[vbim][PF] was prepared by the anion exchange of [vbim][Cl] with KPF. First, [vbim][Cl] (1 mol) was dissolved in distilled water and cooled to less than 8°C with crushed ice. Next, an aqueous KPF(1 mol) solution was added gradually to the aqueous [vbim][Cl] solution under vigorous stirring. When KPFwas added, the solution separated into two phases. The lower part of the light-yellow solution was precipitated immediately by the addition of dichloromethane. The precipitates were washed with a large amount of water and lyophilized for 3 days, resulting in a light-brown solid in 85% yield. The final product was characterized byH NMR and elemental analyses ( Figures S2 and S3 ).

[vbim][Cl] was prepared by the N-alkylation of 1-vinylimidazole with 1-chlorobutane. In an atmosphere of dry nitrogen and under vigorous stirring, 1-chlorobutane (1.5 mol) was added in a dropwise manner over a period of 2 hr to 1-vinylimidazole (1.5 mol). The temperature was maintained at less than 8°C with an ice bath. After the process was complete, the reaction mixture was refluxed for 48 hr at 60°C. The crude product was washed with ethyl acetate for the removal of any unreacted starting materials. The washed material was vacuum evaporated at 80°C for the removal of excess ethyl acetate. Finally, the material was lyophilized for 3 days, resulting in a brown solid in a yield of 72%. The obtained material was characterized byH NMR and elemental analyses ( Figures S1 and S3 ), which were performed by UBE Scientific Analysis Laboratory and A Rabbit Science, respectively.

The ILGs ware prepared by the following thermal and photoinduced polymerization processes. Solid [vbim][PF 6 ] was heated to 80°C, at which point it transformed into a liquid and thus could be handled easily. For the thermal polymerization process, [vbim][PF 6 ] (100 μL), TEGDMA (50 μL, Tokyo Chemical Industry), and BPO (50 mg, Wako) were mixed in a mortar that contained [bmim][PF 6 ] (100 μL, Tokyo Chemical Industry). The mixture was then heated at 80°C for 1 hr. For the photoinduced polymerization process, [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (100 μL), and 2-hydroxy-2-methylpropiophenone (2H2MPP, 100 μL, Tokyo Chemical Industry) were mixed vigorously in a mortar and then illuminated with a hand-held 254-nm UV lamp (SLUV-4, AS ONE). Different types of ILGs were synthesized with [hmim][PF 6 ], [omim][PF 6 ], [bmim][Tf 2 N], [hmim][Tf 2 N], and [omim][Tf 2 N] (all Tokyo Chemical Industry) instead of [bmim][PF 6 ]. However, the protocol used was the same.

Characterization of ILGs

The thermal stabilities of the ILGs were analyzed by thermogravimetric measurements, which were performed at a heating rate of 10°C min−1 in flowing O 2 in a TGA Q 500 thermal analyzer (TA Instruments). The ILG sample was prepared by the thermal polymerization process using [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (50 or 100 μL), and BPO (50 mg).

The volatilities of the ILGs, as well as that of agarose gel, were investigated via observation of the changes in their morphology after they were placed in a vacuum. The ILG sample tested was synthesized by thermal polymerization of [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (50 μL), and BPO (50 mg). A 1% agarose gel sample was also prepared with an agarose tablet (Agarose S, Wako) and distilled water (50 mL). The tablet was dissolved in the distilled water in a microwave oven (NE-S200F, Panasonic), and the solution was cooled to room temperature as per the manufacturer's instructions to yield a homogeneous agarose gel. The ILG and agarose gel samples were placed in a polystyrene-type transparent vacuum desiccator (AS ONE) under a reduced pressure of 10−3 Pa for 3 days.

Compression, UV-induced viscoelasticity, and probe tacking tests were performed by Mitsui Chemical Analysis & Consulting Service. The load-displacement curve of the ILGs was obtained from the compression test. An ILG sample composed of [vbim][PF 6 ] (200 μL), [bmim][PF 6 ] (100 μL), and TEGDMA (100 μL) was prepared by thermal polymerization with BPO (50 mg) in a 35-mm polystyrene dish (Thermo Fisher Scientific). This ILG sample was cut into cubes (average length, 6 mm; average width, 6 mm; average thickness, 6 mm), which were used for the compression test. The mechanical strength of the ILG cubes (n = 5) was measured with a tensile tester (AG-X, Shimadzu) by uniaxial compression tests performed at a rate of 1 mm min−1.

The viscoelasticity of the ILG samples (n = 3) under UV irradiation was analyzed with a rheometer (MCR302, Anton Paar) at a frequency of 1 Hz. The samples were irradiated with a 200 W Hg/Xe lamp at 100 mW cm−2 for 30 min in a N 2 atmosphere. The ILG tested consisted of [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (50 μL), and 2H2MPP (50 μL).

Adhesive tests were performed on ILG sheets (n = 10) with a tack tester with a 5-mm-diameter probe (TAC-II; RHESCA). A reaction mixture consisting of [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (50 μL), and BPO (50 mg) was poured into a polydimethylsiloxane (PDMS) frame with a square hole (average length, 2 cm; average width, 2 cm; and average thickness, 1 mm) on a slide glass and then heated at 80°C for 1 hr. The PDMS frame was peeled off from the glass substrate before the measurements. The immersion speed, preload, test speed, press time, and distance were 120 mm min−1, 400 gf, 120 mm min−1, 20 s, and 2 mm, respectively. The tackiness of the ILGs was also investigated with two coins. A mixture of [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (1 μL), and 2H2MPP (10 μL) was placed on a coin and then irradiated by UV light. The prepared gel was sandwiched between two coins. The lift-and-stick-again stickiness of the ILG was evaluated by repetitive removal of the coins 30 times.

The contact angles of water droplets placed on ILG sheets (n = 5) were measured with a contact angle goniometer (CA-D, Kyowa Interface Science) at room temperature. A mixture of [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (100 μL), and 2H2MPP (100 μL) was spin coated on a slide glass (Iwaki) at 3,000 rpm for 40 s with a spin coater (MS-A1000, Mikasa). After UV irradiation at 254 nm for 10 min with a handheld UV lamp, water (5 μL) containing rhodamine B (1 μM; Wako), which was added to ease visual inspection by the naked eye, was placed on the ILG sheets. In addition, side-view images of the water droplets on the ILG sheets were taken with a high-definition digital camera.

Biocompatibility of ILG was investigated as follows. Mouse embryonic fibroblast cell lines (NIH/3T3 cells) were preseeded in 96-well plates at a density of 5 × 104 cells well−1 in 200 μL of Dulbecco's modified Eagle medium (DMEM) and cultured for 24 hr. The cells were then incubated with a photopolymerized ILG droplet at final volumes 1, 3, and 5 μL for 24 hr. The ILG droplet was prepared from a mixture of [vbim][PF 6 ] (100 μL), [bmim][PF 6 ] (100 μL), TEGDMA (50 μL), and 2H2MPP (50 μL). After being washed with fresh DMEM, cells were incubated with WST-1 cell proliferation reagent (10 μL per well; Dojindo Chemical Laboratory) in a final volume of 100 μL of culture medium per well for 1 hr in a humidified atmosphere (37°C and 5% CO 2 ). The optical absorbance of the samples was measured against a background control (blank) with a microplate reader (model 680, Bio-Ras) at 450 nm. The reference wavelength was 620 nm.