Terminal-Functionality Effect of Poly(N-isopropylacrylamide) Brush Surfaces on Temperature-Controlled Cell Adhesion/Detachment

Surface-Initiated PET-RAFT via the Z-Group Approach

Surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) is a user-friendly and versatile approach for polymer brush engineering. For SI-RAFT, synthetic strategies follow either surface-anchoring of radical initiators (e.g., azo compounds) or anchoring RAFT chain transfer agents (CTAs) onto a substrate. The latter can be performed via the R-group or Z-group of the CTA, with the previous scientific focus in literature skewed heavily toward work on the R-group approach. This contribution investigates the alternative: a Z-group approach toward light-mediated SI photoinduced electron transfer RAFT (SI-PET-RAFT) polymerization. An appropriate RAFT CTA is synthesized, immobilized onto SiO2, and its ability to control the growth (and chain extension) of polymer brushes in both organic and aqueous environments is investigated with different acrylamide and methacrylate monomers. O2 tolerance allows Z-group SI-PET-RAFT to be performed under ambient conditions, and patterning surfaces through photolithography is illustrated. Polymer brushes are characterized via X-ray photoelectron spectroscopy (XPS), ellipsometry, and water contact angle measurements. An examination of polymer brush grafting density showed variation from 0.01 to 0.16 chains nm-2. Notably, in contrast to the R-group SI-RAFT approach, this chemical approach allows the growth of intermittent layers of polymer brushes underneath the top layer without changing the properties of the outermost surface.

Fabrication of electrospun nanofibrous thermoresponsive semi-interpenetrating poly(N-isopropylacrylamide)/polyvinyl alcohol networks containing ZnO nanoparticle mats: characterization and antibacterial and cytocompatibility evaluation

Thermoresponsive nanofiber composites comprising biopolymers and ZnO nanoparticles with controlled release and antibacterial activity are fascinating scientific research areas. Herein, poly(N-isopropylacrylamide) (PNIPAm) was prepared and mixed with poly(vinyl alcohol) (PVA) in 75/25 and 50/50 weight ratios together with ZnO (0, 1, and 2 phr) to construct nanofiber composites. The morphology of the crosslinked nanofiber composites, ZnO content, and their mechanical behavior were assessed by SEM, EDX, and tensile analyses. The wettability results show an increment in nanofiber surface hydrophobicity by increasing the temperature above the LCST of PNIPAm. The in vitro ZnO release exhibits a faster release profile for the sample with 50 wt% PNIPAm (lower crosslinking density) compared to the one with 25 wt%. Besides, a strong interaction between PVA hydroxyl groups and ZnO can restrict the release content. However, by increasing the temperature from 28 to 32 °C, the relative ZnO release becomes half for both compositions. All crosslinked nanofiber composites demonstrated reliable biocompatibility against L929 fibroblast cells. Agar disc-diffusion and optical density methods showed thermo-controllable antibacterial activity against Staphylococcus aureus upon temperature variation between 28 and 32 °C. Furthermore, in vivo and histological results indicate the potentiality of the prepared multidisciplinary wound dressing for robust wound healing and skin tissue engineering.

Design of biointerfaces composed of soft materials using controlled radical polymerizations

Soft interface materials have an immense potential for the improvement of biointerfaces, which are the interface of biological and artificially designed materials. Controlling the chemical and physical structures of the interfaces at the nanometer level plays an important role in understanding the mechanism of the functioning and its applications. Controlled radical polymerization (CRP) techniques, including atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization, have been developed in the field of precision polymer chemistry. It allows the formation of well-defined surfaces such as densely packed polymer brushes and self-assembled nanostructures of block copolymers. More recently, a novel technique to prepare polymers containing biomolecules, called biohybrids, has also been developed, which is a consequence of the advancement of CRP so as to proceed in an aqueous media with oxygen. This review article summarizes recent advances in CRP for the design of biointerfaces.

Fast Thermoresponsive Poly(oligoethylene glycol methacrylate) (POEGMA)-Based Nanostructured Hydrogels for Reversible Tuning of Cell Interactions

Reactive electrospinning is demonstrated as a viable method to create fast-responsive and degradable macroporous thermoresponsive hydrogels based on poly(oligoethylene glycol methacrylate) (POEGMA). Hydrazide- and aldehyde-functionalized POEGMA precursor polymers were coelectrospun to create hydrazone cross-linked nanostructured hydrogels in a single processing step that avoids the need for porogens, phase separation-driving additives, or scaffold postprocessing. The resulting nanostructured hydrogels can respond reversibly and repeatedly to changes in external temperature within seconds, in contrast to the minutes-to-hours response time observed with bulk hydrogels. Furthermore, nearly quantitative cell delamination can be achieved within 2 min of incubation at 4 °C, resulting in the recovery of as many or more (as well as more proliferatively active) cells from the substrate relative to the conventional trypsinization protocol. The combined macroporosity, nanoscale feature size, and interfacial switching potential of these nanostructured hydrogels thus offer promise for manipulating cell-hydrogel interactions as well as other applications in which rapid responses to external stimuli are desirable.

Thermoresponsive interfaces obtained using poly(N-isopropylacrylamide)-based copolymer for bioseparation and tissue engineering applications

Poly(N-isopropylacrylamide) (PNIPAAm) is the most well-known and widely used stimuli-responsive polymer in the biomedical field owing to its ability to undergo temperature-dependent hydration and dehydration with temperature variations, causing hydrophilic and hydrophobic alterations. This temperature-dependent property of PNIPAAm provides functionality to interfaces containing PNIPAAm. Notably, the hydrophilic and hydrophobic alterations caused by the change in the temperature-responsive property of PNIPAAm-modified interfaces induce temperature-modulated interactions with biomolecules, proteins, and cells. This intrinsic property of PNIPAAm can be effectively used in various biomedical applications, particularly in bioseparation and tissue engineering applications, owing to the functionality of PNIPAAm-modified interfaces based on the temperature modulation of the interaction between PNIPAAm-modified interfaces and biomolecules and cells. This review focuses on PNIPAAm-modified interfaces in terms of preparation method, properties, and their applications. Advances in PNIPAAm-modified interfaces for existing and developing applications are also summarized.

Terminal cationization of poly(N-isopropylacrylamide) brush surfaces facilitates efficient thermoresponsive control of cell adhesion and detachment

A variety of poly(N-isopropylacrylamide) (PIPAAm)-grafted surfaces have been reported for temperature-controlled cell adhesion/detachment. However, the surfaces reported to date need further improvement to achieve good outcomes for both cell adhesion and detachment, which are inherently contradictory behaviors. This study investigated the effects of terminal cationization and length of grafted PIPAAm chains on temperature-dependent cell behavior. PIPAAm brushes with three chain lengths were constructed on glass coverslips via surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization. Terminal substitution of the grafted PIPAAm chains with either monocationic trimethylammonium or nonionic isopropyl moieties was performed through the reduction of terminal RAFT-related groups and subsequent thiol-ene reaction with the corresponding acrylamide derivatives. Although the thermoresponsive properties of the PIPAAm brush surfaces were scarcely affected by the terminal functional moiety, the zeta potentials of the cationized PIPAAm surfaces were higher than those of the nonionized ones, both below and above the phase transition temperature of PIPAAm (30°C). When bovine endothelial cells were cultured on each surface at 37°C, the number of adherent cells decreased with longer PIPAAm. Notably, cell adhesion on the cationized PIPAAm surfaces was higher than that on the nonionized surfaces. This terminal effect on cell adhesion gradually weakened with increasing PIPAAm length. In particular, long-chain PIPAAm brushes virtually showed cell repellency even at 37°C, regardless of the termini. Interestingly, moderately long-chain PIPAAm brushes promoted cell detachment at 20°C, with negligible terminal electrostatic interruption. Consequently, both cell adhesion and detachment were successfully improved by choosing an appropriate PIPAAm length with terminal cationization.

Design of Temperature-Responsive Cell Culture Surfaces for Cell Sheet Engineering

Temperature-responsive cell culture surfaces, which modulate cell attachment/detachment characteristics with temperature, have been used to fabricate cell sheets. Extensive study on fabrication of cell sheet with the temperature-responsive cell culture surface, manipulation, and transplantation of the cell sheet has established the interdisciplinary field of cell sheet engineering, in which engineering, biological, and medical fields closely collaborate. Such collaboration has pioneered cell sheet engineering, making it a promising and attractive technology in tissue engineering and regenerative medicine. This review introduces concepts of cell sheet engineering, followed by designs for the fabrication of various types of temperature-responsive cell culture surfaces and technologies for cell sheet manipulation. The development of various methods for the fabrication of temperature-responsive cell culture surfaces was also summarized. The availability of cell sheet engineering for the treatment and regeneration of damaged human tissue has also been described, providing examples of the clinical application of cell sheet transplantation in humans.

Tri-functional platform for the facile construction of dual-functional surfaces via a one-pot strategy

The scope of simultaneously introducing two new functionalities into the same polymeric substrate is largely limited to facile grafting approaches. Here, we designed a novel tri-functional platform and facilely constructed dual-functional surfaces in one pot by combining the "sulfur(vi)-fluoride exchange" (SuFEx) click reaction, photoinitiated polymerization and benzophenone photochemistry.

In Situ Subcellular Detachment of Cells Using a Cell-Friendly Photoresist and Spatially Modulated Light

Dynamic adhesion and detachment of subcellular regions occur during cell migration, thus a technique allowing precise control of subcellular detachment of cells will be useful for cell migration study. Previous methods for cell detachment were developed either for harvesting cells or cell sheets attached on surfaces with low resolution patterning capability, or for detaching subcellular regions located on predefined electrodes. In this paper, a method that allows in situ subcellular detachment of cells with ≈1.5 µm critical feature size while observing cells under a fluorescence microscope is introduced using a cell-friendly photoresist and spatially modulated light. Using this method, a single cell, regions in cell sheets, and a single focal adhesion complex within a cell are successfully detached. Furthermore, different subcellular regions of migrating cells are detached and changes in cell polarity and migration direction are quantitatively analyzed. This method will be useful for many applications in cell detachment, in particular when subcellular resolution is required.

A rapid one-step surface functionalization of polyvinyl chloride by combining click sulfur(vi)-fluoride exchange with benzophenone photochemistry

We demonstrated a rapid one-step strategy for polyvinyl chloride surface functionalization by combining click “sulfur(vi)-fluoride exchange” (SuFEx) reaction with benzophenone photochemistry.

Use of porous membranes in tissue barrier and co-culture models

Porous membranes enable the partitioning of cellular microenvironments in vitro, while still allowing physical and biochemical crosstalk between cells, a feature that is often necessary for recapitulating physiological functions. This article provides an overview of the different membranes used in tissue barrier and cellular co-culture models with a focus on experimental design and control of these systems. Specifically, we discuss how the structural, mechanical, chemical, and even the optical and transport properties of different membranes bestow specific advantages and disadvantages through the context of physiological relevance. This review also explores how membrane pore properties affect perfusion and solute permeability by developing an analytical framework to guide the design and use of tissue barrier or co-culture models. Ultimately, this review offers insight into the important aspects one must consider when using porous membranes in tissue barrier and lab-on-a-chip applications.

Aggregation of Culture Expanded Human Mesenchymal Stem Cells in Microcarrier-based Bioreactor

Three-dimensional aggregation of human mesenchymal stem cells (hMSCs) has been used to enhance their therapeutic properties but current fabrication protocols depend on laboratory methods and are not scalable. In this study, we developed thermal responsive poly(N-isopropylacrylamide) grafted microcarriers (PNIPAM-MCs), which supported expansion and thermal detachment of hMSCs at reduced temperature (23.0 °C). hMSCs were cultured on the PNIPAM-MCs in both spinner flask (SF) and PBS Vertical-Wheel (PBS-VW) bioreactors for expansion. At room temperature, hMSCs were detached as small cell sheets, which subsequently self-assembled into 3D hMSC aggregates in PBS-VW bioreactor and remain as single cells in SF bioreactor owing to different hydrodynamic conditions. hMSC aggregates generated from the bioreactor maintained comparable immunomodulation and cytokine secretion properties compared to the ones made from the AggreWell^®. The results of the current study demonstrate the feasibility of scale-up production of hMSC aggregates in the suspension bioreactor using thermal responsive microcarriers for integrated cell expansion and 3D aggregation in a close bioreactor system and highlight the critical role of hydrodynamics in self-assembly of detached hMSC in suspension.

Poly(N-isopropylacrylamide)-based thermoresponsive surfaces provide new types of biomedical applications

Thermoresponsive surfaces, prepared by grafting of poly(N-isopropylacrylamide) (PIPAAm) or its copolymers, have been investigated for biomedical applications. Thermoresponsive cell culture dishes that show controlled cell adhesion and detachment following external temperature changes, represent a promising application of thermoresponsive surfaces. These dishes can be used to fabricate cell sheets, which are currently used as effective therapies for patients. Thermoresponsive microcarriers for large-scale cell cultivation have also been developed by taking advantage of the thermally modulated cell adhesion and detachment properties of thermoresponsive surfaces. Furthermore, thermoresponsive bioseparation systems using thermoresponsive surfaces for separating and purifying pharmaceutical proteins and therapeutic cells have been developed, with the separation systems able to maintain their activity and biological potency throughout the procedure. These applications of thermoresponsive surfaces have been improved with progress in preparation techniques of thermoresponsive surfaces, such as polymerization methods, and surface modification techniques. In the present review, the various types of PIPAAm-based thermoresponsive surfaces are summarized by describing their preparation methods, properties, and successful biomedical applications.

Intelligent Textiles with Comfort Regulation and Inhibition of Bacterial Adhesion Realized by Cross-Linking Poly(n-isopropylacrylamide-co-ethylene glycol methacrylate) to Cotton Fabrics

Comfort regulation and inhibition of bacterial adhesion to textiles is realized by cross-linking thermoresponsive random copolymer to the cotton fabrics. By introduction of ethylene glycol methacrylate (EGMA) monomers into n-isopropylacrylamide (NIPAM) with a molar ratio of 2:18, the obtained random copolymer poly(n-isopropylacrylamide-co-ethylene glycol methacrylate), abbreviated as P(NIPAM-co-EGMA), presents a transition temperature (TT) of 40 °C in an aqueous solution with a concentration of 1 mg/mL. Because of the additional EGMA in the copolymer, the obtained P(NIPAM-co-EGMA) shows a glass transition temperature (T_g) of 0 °C, which is much lower than that of pure PNIPAM (T_g = 140 °C). Therefore, the introduction of P(NIPAM-co-EGMA) into the cotton fabrics will have little influence on the softness of the fabrics. Due to the cross-linked P(NIPAM-co-EGMA) layer on the cotton fabrics, the porosity of the polymer layer can be adjusted by varying the external temperature below or above TT, showing that regulation of the air and moisture permeability as well as the body comfort are feasible in the cotton fabrics cross-linked with P(NIPAM-co-EGMA). In addition, the cross-linked P(NIPAM-co-EGMA) layer is capable of absorbing moisture in the ambient atmosphere to form a hydrated layer on top, which can inhibit bacterial adhesion to the textiles.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Dynamic electrical behaviour of a thermoresponsive polymer in well-defined poly(N-isopropylacrylamide)-grafted semiconductor devices

Herein, we found that the phase transition behaviour from swelling state to deswelling state in response to temperature change was electrically detected in real time by using the poly(N-isopropylacrylamide)-grafted gate field effect transistor.

Graphical analysis of mammalian cell adhesion in vitro

Short-term (<2h) cell adhesion kinetics of 3 different mammalian cell types: MDCK (epithelioid), MC3T3-E1 (osteoblastic), and MDA-MB-231 (cancerous) on 7 different substratum surface chemistries spanning the experimentally-observable range of water wettability (surface energy) are graphically analyzed to qualitatively elucidate commonalities and differences among cell/surface/suspending media combinations. We find that short-term mammalian cell attachment/adhesion in vitro correlates with substratum surface energy as measured by water adhesion tension, τ≡γ_lvcosθ, where γ_lv is water liquid-vapor interfacial energy (72.8   mJ/m²) and cosθ is the cosine of the advancing contact angle subtended by a water droplet on the substratum surface. No definitive functional relationships among cell-adhesion kinetic parameters and τ were observed as in previous work, but previously-observed general trends were reproduced, especially including a sharp transition in the magnitude of kinetic parameters from relatively low-to-high near τ=0mJ/m², although the exact adhesion tension at which this transition occurs is difficult to accurately estimate from the current data set. We note, however, that the transition is within the hydrophobic range based on the τ=30mJ/m² surface-energetic dividing line that has been proposed to differentiate "hydrophobic" surfaces from "hydrophilic". Thus, a rather sharp hydrophobic/hydrophilic contrast is observed for cell adhesion for disparate cell/surface combinations.

Artificial cilia as autonomous nanoactuators: Design of a gradient self-oscillating polymer brush with controlled unidirectional motion

A gradient self-oscillating polymer brush surface with ordered, autonomous, and unidirectional ciliary motion has been designed. The self-oscillating polymer is a random copolymer composed of N-isopropylacrylamide and ruthenium tris(2,2'-bipyridine) [Ru(bpy)3], which acts as a catalyst for an oscillating chemical reaction, the Belousov-Zhabotinsky reaction. The target polymer brush surface was designed to have a thickness gradient by using sacrificial-anode atom transfer radical polymerization. The gradient structure of the polymer brush was confirmed by x-ray photoelectron spectroscopy, atomic force microscopy, and ultraviolet-visible spectroscopy. These analyses revealed that the thickness of the polymer brush was in the range of several tens of nanometers, and the amount of Ru(bpy)3 increased as the thickness increased. The gradient polymer brush induced a unidirectional propagation of the chemical wave from the region with small Ru(bpy)3 amounts to the region with large Ru(bpy)3 amounts. This spatiotemporal control of the ciliary motion would be useful in potential applications to functional surface such as autonomous mass transport systems.

Thermoresponsive-polymer-based materials for temperature-modulated bioanalysis and bioseparations

In this review, bioseparations using thermoresponsive polymers are summarized. Thermoresponsive chromatography for separating bioactive compounds and proteins, and cell separations using thermoresponsive polymers and their properties are reviewed.

Smart surfaces based on thermo-responsive polymer brushes prepared from L-alanine derivatives for cell capture and release

Two novel thermo-responsive polymer brushes were prepared from L-alanine derivatives using the surface-initiated atom transfer radical polymerization (SI-ATRP) technique. The temperature-induced cell capture and release on both polymer brush modified substrates were further explored.

Preparation of upper critical solution temperature (UCST) responsive diblock copolymers bearing pendant ureido groups and their micelle formation behavior in water

Poly(2-ureidoethyl methacrylate) (PUEM) was prepared via reversible addition-fragmentation chain transfer (RAFT) controlled radical polymerization and a post-modification reaction. PUEM shows upper critical solution temperature (UCST) behavior in aqueous solution. Although PUEM can dissolve in water above the UCST, it cannot dissolve in water below the UCST. Diblock copolymers (MmUn) composed of a biocompatible hydrophilic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) block and a PUEM block with different compositions were prepared via RAFT radical polymerization and a post-modification reaction. "M" and "U" represent PMPC and PUEM blocks, respectively, and the subscripts represent the degree of polymerization of each block. M95U149 and M20U163 formed polymer micelles comprising a PUEM core and a PMPC shell below the critical aggregation temperature (Tc) in aqueous solution. Polymer micelles were formed from M20U163 below 32 °C, which can incorporate guest molecules into the core.

Electrostatic Self-Assembly of Au Nanoparticles onto Thermosensitive Magnetic Core-Shell Microgels for Thermally Tunable and Magnetically Recyclable Catalysis

A facile route to fabricate a nanocomposite of Fe3O4@poly[N-isopropylacrylamide (NIPAM)-co-2-(dimethylamino)ethyl methacrylate (DMAEMA)]@Au (Fe3O4@PND@Au) is developed for magnetically recyclable and thermally tunable catalysis. The negatively charged Au nanoparticles with an average diameter of 10 nm are homogeneously loaded onto positively charged thermoresponsive magnetic core-shell microgels of Fe3O4@poly(NIPAM-co-DMAEMA) (Fe3O4@PND) through electrostatic self-assembly. This type of attachment offers perspectives for using charged polymeric shell on a broad variety of nanoparticles to immobilize the opposite-charged nanoparticles. The thermosensitive PND shell with swollen or collapsed properties can be as a retractable Au carrier, thereby tuning the aggregation or dispersion of Au nanoparticles, which leads to an increase or decrease of catalytic activity. Therefore, the catalytic activity of Fe3O4@PND@Au can be modulated by the volume transition of thermosensitive microgel shells. Importantly, the mode of tuning the aggregation or dispersion of Au nanoparticles using a thermosensitive carrier offers a novel strategy to adjust and control the catalytic activity, which is completely different with the traditional regulation mode of controlling the diffusion of reactants toward the catalytic Au core using the thermosensitive poly(N-isopropylacrylamide) network as a nanogate. Concurrent with the thermally tunable catalysis, the magnetic susceptibility of magnetic cores enables the Fe3O4@PND@Au nanocomposites to be capable of serving as smart nanoreactors for thermally tunable and magnetically recyclable catalysis.

Controlled decoration of the surface with macromolecules: polymerization on a self-assembled monolayer (SAM)

Polymer functionalized surfaces are important components of various sensors, solar cells and molecular electronic devices. In this context, the use of self-assembled monolayer (SAM) formation and subsequent reactions on the surface have attracted a lot of interest due to its stability, reliability and excellent control over orientation of functional groups. The chemical reactions to be employed on a SAM must ensure an effective functional group conversion while the reaction conditions must be mild enough to retain the structural integrity. This synthetic constraint has no universal solution; specific strategies such as "graft from", "graft to", "graft through" or "direct" immobilization approaches are employed depending on the nature of the substrate, polymer and its area of applications. We have reviewed current developments in the methodology of immobilization of a polymer in the first part of the article. Special emphasis has been given to the merits and demerits of certain methods. Another issue concerns the utility - demonstrated or perceived - of conjugated or non-conjugated macromolecules anchored on a functionally decorated SAM in the areas of material science and biotechnology. In the last part of the review article, we looked at the collective research efforts towards SAM-based polymer devices and identified major pointers of progress (236 references).

Effects of terminal group and chain length on temperature-responsive chromatography utilizing poly(N-isopropylacrylamide) synthesized via RAFT polymerization

Even using the same homo poly(N-isopropylacrylamide) immobilized silica beads as stationary phases, terminal functional group and chain length significantly affected temperature-dependent elution behavior of steroids.

Adhesion switch on a gecko-foot inspired smart nanocupule surface

A gecko-foot inspired nanocupule surface prepared by an AAO template covering method was composed of poly(N-isopropylacrylamide) and polystyrene blend. Both superhydrophobicity and high adhesion force were exhibited on the PNIPAm/PS film at room temperature. Moreover, by controlling the temperature, the wettability of the film could be switched between 138.1 ± 5.5° and 150.6 ± 1.5°, and the adhesion force could also be correspondingly tuned accurately by temperature. This reversibility in both wettability and adhesion force could be used to construct smart devices for fine selection of water droplets. The proof-of-concept was demonstrated by the selective catching of precise weight controlled water droplets at different temperatures. This work could help us to design new type of devices for blood bioanalysis or lossless drug transportation.