target molecules to the —OH (or other reactive groups) following standard protocols. However, the underlying glass-like layer remains brittle, limiting applications where elasticity is required, and the process requires multiple reaction steps, which can be costly, inefficient, and generate waste in the form of organic solvent.
Physical entanglement of amphiphilic copolymers containing a PDMS chain to serve as an anchor group is achieved via a swelling-deswelling method. In this approach, a cross-linked PDMS monolith is placed in an organic solvent, such as choloroform, resulting in swelling. In the swollen state low molecular weight amphipilic copolymers may penetrate the PDMS surface. Exchanging the solvent for a polar solvent returns the PDMS monolith to its original size. The aim is to embed/anchor the amphiphilic copolymers on the surface of a cross-linked PDMS. Copolymers that may penetrate the surface of the swollen PDMS monolith are likely held in place by van der Waals force and hydrophobic interactions between the PDMS monolith and PDMS segments in block copolymer amphiphiles. The aim is produce a stable hydrophilic surface on PDMS. However, this method is very time consuming and requires an organic solvent such as chloroform to sufficiently swell the PDMS.
From a review of the current literature, there is a need for a simple and cost efficient technique to form a silicon elastomer, namely, polydimethylsiloxane exhibiting hydrophilic character that can be tuned by the preparation conditions and subsequent treatments and exposure environments.