Mimicking a Cytoskeleton by Coupling Poly(N-isopropylacrylamide) to the Inner Leaflet of Liposomal Membranes:  Effects of Photopolymerization on Vesicle Shape and Polymer Architecture

Abstract

Networks of N-isopropylacrylamide (NIPAM) copolymers, coupled to spherical phospholipid bilayers, are suitable as a model for the study of the interaction between the cytoskeleton and cellular membranes, as well as for promising new drug delivery systems with triggerable drug release properties and improved stability. In this article, we describe a simple preparation technique for liposomes from egg phosphatidyl choline (EPC) encapsulating a cross-linked NIPAMminus signTEGDM copolymer skeleton (tetraethylene glycol dimethacrylate, TEGDM) which is coupled only to the inner monolayer by a novel membrane anchor monomer. Polymerization in the lipid vesicles was initiated at the inner membrane surface by the radical initiator 2,2-diethoxy-acetophenone (DEAP) permeating through the membrane from the outside. The effects of photopolymerization and polymer formation on vesicle shape and membrane integrity were studied by transmission electron microscopy (TEM), cryo-TEM, and atomic force microscopy (AFM). Upon UV irradiation, approximately 100% of the vesicles contained a polymer gel and only occasional changes in the spherical shape of the liposomes were observed. The architecture of the polymer network inside the liposomal compartment was determined by the conditions of the photopolymerization. Composite structures of polymer hollow spheres or solid spheres, respectively, tethered to spherical membrane vesicles were produced. The increased stability of the polymer-tethered lipid bilayers against solubilization by sodium cholate, compared to pure EPC vesicles, was determined by radiolabeling the lipid membrane.

Selecting analytical tools for characterization of polymersomes in aqueous solution

“…The holes could reflect deflated interiors of some polymersomes, or be due to cantilever-induced artifacts. AFM analysis did not reveal any information about polymersome interior (multilamellarity or multivesicularity) in contrast to what has been achieved with liposomes 113 . The contrast and contour of FF-TEM visualized polymersomes was higher than the ones visualized using AFM, which is in agreement with other studies 40 .…”

“…For encapsulation, NS-TEM has advantages over Cryo-TEM due to the possibility of differential staining 113 . However, results from NS-TEM analysis needs to be substantiated by data from other methods, due to osmotic shrinkage caused by the staining agent.…”

“…For liposomes several analytical tools have been investigated 53,60 , where CLSM stands out as a particularly popular method. There have also been extensive studies related to specific types of analysis such as EM 64,111 , fluorescence microscopy 112 , mechanical manipulation 79,80 , single particle techniques like NTA 37 , AFM 40,113 and scanning tunneling microscopy (STM) 70 . Comparative analysis reviews have been presented for liposomes 39,54 , whereas for some polymersome reviews, characterization methods generally only constitute a smaller part of the review, see 8,55 .…”

Formation of interfacial network structure via photo-crosslinking in carbon fiber/epoxy composites

“…Among the physical and chemical crosslinking methods [14,15], photo-crosslinking exhibits considerable advantages, including mild reaction conditions, minimum byproduct formation, absence of potential toxic catalysts and initiators and easily controlled processing [16]. Consequently, in recent years there has been an increasing interest in utilizing photo-crosslinking as a means of biomaterial preparation in the field of medicinal and biomedical science [17][18][19][20][21]. However, to our knowledge, introduction of photosensitive materials into the interface between carbon fibers and resin matrix has never been reported.…”

Responsive Giant Vesicles Filled with Poly(N-isopropylacrylamide) Sols or Gels

“…Apart studies concerning gel beads coated with lipids [3][4][5][6][7][8][9], lipid vesicles were mainly combined with physical gels, used as external coating (rigid actin filaments [10]) or internalized in the vesicle volume (rigid actin networks [11,12], agarose gels [13], solutions of aggregated Poly(Nisopropylacrylamide) (PolyNipam) [14]). A recent work dealt with controlled covalent PolyNipam gels but concerned submicronic liposomes [15,16].…”