Tuning the thickness of polymer brushes grafted from nonlinearly growing multilayer assemblies

Switching it Up: The Promise of Stimuli-Responsive Polymer Systems in Biomedical Science

Responsive polymer systems have the ability to change properties or behavior in response to external stimuli. The properties of responsive polymer systems can be fine-tuned by adjusting the stimuli, enabling tailored responses for specific applications. These systems have applications in drug delivery, biosensors, tissue engineering, and more, as their ability to adapt and respond to dynamic environments leads to improved performance. However, challenges such as synthesis complexity, sensitivity limitations, and manufacturing issues need to be addressed for successful implementation. In our review, we provide a comprehensive summary on stimuli-responsive polymer systems, delving into the intricacies of their mechanisms and actions. Future developments should focus on precision medicine, multifunctionality, reversibility, bioinspired designs, and integration with advanced technologies, driving the dynamic growth of sensitive polymer systems in biomedical applications.

Transfer of functional thermoresponsive poly(glycidyl ether) coatings for cell sheet fabrication from gold to glass surfaces

Thermoresponsive polymer coatings can facilitate cell sheet fabrication under mild conditions by promoting cell adhesion and proliferation at 37 °C. At lower temperatures the detachment of confluent cell sheets is triggered without enzymatic treatment. Thus, confluent cell sheets with intact extracellular matrix for regenerative medicine or tissue engineering applications become available. Herein, we applied the previously identified structural design parameters of functional, thermoresponsive poly(glycidyl ether) brushes on gold to the more application-relevant substrate glass via the self-assembly of a corresponding block copolymer (PGE-AA) with a short surface-reactive, amine-presenting anchor block. Both, physical and covalent immobilization on glass via either multivalent ionic interactions of the anchor block with bare glass or the coupling of the anchor block to a polydopamine (PDA) adhesion layer on glass resulted in stable coatings. Atomic force microscopy revealed a high degree of roughness of covalently attached coatings on the PDA adhesion layer, while physically attached coatings on bare glass were smooth and in the brush-like regime. Cell sheets of primary human dermal fibroblasts detached reliably (86%) and within 20 ± 10 min from physically tethered PGE-AA coatings on glass when prepared under cloud point grafting conditions. The presence of the laterally inhomogeneous PDA adhesion layer, however, hindered the spontaneous temperature-triggered cell detachment from covalently grafted PGE-AA, decreasing both detachment rate and reliability. Despite being only physically attached, self-assembled monolayer brushes of PGE-AA block copolymers on glass are functional and stable thermoresponsive coatings for application in cell sheet fabrication of human fibroblasts as determined by X-ray photoelectron spectroscopy.

Effects of Methacrylate-Based Thermoresponsive Polymer Brush Composition on Fibroblast Adhesion and Morphology

Thermoresponsive polymers are being used increasingly in cell culture applications due to their temperature dependent surface properties. Poly(MEO2MA-co-OEGMA) (PMO) brushes offer tunable physical properties via variation in the copolymer ratio, but the effects of composition on cell-substrate interactions is unclear. To this end, a series of PMO brushes (0-8% OEGMA) was fabricated and L-929 fibroblast adhesion and morphology was quantified in the presence of serum (FBS) or after functionalization via the adsorption of fibronectin (FN) and vitronectin (VN). Quantification of the adsorption of model proteins, bovine serum albumin and FN, revealed that the extent of adsorption was correlated to the amount MEO2MA content, which represents the more hydrophobic component in PMO brushes. Cells exhibited delayed attachment and spreading on all PMO substrates in the presence of FBS. After 24 h, cell attachment was comparable; however, increased spreading was correlated with increased MEO2MA content. Adsorption of FN significantly increased initial cell attachment to all PMO surfaces after 2 h. This was not observed with VN; however, both FN and VN increased cell spreading/decreased cell circularity for all PMO substrates relative to FBS. Pure MEO2MA brushes with FN exhibited increased cell spreading/decreased cell circularity relative to other PMO substrates after 2 h, and elicited the highest cell density after 24 h. These results demonstrate that increased MEO2MA content in PMO substrates facilitates cell attachment and spreading, which can be further enhanced by adsorbing FN in the absence of other proteins.

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.

Temperature responsive behavior of polymer brush/polyelectrolyte multilayer composites

The complex interaction of polyelectrolyte multilayers (PEMs) physisorbed onto end-grafted polymer brushes with focus on the temperature-responsive behavior of the system is addressed in this work. The investigated brush/multilayer composite consists of a poly(styrene sulfonate)/poly(diallyldimethylammonium chloride) (PSS/PDADMAC) multilayer deposited onto the poly(N-isopropylacrylamide-b-dimethylaminoethyl methacrylate) P(NIPAM-b-DMAEMA) brush. Ellipsometry and neutron reflectometry were used to monitor the brush collapse with the thickness decrease as a function of temperature and the change in the monomer distribution perpendicular to the substrate at temperatures below, across and above the phase transition, respectively. It was found that the adsorption of PEMs onto polymer brushes had a hydrophobization effect on PDMAEMA, inducing the shift of its phase transition to lower temperatures, but without suppressing its temperature-responsiveness. Moreover, the diffusion of the free polyelectrolyte chains inside the charged brush was proved by comparing the neutron scattering length density profile of pure and the corresponding PEM-capped brushes, eased by the enhanced contrast between hydrogenated brushes and deuterated PSS chains. The results presented herein demonstrate the possibility of combining a temperature-responsive brush with polyelectrolyte multilayers without quenching the responsive behavior, even though significant interpolyelectrolyte interactions are present. This is of importance for the design of multicompartment coatings, where the brush can be used as a reservoir for the controlled release of substances and the multilayer on the top as a membrane to control the diffusion in/out by applying different stimuli.

In-depth analysis of switchable glycerol based polymeric coatings for cell sheet engineering

Scaffold-free cell sheet engineering using thermoresponsive substrates provides a promising alternative to conventional tissue engineering which in general employs biodegradable scaffold materials. We have previously developed a thermoresponsive coating with glycerol based linear copolymers that enables gentle harvesting of entire cell sheets. In this article we present an in-depth analysis of these thermoresponsive linear polyglycidyl ethers and their performance as coating for substrates in cell culture in comparison with commercially available poly(N-isopropylacrylamide) (PNIPAM) coated culture dishes. A series of copolymers of glycidyl methyl ether (GME) and glycidyl ethyl ether (EGE) was prepared in order to study their thermoresponsive properties in solution and on the surface with respect to the comonomer ratio. In both cases, when grafted to planar surfaces or spherical nanoparticles, the applied thermoresponsive polyglycerol coatings render the respective surfaces switchable. Protein adsorption experiments on copolymer coated planar surfaces with surface plasmon resonance (SPR) spectroscopy reveal the ability of the tested thermoresponsive coatings to be switched between highly protein resistant and adsorptive states. Cell culture experiments demonstrate that these thermoresponsive coatings allow for adhesion and proliferation of NIH 3T3 fibroblasts comparable to TCPS and faster than on PNIPAM substrates. Temperature triggered detachment of complete cell sheets from copolymer coated substrates was accomplished within minutes while maintaining high viability of the harvested cells. Thus such glycerol based copolymers present a promising alternative to PNIPAM as a thermoresponsive coating of cell culture substrates.

Engineering adhesion to thermoresponsive substrates: effect of polymer composition on liquid-liquid-solid wetting

Adhesion control in liquid-liquid-solid systems represents a challenge for applications ranging from self-cleaning to biocompatibility of engineered materials. By using responsive polymer chemistry and molecular self-assembly, adhesion at solid/liquid interfaces can be achieved and modulated by external stimuli. Here, we utilize thermosensitive polymeric materials based on random copolymers of di(ethylene glycol) methyl ether methacrylate (x = MEO2MA) and oligo(ethylene glycol) methyl ether methacrylate (y = OEGMA), that is, P(MEO2MAx-co-OEGMAy), to investigate the role of hydrophobicity on the phenomenon of adhesion. The copolymer ratio (x/y) dictates macromolecular changes enabling control of the hydrophilic-to-lipophilic balance (HBL) of the polymer brushes through external triggers such as ionic strength and temperature. We discuss the HBL of the thermobrushes in terms of the surface energy of the substrate by measuring the contact angle at water-decane-P(MEO2MAx-co-OEGMAy) brush contact line as a function of polymer composition and temperature. Solid supported polyelectrolyte layers grafted with P(MEO2MAx-co-OEGMAy) display a transition in the wettability that is related to the lower critical solution temperature of the polymer brushes. Using experimental observation of the hydrophilic to hydrophobic transition by the contact angle, we extract the underlying energetics associated with liquid-liquid-solid adhesion as a function of the copolymer ratio. The change in cellular attachment on P(MEO2MAx-co-OEGMAy) substrates of variable (x/y) composition demonstrates the subtle role of compositional tuning on the ability to control liquid-liquid-solid adhesion in biological applications.

Flexible thermoresponsive nanomembranes at the aqueous–air interface

Thermoresponsive freestanding nanomembranes were grown by surface-initiated polymerization at the aqueous–air interface of a pendant drop. We demonstrate formation of liquid-like interfaces supporting anisotropic stress and mechanical flexibility.

Poly(2-substituted-2-oxazoline) surfaces for dermal fibroblasts adhesion and detachment

The thermoresponsive surfaces of brush structure (linear polymer chains tethered on the surface) based on poly(2-isopropyl-2-oxazoline)s and copolymers of 2-ethyl-2-oxazoline and 2-nonyl-2-oxazoline were obtained using the grafting-to method. The living oxazoline (co)polymers have been synthesized by cationic ring-opening polymerization and subsequently terminated by the reactive amine groups present on the surface. The changes in the surface morphology, philicity and thickness occurring during surface modification were monitored via atomic force microscopy, contact angle and ellipsometry. The thickness of the (co)poly(2-substituted-2-oxazoline) layers ranged from 4 to 11 nm depending on the molar mass of immobilized polymer and reversibly varied with the temperature changes. This confirmed thermoresponsive properties of obtained surfaces. The obtained polymer surfaces were used as a support for dermal fibroblast culture and detachment. The fibroblasts' adhesion and proliferation on the polymer surfaces were observed when the culture temperature was above the cloud point temperature of the immobilized polymer. Lowering the temperature resulted in the detachment of the dermal fibroblast sheets from the polymer layers, which makes these surfaces suitable for the treatment of wounds and in skin tissue engineering.

Surface modification with thermoresponsive polymer brushes for a switchable electrochemical sensor

Thermoresponsive polymer brushes' reversible phase transition is evidenced with E-QCM-D and electrochemistry, and acts as an electrochemical switch.

Effect of colloidal substrate curvature on pH-responsive polyelectrolyte brush growth

Coatings consisting of polymer brushes are an effective way to modify solid interfaces. Polymer brush-modified hybrid particles have been prepared by surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP) of 2-(diethylamino)ethyl methacrylate (DEA) on silica particles. We have optimized the synthesis with respect to changing the reducing agent, temperature, and reaction solvent from an aqueous ethanol mixture to an aqueous methanol mixture. Our flexible electrostatically adsorbed macroinitiator approach allows for the modification of a variety of surfaces. Polybasic brushes have been grown on silica particles of different sizes, from 120 to 840 nm in diameter, as well as on wafers, and a comparison of the products has allowed the effect of surface curvature to be elucidated. An examination of the thickness of the dry brush and the aqueous hydrodynamic brush at both pH 7 and at 4 demonstrated that growth increased substantially with substrate curvature for particles with a diameter below 450 nm. This is attributed to the increasing separation between active chain ends, reducing the rate of termination. This is believed to be the first time that this effect has been demonstrated experimentally. Furthermore, we have seen that polymer brush growth on planar wafers was significantly reduced when the reaction mixture was stirred.

Poly[tri(ethylene glycol) ethyl ether methacrylate]-coated surfaces for controlled fibroblasts culturing

Well-defined thermosensitive poly[tri(ethylene glycol) monoethyl ether methacrylate] (P(TEGMA-EE)) brushes were synthesized on a solid substrate by the surface-initiated atom transfer radical polymerization of TEGMA-EE. The polymerization reaction was initiated by 2-bromo-2-methylpropionate groups immobilized on the surface of the wafers. The changes in the surface composition, morphology, philicity, and thickness that occurred at each step of wafer functionalization confirmed that all surface modification procedures were successful. Both the successful modification of the surface and bonding of the P(TEGMA-EE) layer were confirmed by X-ray photoelectron spectroscopy (XPS) measurements. The thickness of the obtained P(TEGMA-EE) layers increased with increasing polymerization time. The increase of environmental temperature above the cloud point temperature of P(TEGMA-EE) caused the changes of surface philicity. A simultaneous decrease in the polymer layer thickness confirmed the thermosensitive properties of these P(TEGMA-EE) layers. The thermosensitive polymer surfaces obtained were evaluated for the growth and harvesting of human fibroblasts (basic skin cells). At 37 °C, seeded cells adhered to and spread well onto the P(TEGMA-EE)-coated surfaces. A confluent cell sheet was formed within 24 h of cell culture. Lowering the temperature to an optimal value of 17.5 °C (below the cloud point temperature of the polymer, TCP, in cell culture medium) led to the separation of the fibroblast sheet from the polymer layer. These promising results indicate that the surfaces produced may successfully be used as substrate for engineering of skin tissue, especially for delivering cell sheets in the treatment of burns and slow-healing wounds.

Thermoresponsive PEG-based polymer layers: surface characterization with AFM force measurements

Thermoresponsive polymer-coated surfaces based on poly(2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate) [P(MEO(2)MA-co-OEGMA)] allow switching between cell attachment and detachment. Here, we investigate the temperature-dependent surface interactions between the polymer coating and a colloidal probe in an aqueous medium by means of atomic force microscopy (AFM) force-distance measurements. The analysis of the adhesion forces from AFM retraction curves identifies two kinds of regimes for the copolymer at temperatures below and above the lower critical solution temperature (LCST). Whereas at 25 degrees C the surface interactions with the polymer in the swollen state are dominated by repulsive forces, at 37 degrees C the surface interactions switch to attractive forces and a stronger adhesion is detected by AFM. Running several heating/cooling cycles repeatedly shows that switching the surface properties provides reproducible adhesion force values. Time-dependent measurements give insight into the switching kinetics, demonstrating that the cell response is coupled to the polymer kinetics but probably limited by the cellular rearrangements.

PEGylated Chromatography: Efficient Bioseparation on Silica Monoliths Grafted with Smart Biocompatible Polymers

Novel oligo(ethylene glycol)-based thermoresponsive stationary phases have been studied for the separation of bioanalytes. Well-defined copolymers of (2-methoxyethoxy)ethyl methacrylate and oligo(ethylene glycol) methacrylate were synthesized by atom-transfer radical polymerization in the presence of an N-succinimidyl-functionalized initiator. The reactive chain ends of these copolymers were then reacted with amino-functionalized silica monoliths. The formed composites were studied as chromatography materials. For instance, it was demonstrated that thermoresponsive oligo(ethylene glycol)-based stationary phases allow rapid, efficient separation of steroid and protein mixtures in pure water under isocratic high-performance liquid chromatographic elution.