Ablation and Deposition of Poly(dimethylsiloxane) with X‐Rays

Abstract

Poly(dimethylsiloxane) [PDMS] is the most widely used siliconbased organic polymer due to its excellent mechanical, chemical, and optical properties for microfluidics, microchips, microlithography, electrophoresis, and biomedical applications. One challenge for the applications is to modify the surface wettability with long-term stability in air. Herein we present a highly efficient protocol for the surface modification of PDMS using Xray irradiation, which enables not only to “ablate” hydrophobic molecules from PDMS, rendering the surface hydrophilic in atmosphere and room temperature conditions, but also to “deposit” them on flat and nanostructure surfaces, inducing hydrophobic surfaces. In spite of the development of many interesting applications, microfluidic devices based on PDMS are not yet standard equipment observed in most biologists’ toolboxes. One of the obstacles is the difficulty in making PDMS-based devices. For instance, soft lithography, the main fabrication process, requires using a clean room, which makes the traditional soft lithography approach painfully slow and difficult. The intrinsic hydrophobicity of PDMS that comes from Si CH3 end-bonds is another big problem in applying as an ideal flexible substrate for microfluidic channels. 4] The PDMS surface can be rendered hydrophilic using a variety of methods (see a review) such as oxygen plasmas, corona discharges, and ultraviolet lights, and exposures of lasers or excimer lamps. The hydrophilic property, however, quickly diminishes through the sequent hydrophobic recovery. 4] Such physical protocols usually require an additional sophisticated chemical process to stabilize the hydrophilic SiO2 layer. [4] Laser irradiation that enables one not only to modify, but also to ablate the PDMS surface through photothermal and photochemical effects may have a limitation in applications to organic polymers such as PDMS because of thermal damage. It is thus required to develop a versatile and robust protocol to modify and ablate polymers simultaneously with a finely tunable, nonthermal irradiation technique. Contrary to laser beams, ionizing radiation can induce photochemical scission of PDMS without thermal damage. For instance, electron irradiation, revealed by Miller as early as 1960, causes Si CH3 and SiCH2 H scissions, generating H2, CH4, and C2H6 as gaseous products. [10,11] Despite the well-established radiation chemistry of PDMS, there are few reports to demonstrate highly efficient wettability changes in PDMS using ionizing radiation. From studies on various soft matter and inorganic systems using hard X-rays (10–60 keV), we recently developed an X-ray ablation technique using rapid depolymerization of organic polymers. Here our results of PDMS demonstrate that X-ray irradiation is a highly efficient and feasible protocol for surface modification through PDMS ablation. The X-ray-induced ablation enables one not only to oxidize the PDMS surface (hydrophilicity) but also to deposit the ablated PDMS (hydrophobicity) on other hydrophilic surfaces. The evidence of the X-ray-induced PDMS ablation and deposition is shown using contact-angle and attenuated total reflection (ATR) FT–IR measurements, which are typically used to prove the mechanism. We suppose that the X-ray-induced ablation–deposition process occurs as illustrated in Scheme 1. A rapid chain scission initially occurs in the irradiated PDMS substrate under hard X-ray irradiation. Once the chain scission at the Si CH3 (or SiCH2 H) bonds occurs, the irradiated surface can easily be oxidized, as commonly observed in laser ablation, and