Cationic temperature-responsive poly(N-isopropyl acrylamide) graft copolymers: from triggered association to gelation

Hollow colloidosomes prepared using accelerated solvent evaporation

We demonstrate a new, scalable, simple, and generally applicable two-step method to prepare hollow colloidosomes. First, a high volume fraction oil-in-water emulsion was prepared. The oil phase consisted of CH2Cl2 containing a hydrophobic structural polymer, such as polycaprolactone (PCL) or polystyrene (PS), which was fed into the water phase. The water phase contained poly(vinylalcohol), poly(N-isopropylacrylamide), or a range of cationic graft copolymer surfactants. The emulsion was rotary evaporated to rapidly remove CH2Cl2. This caused precipitation of PCL or PS particles which became kinetically trapped at the periphery of the droplets and formed the shell of the hollow colloidosomes. Interestingly, the PCL colloidosomes were birefringent. The colloidosome yield increased and the polydispersity decreased when the preparation scale was increased. One example colloidosome system consisted of hollow PCL colloidosomes stabilized by PVA. This system should have potential biomaterial applications due to the known biocompatibility of PCL and PVA.

Thermally reversible colloidal gels for three-dimensional chondrocyte culture

Healthy cells are required in large numbers to form a tissue-engineered construct and primary cells must therefore be increased in number in a process termed 'expansion'. There are significant problems with existing procedures, including cell injury and an associated loss of phenotype, but three-dimensional culture has been reported to offer a solution. Reversible gels, which allow for the recovery of cells after expansion would therefore have great value in the expansion of chondrocytes for tissue engineering applications, but they have received relatively little attention to date. In this study, we examined the synthesis and use of thermoresponsive polymers that form reversible three-dimensional gels for chondrocyte cell culture. A series of polymers comprising N-isopropylacrylamide (NIPAM) and styrene was synthesized before studying their thermoresponsive solution behaviour and gelation. A poly(NIPAM-co-styrene-graft-N-vinylpyrrolidone) variant was also synthesized in order to provide increased water content. Both random- and graft-copolymers formed particulate gels above the lower critical solution temperature and, on cooling, re-dissolved to allow enzyme-free cell recovery. Chondrocytes remained viable in all of these materials for 24 days, increased in number and produced collagen type II and glycosaminoglycans.

Thermally triggered assembly of cationic graft copolymers containing 2-(2-methoxyethoxy)ethyl methacrylate side chains

Thermoresponsive copolymers continue to attract a great deal of interest in the literature. In particular, those based on ethylene oxide-containing methacrylates have excellent potential for biomaterial applications. Recently, some of us reported a study of thermoresponsive cationic graft copolymers containing poly(N-isopropylacrylamide), PNIPAm, (Liu et al., Langmuir, 24, 7099). Here, we report an improved version of this new family of copolymers. In the present study, we replaced the PNIPAm side chains with poly(2-(2-methyoxyethoxy)ethylmethacrylate), PMeO(2)MA. These new, nonacrylamide containing, cationic graft copolymers were prepared using atom transfer radical polymerization (ATRP) and a macroinitiator. They contained poly(trimethylamonium)-aminoethyl methacrylate and PMeO(2)MA, i.e., PTMA(+)(x)-g-(PMeO(2)MA(n))(y). They were investigated using variable-temperature turbidity, photon correlation spectroscopy (PCS), electrophoretic mobility, and (1)H NMR measurements. For one system, four critical temperatures were measured and used to propose a mechanism for the thermally triggered changes that occur in solution. All of the copolymers existed as unimolecular micelles at 20 °C. They underwent reversible aggregation with heating. The extent of aggregation was controlled by the length of the side chains. TEM showed evidence of micellar aggregates. The thermally responsive behaviors of our new copolymers are compared to those for the cationic PNIPAm graft copolymers reported by Liu et al. Our new cationic copolymers retained their positive charge at all temperatures studied, have high zeta potentials at 37 °C, and are good candidates for conferring thermoresponsiveness to negatively charged biomaterial surfaces.

Responsive nanostructures from aqueous assembly of rigid-flexible block molecules

During the past decade, supramolecular nanostructures produced via self-assembly processes have received considerable attention because these structures can lead to dynamic materials. Among these diverse self-assembly systems, the aqueous assemblies that result from the sophisticated design of molecular building blocks offer many potential applications for producing biocompatible materials that can be used for tissue regeneration, drug delivery, and ion channel regulation. Along this line, researchers have synthesized self-assembling molecules based on ethylene oxide chains and peptide building blocks to exploit water-soluble supramolecular structures. Another important issue in the development of systems that self-assemble is the introduction of stimuli-responsive functions into the nanostructures. Recently, major efforts have been undertaken to develop responsive nanostructures that respond to applied stimuli and dynamically undergo defined changes, thereby producing switchable properties. As a result, this introduction of stimuli-responsive functions into aqueous self-assembly provides an attractive approach for the creation of novel nanomaterials that are capable of responding to environmental changes. This Account describes recent work in our group to develop responsive nanostructures via the self-assembly of small block molecules based on rigid-flexible building blocks in aqueous solution. Because the rigid-flexible molecules self-assemble into nanoscale aggregates through subtle anisometric interactions, the small variations in local environments trigger rapid transformation of the equilibrium features. First, we briefly describe the general self-assembly of the rod amphiphiles based on a rigid-flexible molecular architecture in aqueous solution. We then highlight the structural changes and the optical/macroscopic switching that occurs in the aqueous assemblies in response to the external signals. For example, the aqueous nanofibers formed through the self-assembly of the rod amphiphiles respond to external triggers by changing their shape into nanostructures such as hollow capsules, planar sheets, helical coils, and 3D networks. When an external trigger is applied, supramolecular rings laterally associate and merge to form 2D networks and porous capsules with gated lateral pores. We expect that the combination of self-assembly principles and responsive properties will lead to a new class of responsive nanomaterials with many applications.

Thermoresponsive surfaces prepared using adsorption of a cationic graft copolymer: a versatile method for triggered particle capture

In this study we investigate triggered particle capture at substrates containing adsorbed thermally responsive graft copolymers. The copolymers used were PDMA(x)(+)-g-(PNIPAm(n))(y), where DMA(+) is quaternized N,N-dimethylaminoethyl methacrylate and NIPAm is N-isopropylacrylamide. The x and y values originate from the macroinitiator used for copolymer preparation. In this study the copolymers are adsorbed onto two different substrates: quartz microscope slides and microporous, high surface area carbon foam. The substrates were coated with a layer of calcined laponite. The laponite acted as a conditioning layer and promoted strong adsorption of the copolymer. The hydrophobicity of the thermoresponsive surfaces was probed using variable-temperature contact angle measurements. The contact angles generally increased considerably upon increasing the temperature to above the lower critical solution temperature (LCST) of the copolymers. The ability of the thermoresponsive surfaces to capture dispersed particles was investigated using anionic and cationic polystyrene (PS) particles. PDMA(30)(+)-g-(PNIPAm(210))(14) was the most effective copolymer in terms of providing high capture efficiencies of anionic PS particles using temperature as the trigger. The thermoresponsive surfaces strongly held the anionic PS particles even when cooled to below the LCST. The relationships between copolymer structure and particle capture efficiency are discussed. The new approach used here for preparation thermoresponsive surfaces is potentially scalable to high volume applications.

Temperature-triggered gelation of aqueous laponite dispersions containing a cationic poly(N-isopropyl acrylamide) graft copolymer

In this work, temperature-triggered gelation of aqueous laponite dispersions containing a cationic poly(N-isopropylacrylamide) (PNIPAm) graft copolymer was investigated. The copolymer used was PDMA(+)(30)-g-(PNIPAm(210))(14) [Liu et al. Langmuir 2008, 24, 7099]. DMA(+) is quarternarized N,N-dimethylaminoethyl methacrylate. The presence of small concentrations of laponite enabled temperature-triggered gel formation to occur at low copolymer concentrations (e.g., 1 wt %). Dynamic rheological measurements of the gels showed that they had storage modulus values of up to 400 Pa when the total solid volume fraction (polymer and laponite) was only about 0.02. The storage modulus was dependent on both the temperature and the composition of the dispersion used for preparation. The key component that provided the temperature-triggered gels with their elasticity was found to be self-assembled nanocomposite (NC) sheets. These NC sheets spontaneously formed at room temperature upon addition of laponite to the copolymer solution. The NC sheets had lateral dimensions on the order of hundreds of micrometers and a thickness of a few micrometers. The NC sheets were present within the temperature-triggered gels and formed elastically effective chains. The NC sheets exhibited temperature-triggered contraction with a contraction onset temperature of 27 degrees C. A conceptual model is proposed to qualitatively explain the relationship between gel elasticity and dispersion composition.