Temperature controlled transformations of giant unilamellar vesicles of amphiphilic triblock copolymers synthesized via microfluidic mixing

Abstract

We report on a simple approach for synthesis of temperature-responsive giant unilamellar vesicles (GUVs) from poly(N-vinylcaprolactam)₁₅-block-poly(dimethylsiloxane)₆₅-block-poly(N-vinylcaprolactam)₁₅ (PVCL₁₅-PDMS₆₅-PVCL₁₅) triblock copolymer and non-temperature responsive small and giant vesicles from novel poly(N-vinylpyrrolidone)-block-poly(dimethylsiloxane)-block-poly(N-vinylpyrrolidone) (PVPON₁₅-PDMS₆₅-PVPON₁₅ and PVPON₆-PDMS₃₀-PVPON₆) triblock copolymers using microfluidic mixing at 25 °C. We show that temperature-responsive PVCL₁₅-PDMS₆₅-PVCL₁₅ GUVs with the average diameter of 1.4 ± 0.2 µm while being stable at room temperature for at least 14 days, transformed irreversibly into small vesicles of 168 ± 40 nm after incubation of their aqueous solution at 42 °C for 24 h. We hypothesized that this transformation is induced by local compressive stresses of the vesicle membrane due to the collapse of PVCL blocks above the copolymer lower critical solution temperature (LCST) leading to the decrease of the vesicle membrane thickness. Consequently, we found that the temperature-induced size transformation of the PVCL-based GUVs at 42°C can be suppressed by substituting PVCL with its hydrophilic homologue PVPON, or by suppressing the PVCL's LCST behavior through hydrogen-bonding with tannic acid molecules. In the former case, novel PVPON_n-PDMS_m-PVPON_n triblock copolymers (n = 15, m = 65 and n = 6, m = 30) assemble into vesicles stable from 25 °C to 55 °C as confirmed by optical and electron microscopy, dynamic light scattering (DLS) and small-angle neutron scattering (SANS). In the latter, hydrogen bonding interactions of PVCL with the polyphenol tannic acid (TA) at room temperature resulted in stable PVCL-based GUVs at 42°C as confirmed by optical, electron, and atomic force microscopies. We also found that physical crosslinking of the PVCL corona through hydrogen bonding with TA in PVCL₁₅-PDMS₆₅-PVCL₁₅ GUVs will delay their low pH-induced degradation at 37°C by 48 h compared to non-modified GUVs. Our findings open opportunities for the development of temperature-regulated stable micro-vehicles that would change their structural characteristics in the physiologically relevant temperature range from 25 to 42°C and can be utilized for cell mimicking studies. The developed GUVs also have potential in theranostic drug delivery as substitutes for polymer microcapsules and lipid microbubbles as well as for stimuli-triggered sensing, protection, and rapid response in an aqueous environment.