Perfluoroalkyl-Functionalized Hyperbranched Polyglycerol as Pore Forming Agents and Supramolecular Hosts in Polymer Microspheres

Perfluoroalkyl-functionalized, hyperbranched polyglycerols that produce stable microbubbles are integrated into a microfluidic emulsion to create porous microspheres. In a previously-presented work a dendrimer with a perfluorinated shell was used. By replacing this dendrimer core with a hyperbranched core and evaluating different core sizes and degrees of fluorinated shell functionalization, we optimized the process to a more convenient synthesis and higher porosities. The new hyperbranched polyglycerol porogens produced more pores and can be used to prepare microspheres with porosity up to 12% (v/v). The presented preparation forms pores with a perfluoroalkyl-functionalized surface that enables the resulting microspheres to act as supramolecular host systems. The microspheres can incorporate gases into the pores and actives in the polymer matrix, while the perfluoroalkylated pore surface can be used to immobilize perfluoro-tagged molecules onto the pores by fluorous-fluorous interaction.


Synthesis of Perfluoro-Tagged Disperse Red 1 (F-DR)
A mixture of Disperse Red 1 (426 mg, 1.36 mmol) and heptadecafluoroundecanoic acid (669 mg, 1.359 mmol) was dissolved in DCM (50 mL). ECDI (519 mg, 2.72 mmol) and DMAP (208 mg, 1.70 mmol) were added to the solution and stirred for 72 h at rt. After the mixture was repeatedly washed with pure water the solvent was removed under vacuum to yield a dark red residue. For further purification the raw product was washed with DMF and yielded the product as a red solid (540 mg, 50%).

Materials
Commercially available chemicals were purchased from reliable sources and used as delivered. Poly(dl-lactic acid) (PLA, MW = 15,000 g·mol −1 , Polysciences, Inc., Warrington, PA, USA) was used as matrix-forming polymer for microspheres. Poly(vinyl alcohol) (PVA, MW = 13,000-23,000 g·mol −1 , 98% hydrolyzed, Aldrich) was used as surfactant (5% w/v) for the outer aqueous phase. Nile Red was used as a hydrophobic dye for the inner oil phase. Dichloromethane (DCM, 99.8%, Mallinckrodt) served as an organic solvent for PLA and Nile Red. The square microcapillaries were purchased from Atlantic International Technologies (AIT, Rockaway, NJ, USA). The round glass microcapillaries were purchased from World Precision Instruments, Inc. (Sarasota, FL, USA) and tapered using a micropipette puller from Shutter Instruments Co. (Novato, CA, USA). All aqueous solutions were filtered by Acrodisc 32 mm syringe filters with 5 µm Supor membrane before use.

Device Fabrication
The microfluidic devices that were used for the microsphere fabrication consisted of a glass slide, PE or Teflon tubes, two glass capillaries and a syringe tip. The device was fabricated by inserting the round capillary into the square capillary and mixing both with epoxy adhesive to the glass slide. Less than a centimeter of tube for the inner oil phase was inserted into the square capillary and sealed with epoxy glue. The direct tubing of the oil phase inlet without syringe tip avoids the formation of unwanted air reservoirs that cause pulsing and breaks of the continuous oil phase stream. To avoid wetting on glass surfaces by DCM, the capillaries were coated with the hydrophilic compound 2-[methoxy(polyethyleneoxy)-propyl]trimethoxysilane. The syringe tip was adjusting to the interconnection of the round capillary and the square capillary, followed by sealing with epoxy adhesive.

Microsphere Fabrication
The two phases-outer aqueous phase and inner oil phase-were infused at independently adjustable flow rates by syringe pumps connected to the device by tubing. An aqueous 10 wt % PVA solution was used as the outer aqueous phase and a 5 wt % PLA solution in DCM with Nile Red (0.1-2 mM) was used as the inner oil phase. For the collection phase, an aqueous 2 wt % PVA solution was used. The collected DCM droplets in PVA solution were dried in at 700 mbar vacuum for 48 h to evaporate the DCM and to obtain solid microspheres. The spheres were washed with purified water to remove residual PVA. Figure S12. SEM pictures of multiple porous microspheres displaying their monodispersity.

Statistical Calculation of Diameters and Porosity
Number per area (20,000 µm 2 ), average diameter and coefficient of variation (CV) of the micron-sized bubbles were calculated from the confocal microscopy images. The value of CV is defined by CV = δ/[M] × 100, where δ is the standard deviation and [M] is the average diameter. The necessary contrast was created by using Nile Red in solution. Black air bubbles on red background were analyzed via picture threshold setting and particle measurement using ImageJ 1.0. Measurement values (sensitivity and range) were set to: circularity = 0.8-1, range = 0.5-5000 µm 2 . The porosity of fabricated spheres was determined by cross section confocal images. The dried PLA particles were recorded in formamide for a matching refractional index and imaged in various depths within one sphere. The average porosity was determined by calculating the air to total volume ratio of 20 cross sections of multiple spheres randomly selected of each sample. Figure 4 shows an example of a cross section after area calculation. All measurements were done in triplicate.