One-step Reversal Technique towards Hydrophilic Surface Modification of Polydimethylsiloxane-co-Polyurethane (PDMS-PU)

Polydimethylsiloxane (PDMS) has in the last few decades been considered as the most prevalent molding materials for the fabrication of various micro devices for biological and biomedical research and applications, due to the prominent features of non-toxicity, low cost, chemical inertness, gas permeability, optical transparency, functionalized surface and mold ability [1-4]. Despite the above appealing properties, PDMS is naturally hydrophobic and this confines its wide use in several applications including biocompatible devices, due to its tendency to absorb some hydrophobic molecules, cells and proteins onto its surface. Therefore, there is pressing need to improve the hydrophilicity of PDMS materials’ surface.

adhesion through hydrophilic modification and interfacial segregation by utilizing a combination of plasma treatments and rheological modifier. They showed that, the adhesion between oxygen plasma-treated PDMS and the hydrogel increased with time and reached a stable value after six days and the WCA remained constant during that time, suggesting that the presence of the hydrogel suppressed the hydrophobic recovery of plasma-treated PDMS, but this method is tedious and time consuming. Chou et al. [16] grafted zwitterionic polymers by in situ self-assembling coating (ISC) which comprised polymerization and simultaneously coating of a copolymer onto the surface in order to resolve the solubility problem of zwitterionic copolymers. They applied ISC method to copolymers composed of hydrophobic styrene (ST) and hydrophilic sulfobetaine methacrylate (SBMA). They not only found that the coated surfaces resisted befouling but also improved wett ability.
Nevertheless, this method required optimization of operating conditions including initial solid content in the polymerization bath, the monomer ratio and the reaction time.
In our lab group, 2-(trimethylsiloxy) ethyl methacrylate (TMSE-MA) has been found to be a suitable reagent for hydrophilic surface medication of PDMS via hydrophilic reversal technique [17]. The reversal technique is a one-step solution soaking treatment that does not require any pretreatment or use of organic solvents. TM-SEMA is mostly used as a protecting agent for hydroxyl groups in hydroxyethylmethacrylate due to the trimethylsiloxy groups contained in its structure. Mecerreyes et al. [18] fabricated the poly (alkyl methacrylate)-graft-polylactone via sequential living polymerization by utilization of TMSEMA protective effect on hydroxyl groups. Wang et al. [19] synthesized amphiphilic block copolymer PSt-b-PHEMA by hydrolysis of PSt-b-PTMSEMA that led to the synthesis of PSt-b-PHEMA metal hybrid. Liu et al. [20] prepared polystyrene-block-poly (2-hydroxyethyl methacrylate) with different molar masses via hydrolysis of trimethylsilyl groups and the micelles formed from these polymers in THF/cyclohexane mixtures were investigated. From the studies, it's clear that TMSEMA plays a critical role as a protecting agent for hydroxyl groups. We recently prepared series of PDMS film using (3-acryloxy-2-hydroxypropoxypropyl) terminated polydimethylsiloxane TMSEMA, then subjected the films to alkali hydrolysis to obtain hydrophilic PDMS surface with reduced protein adsorption [17].
The simplicity and convenience of TMSEMA hydrolysis reaction inspired us to apply TMSEMA to the hydrophilic surface modification of PDMS-PU for the first time. Unlike other previous PDMS modification methods, the proposed method renders hydrophilicity to PDMS-HEMA by surface reverse technique. The previously reported procedures have utilized organic solvents such as methanol, THF, cyclohexane etc. in efforts to hydrolyze TMSEMA [18][19][20], but the use of organic solvents results into contamination and increased toxicity which affects the materials if they are to be used in biological applications. Hence, an efficient and environmentally friendly method to hydrolyze TMSEMA is presented in this research.
In this paper, TMSEMA is key to hydrophilic surface modification of PDMS-HEMA films. A series of PDMS-PU films were fabricated using PDMS-HEMA macromer and a hydrophilizing reagent TMSEMA in the presence of a cross linker ethylene glycol dimethacrylate (EGDMA) and 2-Hydroxy-2-methylpiophenome (D-1173). The obtained PDMS-PU films were subjected to reversal technique using a certain wt. % of potassium hydroxide (KOH) solution in order to obtain hydroxyl groups on the surface Figure 1.

Preparation of the PDMS-PU films
The PDMS-PU network was prepared by reacting the prepared PDMS-HEMA reaction mixture with different weight percentage ratios of TMSEMA using D-1173 and EGDMA as the photo-initiator and cross-linker respectively followed by 12h stirring in the dark at room temperature (25 °C). Thereafter, the mixture was injected into the double stack polypropylene molds and UV-initiated (≥15mV/cm 2 ) for 2h at room temperature. The resulting films were then stored in PBS buffer solution for further experiments. Pristine PDMS-PU containing TMSEMA was represented in Table 1.

PDMS-PU surface modification
The PDMS-PU surface was modified via reversal treatment [17].
Under alkaline conditions, the trimethylsiloxy group contained in TMSEMA undergoes hydrolysis to remove trimethylsilyl there by forming the hydroxyl group. The prepared films were treated using the optimum condition. Briefly, the prepared PDMS-PU films were immersed into 5wt% KOH solution and shaken at 150rpm for 3h at 37 °C to obtain the films. The films were washed with deionized water several times to remove the residues produced by the reversal treatment. Phosphate buffered saline was used for storing the obtained films for the next studies.

Surface characterization of PDMS-PU films
The water contact angle of the as-prepared PDMS-PU films was evaluated using a contact angle goniometer (KSV CAM-200, KS-VIns), while attenuated total reflection Fourier transform infrared (Nicolet 5700 FTIR) spectroscopy was used to assess the functional groups difference on the surface of the unmodified and modified films. Furthermore, the surface morphology was examined using a scanning electron microscope (SEM FEI Inspect F50).

Measurement of Equilibrium Water Content (EWC)
The equilibrium water content of the PDMS-PU films was evaluated as follows: Where W wet is the weight of the wet sample and W dry is the weight of the dry sample. The sample was immersed in water for 24 h to obtain the weight of the wet sample.

The mechanical properties
A Labthink XLW (PC) Auto Tensile Tester was used to test the tensile strength and elongation at break of the films at a stretching rate of 50mm/min.

Protein adsorption analysis
For protein adsorption analysis, the protein bovine serum al-

Results and Discussion
In this research, we utilized the condensation reaction and ultraviolet initiated copolymerization reactions to prepare PDMS-PU films. Hydroxyl terminated PDMS was polymerized via condensa-

Water contact angle (WCA) analysis
The

ATR-FTIR surface analysis
The contact angle measurement confirmed that, after the hy-drophilic reversal treatment, the hydrophilicity of the PDMS-PU films improved significantly and this was attributed to the genera-
Therefore, to prove this, the surface of the PDMS-PU film was tested by ATR-FTIR to monitor the generation of the hydroxyl groups on the surface. From Figure 3 it can be seen that for both pristine and modified PDMS-PU, the absorption bands at (~798cm -1 ) and (~1261cm -1 ) are ascribed to SiCH 3 stretching vibrations and bending deformation respectively, while (~1022cm -1 ) and (~1091cm -1 ) are attributed to symmetric and asymmetric -Si-O-Si-stretches, respectively. The absorption peak appearing at (~1727cm -1 ) is due to the -C=O stretching vibration. The modified PDMS-PU had the absorption appearing at (~3387cm -1 ) which is ascribed to the -OH, the peak intensity increased with increased content of TMSEMA.
The principle of the hydrophilic reversal treatment method and its hydrophilic effect is supported by the presence of the hydroxyl groups. Furthermore, the absorption peak at (~1568cm -1 ) was observed and its intensity increased with increasing TMSEMA content. This observation could be ascribed to the reversal reaction not only causing the cleavage of the Si-O bond to form the hydroxyl group, but also resulted in the formation of the ester bond due the hydrolysis of TMSEMA in the alkaline solution to form salt (carboxyl ate) structure.  The pristine and modified PDMS-PU film's surface morphologies were studied by SEM analysis. Figure 5 depicts the surface morphologies of the PDMS-PU before and after reversal treatment.

Equilibrium water content (EWC) analysis
As can be seen from the SEM images, a smooth and flat surface was

Mechanical properties
To study the effect of the KOH solution treatment on the mechanical properties of the modified PDMS-PU films, the tensile strength and elongation at break of both the pristine and modified PDMS-PU films were determined. As can it be seen from Figure 6A  PBS solution pH 7.4, LZM is positively charged while BSA is negatively charged [18,24]. From the IR spectrum in Figure 3, the carboxyl ate and hydroxyl functional groups are generated on the surface of the PDMS-PU after the hydrophilic reversal treatment, which results in the modified PDMS-PU surface being negatively charged.
Further, the IR spectrum shows that the intensity of the carboxyl ate groups on the surface of the surface of the reversal-treated PDMS-PU is significantly enhanced with increasing TMSEMA content, which explains why the relative ratio of BSA adsorption decrease with TMSEMA content, while that of LZM increases. These results are in agreement with those reported by other researchers who studied protein adsorption [18,[24][25][26]. Many researchers have reported that, hydrophilic surfaces are more resistant to protein adsorption compared to hydrophobic surfaces [25,[27][28][29]. However, in the current study, the protein adsorption capacity of the reversal-treated PDMS-PU surface was not only dependent on its hydrophilicity but also on the protein isoelectric point value resulting from the surfaces negatively charged characteristics. Consequently, the lipophilicity of the hydrophilizing reagent TMSEMA provides it with good compatibility with the hydrophobic siloxane structure, and can be added in any proportion in the synthesis of the PDMS-PU film system. TMSEMA's low surface energy properties enables it to migrate to the surface and accumulate on the surface of the film to allow the reverse reaction to take place. Further, the generation of the carboxyl and hydroxyl functional groups on the modified film surface offers more potential and possibility for further modification of PDMS-PU.

Conclusion
A series of PDMS-PU films were prepared via the copolymerization of hydrophilizing reagent TMSEMA and the macromer PDMS-HEMA. The as-prepared PDMS-PU films were subjected to one-step reversal treatment. The WCA measurements on the PDMS-PU surface indicated that the wet ability was greatly improved after the reversal treatment. The improvement of the wettability was further confirmed by ATR-FTIR analysis, which showed the formation of the hydroxyl and carboxyl ate groups on the reversal-treated PDMS-PU surface. Meanwhile, the EWC analysis showed that the water holding capacity of the PDMS-PU did not significantly increase after modification, indicating that the hydrophilic reverse reaction only hydrolyzes the surface while the interior of the PDMS-PU remains the hydrophobic siloxane structure. Further, the

RDMS.000820. 13(4).2020
SEM analysis showed that the alkaline solution soaking treatment caused damage on the sample surface. The damage caused on the surface slightly decreased both the tensile strength and the elongation at break of the PDMS-PU films. The modified PDMS-PU showed different protein adsorption performance towards different protein. The relative ratio of BSA adsorption decreased with increas-ing TMSEMA while the relative ratio of LZM adsorption exhibited a general upward trend. This study explored a non-polluting, simple, efficient and cost effective method for improving the hydrophilicity of PDMS-PU for biomaterial applications. It is hoped that this method will in the near future be applied to hydrophilic surface modification of other PDMS materials.