Bio-functionalized thermoresponsive interfaces facilitating cell adhesion and proliferation

Automated detection of patterned single-cells within hydrogel using deep learning

Single-cell analysis has been widely used in various biomedical engineering applications, ranging from cancer diagnostics, and immune response monitoring to drug screening. Single-cell isolation is fundamental for observing single-cell activities and an automatic finding method of accurate and reliable cell detection with few possible human errors is also essential. This paper reports trapping single cells into photo patternable hydrogel microwell arrays and isolating them. Additionally, we present an object detection-based DL algorithm that detects single cells in microwell arrays and predicts the presence of cells in resource-limited environments at the highest possible mAP (mean average precision) of 0.989 with an average inference time of 0.06 s. This algorithm leads to the enhancement of the high-throughput single-cell analysis, establishing high detection precision and reduced experimentation time.

Functional Thermoresponsive Hydrogel Molecule to Material Design for Biomedical Applications

Temperature-induced, rapid changes in the viscosity and reproducible 3-D structure formation makes thermos-sensitive hydrogels an ideal delivery system to act as a cell scaffold or a drug reservoir. Moreover, the hydrogels’ minimum invasiveness, high biocompatibility, and facile elimination from the body have gathered a lot of attention from researchers. This review article attempts to present a complete picture of the exhaustive arena, including the synthesis, mechanism, and biomedical applications of thermosensitive hydrogels. A special section on intellectual property and marketed products tries to shed some light on the commercial potential of thermosensitive hydrogels.

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.

Thermoresponsive Microgel Coatings as Versatile Functional Compounds for Novel Cell Manipulation Tools

For the effective use of live cells in biomedicine as in vitro test systems or in biotechnology, non-invasive cell processing and characterisation are key elements. Thermoresponsive polymer coatings have been demonstrated to be highly beneficial for controlling the interaction of adherent cells through their cultivation support. However, the widespread application of these coatings is hampered by limitations in their adaptability to different cell types and because the full range of applications has not yet been fully explored. In the work presented here, we address these issues by focusing on three different aspects. With regard to the first aspect, by using well-defined laminar flow in a microchannel, a highly controllable and reproducible shear force can be applied to adherent cells. Employing this tool, we demonstrate that cells can be non-invasively detached from a support using a defined shear flow. The second aspect relates to the recent development of simple methods for patterning thermoresponsive coatings. Here, we show how such patterned coatings can be used for improving the handling and reliability of a wound-healing assay. Two pattern geometries are tested using mouse fibroblasts and CHO cells. In terms of the third aspect, the adhesiveness of cells depends on the cell type. Standard thermoresponsive coatings are not functional for all types of cells. By coadsorbing charged nanoparticles and thermoresponsive microgels, it is demonstrated that the adhesion and detachment behaviour of cells on such coatings can be modulated.

Mixed Copolymer Adlayers Allowing Reversible Thermal Control of Single Cell Aspect Ratio

Dynamic guidance of living cells is achieved by fine-tuning and spatiotemporal modulation on artificial polymer layers enabling reversible peptide display. Adjustment of surface composition and interactions is obtained by coadsorption of mixed poly(lysine) derivatives, grafted with either repellent PEG, RGD adhesion peptides, or T-responsive poly(N-isopropylacrylamide) strands. Deposition of mixed adlayers provides a straightforward mean to optimize complex substrates, which is here implemented to achieve (1) thermal control of ligand accessibility and (2) adjustment of relative adhesiveness between adjacent micropatterns, while preserving cell attachment during thermal cycles. The reversible polarization of HeLa cells along orthogonal stripes mimics guidance along natural matrices.

Thermogelling, ABC Triblock Copolymer Platform for Resorbable Hydrogels with Tunable, Degradation-Mediated Drug Release

Clinical application of injectable, thermoresponsive hydrogels is hindered by lack of degradability and controlled drug release. To overcome these challenges, a family of thermoresponsive, ABC triblock polymer-based hydrogels has been engineered to degrade and release drug cargo through either oxidative or hydrolytic/enzymatic mechanisms dictated by the "A" block composition. Three ABC triblock copolymers are synthesized with varying "A" blocks, including oxidation-sensitive poly(propylene sulfide), slow hydrolytically/enzymatically degradable poly(ε-caprolactone), and fast hydrolytically/enzymatically degradable poly(D,L-lactide-co-glycolide), forming the respective formulations PPS135-b-PDMA152-b-PNIPAAM225 (PDN), PCL85-b-PDMA150-b-PNIPAAM150 (CDN), and PLGA60-b-PDMA148-b-PNIPAAM152 (LGDN). For all three polymers, hydrophilic poly(N,N-dimethylacrylamide) and thermally responsive poly(N-isopropylacrylamide) comprise the "B" and "C" blocks, respectively. These copolymers form micelles in aqueous solutions at ambient temperature that can be preloaded with small molecule drugs. These solutions quickly transition into hydrogels upon heating to 37 °C, forming a supra-assembly of physically crosslinked, drug-loaded micelles. PDN hydrogels are selectively degraded under oxidative conditions while CDN and LGDN hydrogels are inert to oxidation but show differential rates of hydrolytic/enzymatic decomposition. All three hydrogels are cytocompatible in vitro and in vivo, and drug-loaded hydrogels demonstrate differential release kinetics in vivo corresponding with their specific degradation mechanism. These collective data highlight the potential cell and drug delivery use of this tunable class of ABC triblock polymer thermogels.

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.

Free Epoxide Content Mediates Encapsulated Cell Viability and Activity through Protein Interactions in a Thermoresponsive, In Situ Forming Hydrogel

The development of synthetic and well-defined extracellular matrices that are free of xenogeneic components and are capable of inducing desired cellular responses holds great potential for use in vitro as 3D cell culture environments and in vivo as scaffolds for tissue regeneration. In this study, the impact of free and partially occupied epoxide groups on the viability, activity, and behavior of rat fibroblasts encapsulated in thermoresponsive hydrogels based on poly(N-isopropylacrylamide) (pNiPAAm) was investigated. While fibroblasts encapsulated in neat pNiPAAm remained rounded and showed significant toxicity by 5 days, those encapsulated in the epoxide-modified thermogels demonstrated not only high viability but also an ability to proliferate, attach, produce extracellular matrix (ECM) components, and cluster. The results demonstrated that the presence of free epoxide groups led to the local conjugation of available proteins to produce a modified structure in situ, which supported cell viability, activity, and cluster formation within the hydrogel.

Temperature-Controlled Three-Stage Switching of Wetting, Morphology, and Protein Adsorption

The novel polymeric coatings of oligoperoxide-graft-poly(4-vinylpyridine-co-oligo(ethylene glycol)ethyl ether methacrylate246) [oligoperoxide-graft-P(4VP-co-OEGMA246)] attached to glass were successfully fabricated. The composition, thickness, morphology, and wettability of resulting coatings were analyzed using X-ray photoelectron spectroscopy, ellipsometry, atomic force microscopy, and contact angle measurements, respectively. In addition, adsorption of the bovine serum albumin was examined with fluorescence microscopy. The thermal response of wettability and morphology of the coatings followed by that of protein adsorption revealed two distinct transitions at 10 and 23 °C. For the first time, three stage switching was observed not only for surface wetting but also for morphology and protein adsorption. Moreover, the influence of the pH on thermo-sensitivity of modified surfaces was shown.

Self-assembled supramolecular polymers with tailorable properties that enhance cell attachment and proliferation

Self-assembled supramolecular scaffolds, a combination of noncovalent interactions within a biocompatible polymer substrate, can be used for efficient construction of highly-controlled self-organizing hierarchical structures; these newly-developed biomaterials exhibit excellent mechanical properties, tunable surface hydrophilicity, low cytotoxicity and high biodegradability, making them highly attractive for tissue engineering and regenerative medicine applications. Herein, we demonstrate a novel supramolecular poly(ε-caprolactone) (PCL) containing self-complementary sextuple hydrogen-bonded uracil-diamidopyridine (U-DPy) moieties, which undergoes spontaneous self-assembly to form supramolecular polymer networks. Inclusion of various U-DPy contents enhanced the mechanical strength and viscosities of the resulting materials by up to two orders of magnitude compared to control PCL. Surface wettability and morphological studies confirmed physically-crosslinked films can be readily tailored to provide the desired surface properties. Cell viability assays indicated the excellent in vitro biocompatibility of U-DPy-functionalized substrates and indicate the potential of these materials for various biomedical applications. More importantly, mouse fibroblast NIH/3T3 cells cultured on these substrates displayed a more elongated cell morphology and had substantially higher cell densities than cells seeded on control PCL substrate, which indicates that introduction of U-DPy moieties into polymer matrixes could be used to create tissue culture surfaces that enhance cell attachment and proliferation. This new system is suggested as a potential route towards the practical realization of next-generation tissue-engineering scaffolds.

STATEMENT OF SIGNIFICANCE

In this study, we report a significant breakthrough in development of self-assembled supramolecular polymers to form well-defined scaffolds through self-complementary hydrogen-bonding interactions. These newly developed materials exhibited extremely good mechanical properties, fine-tunable hydrophilic characteristics and excellent biocompatibility due to hydrogen-bond-induced physical cross-linking. Importantly, cell adhesion and proliferation assays indicated that these substrates efficiently promoted the growth of mouse embryonic fibroblasts NIH/3T3 cells in vitro. Thus, this finding provides a simple and effective route for the development of next-generation tissue-engineering scaffolds that have improved mechanical properties, increased surface hydrophilicity and can enhance the growth and biological activity of adherent cells.

SiO2/TiO2 Nanocomposite Films on Polystyrene for Light-Induced Cell Detachment Application

Light-induced cell detachment shows much potential in in vitro cell culture and calls for high-performance light-responsive films. In this study, a smooth and dense SiO₂/TiO₂ nanocomposite thin film with thickness of around 250 nm was first fabricated on H₂O₂ treated polystyrene (PS) substrate via a low-temperature sol-gel method. It was observed that the film could well-adhere on the PS surface and the bonding strength became increasingly high with the increase of SiO₂ content. The peeling strength and shear strength reached 3.05 and 30.02 MPa, respectively. It was observed the surface of the film could transform into superhydrophilic upon 20 min illumination of ultraviolet with a wavelength of 365 nm (UV365). In cell culture, cells, i.e., NIH3T3 and MC3T3-E1 cells, cultured on SiO₂/TiO₂ nanocomposite film were easily detached after 10 min of UV365 illumination; the detachment rates reached 90.8% and 88.6%, respectively. Correspondingly, continuous cell sheets with good viability were also easily obtained through the same way. The present work shows that SiO₂/TiO₂ nanocomposite thin film could be easily prepared on polymeric surface at low temperature. The corresponding film exhibits excellent biocompatibility, high bonding strength, and good light responses. It could be a good candidate for the surface of cell culture utensils with light-induced cell detachment property.

Design and cytocompatibility of chitosan-based thermoresponsive cell culture plates

BACKGROUND

The aim of this study was to develop a novel thermoresponsive material suited for tissue engineering and investigate the growth and harmless detachment of cells cultured on the surface of thermoresponsive tissue culture polystyrene (TCPS).

METHODS

Thermoresponsive N-isopropylacrylamide (NIPAAm) and biocompatible chitosan (CS) were grafted onto the surface of TCPS by ultraviolet (UV)-induced graft polymerization. The chemical composition, surface morphology and thermoresponsiveness of the modified TCPS were investigated by X-ray photoelectron spectroscopy (XPS), atom force microscopy (AFM) and contact angle (CA), respectively. Furthermore, the growth and detachment behaviors of mouse fibroblast cells (L929) on the surface of the modified TCPS were studied by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

The modified TCPS exhibited good hydrophobic/hydrophilic property alterations in response to temperature. The cytocompatibility of the materials was improved due to the introduction of CS. Cells could be spontaneously detached from the surface without any damage, by controlling environmental temperature. The viability of cells obtained by temperature induction was higher than that obtained by enzymatic digestion.

CONCLUSIONS

This study developed a simple and economical method to fabricate thermoresponsive cell culture dishes and provided new thoughts and experimental bases for exploring novel material applied in tissue engineering.

Thermoresponsive Polymer Micropatterns Fabricated by Dip-Pen Nanolithography for a Highly Controllable Substrate with Potential Cellular Applications

We report a novel approach for patterning thermoresponsive hydrogels based on N,N-diethylacrylamide (DEAAm) and bifunctional Jeffamine ED-600 by dip-pen nanolithography (DPN). The direct writing of micron-sized thermoresponsive polymer spots was achieved with efficient control over feature size. A Jeffamine-based ink prepared through the combination of organic polymers, such as DEAAm, in an inorganic silica network was used to print thermosensitive arrays on a thiol-silanized silicon oxide substrate. The use of a Jeffamine hydrogel, acting as a carrier matrix, allowed a reduction in the evaporation of ink molecules with high volatility, such as DEAAm, and facilitated the transfer of ink from tip to substrate. The thermoresponsive behavior of polymer arrays which swell/deswell in aqueous solution in response to a change in temperature was successfully characterized by atomic force microscopy (AFM) and Raman spectroscopy: a thermally induced change in height and hydration state was observed, respectively. Finally, we demonstrate that cells can adhere to and interact with these dynamic features and exhibit a change in behavior when cultured on the substrates above and below the transition temperature of the Jeffamine/DEAAm thermoresponsive hydrogels. This demonstrates the potential of these micropatterned hydrogels to act as a controllable surface for cell growth.

Thermoresponsive polyurethane/siloxane membrane for wound dressing and cell sheet transplantation: In-vitro and in-vivo studies

Polyurethane/siloxane based wound dressing for transferring fibroblast cell sheet to wounded skin and ability to provide an optimum condition for cellular activity at damaged tissue was prepared in this research. The dressing was made thermoresponsive, via the introduction of a poly(N-isopropyl acrylamide) copolymer into the backbone of dressing. The ability of membrane for adhesion, growth, and proliferation of fibroblast cells was improved via surface modification with gelatin. The optimized dressing exhibited appropriate tensile strength (4.5MPa) and elongation at break (80%) to protect wound against physical forces. Due to controlled equilibrium water absorption of about 89% and water vapor transmission rate of 2040g/m(2)day, the dressing could maintain the favorable moist environment over moderate to high exuding wounds. The grown cell sheet on dressing membrane could easily roll up from the surface just with lowering the temperature. The in vivo study of the wound dressed with cell loaded membrane confirmed the accelerated healing and production of tissue with complete re-epithelization, enhanced vascularization, and increased collagen deposition on the damaged area.

Progress in Corneal Stromal Repair: From Tissue Grafts and Biomaterials to Modular Supramolecular Tissue-Like Assemblies

Corneal injuries and degenerative conditions have major socioeconomic consequences, given that in most cases, they result in blindness. In the quest of the ideal therapy, tissue grafts, biomaterials, and modular engineering approaches are under intense investigation. Herein, advancements and shortfalls are reviewed and future perspectives for these therapeutic strategies discussed.

Co-immobilization of adhesive peptides and VEGF within a dextran-based coating for vascular applications

Multifunctional constructs providing a proper environment for adhesion and growth of selected cell types are needed for most tissue engineering and regenerative medicine applications. In this context, vinylsulfone (VS)-modified dextran was proposed as a matrix featuring low-fouling properties as well as multiple versatile moieties. The displayed VS groups could indeed react with thiol, amine or hydroxyl groups, be it for surface grafting, crosslinking or subsequent tethering of biomolecules. In the present study, a library of dextran-VS was produced, grafted to aminated substrates and characterized in terms of degree of VS modification (%VS), cell-repelling properties and potential for the oriented grafting of cysteine-tagged peptides. As a bioactive coating of vascular implants, ECM peptides (e.g. RGD) as well as vascular endothelial growth factor (VEGF) were co-immobilized on one of the most suitable dextran-VS coating (%VS=ca. 50% of saccharides units). Both RGD and VEGF were efficiently tethered at high densities (ca. 1nmol/cm(2) and 50fmol/cm(2), respectively), and were able to promote endothelial cell adhesion as well as proliferation. The latter was enhanced to the same extent as with soluble VEGF and proved selective to endothelial cells over smooth muscle cells. Altogether, multiple biomolecules could be efficiently incorporated into a dextran-VS construct, while maintaining their respective biological activity.

STATEMENT OF SIGNIFICANCE

This work addresses the need for multifunctional coatings and selective cell response inherent to many tissue engineering and regenerative medicine applications, for instance, vascular graft. More specifically, a library of dextrans was first generated through vinylsulfone (VS) modification. Thoroughly selected dextran-VS provided an ideal platform for unbiased study of cell response to covalently grafted biomolecules. Considering that processes such as healing and angiogenesis require multiple factors acting synergistically, vascular endothelial growth factor (VEGF) was then co-immobilized with the cell adhesive RGD peptide within our dextran coating through a relevant strategy featuring orientation and specificity. Altogether, both adhesive and proliferative cues could be incorporated into our construct with additive, if not synergetic, effects.

Thermo-responsive poly(N-isopropylacrylamide)-grafted hollow fiber membranes for osteoblasts culture and non-invasive harvest

Hollow fiber membrane (HFM) culture system is one of the most important bioreactors for the large-scale culture and expansion of therapeutic cells. However, enzymatic and mechanical treatments are traditionally applied to harvest the expanded cells from HFMs, which inevitably causes harm to the cells. In this study, thermo-responsive cellulose acetate HFMs for cell culture and non-invasive harvest were prepared for the first time via free radical polymerization in the presence of cerium (IV). ATR-FTIR and elemental analysis results indicated that the poly(N-isopropylacrylamide) (PNIPAAm) was covalently grafted on HFMs successfully. Dynamic contact angle measurements at different temperatures revealed that the magnitude of volume phase transition was decreased with increasing grafted amount of PNIPAAm. And the amount of serum protein adsorbed on HFMs surface also displayed the same pattern. Meanwhile osteoblasts adhered and spread well on the surface of PNIPAAm-grafted HFMs at 37 °C. And Calcein-AM/PI staining, AB assay, ALP activity and OCN protein expression level all showed that PNIPAAm-grafted HFMs had good cell compatibility. After incubation at 20 °C for 120 min, the adhering cells on PNIPAAm-grafted HFMs turned to be round and detached after being gently pipetted. These results suggest that thermo-responsive HFMs are attractive cell culture substrates which enable cell culture, expansion and the recovery without proteolytic enzyme treatment for the application in tissue engineering and regenerative medicine.

Thermoresponsive polymers: insights into decisive hydrogel characteristics, mechanisms of gelation, and promising biomedical applications

Thermally induced gelling systems have gained enormous attention over the last decade. They consist of hydrophilic homopolymers or block copolymers in water that present a sol at room temperature and form a gel after administration into the body. This article reviews the main types of thermoresponsive polymers, with special focus on decisive hydrogel characteristics, mechanisms of gelation, and biocompatibility. Promising biomedical applications are described with a focus on injectable formulations, which include solubilization of small hydrophobic drugs, controlled release, delivery of labile biopharmaceutics, such as proteins and genes, cell encapsulation, and tissue regeneration. Furthermore, combinations of thermoresponsive hydrogels and various nanocarriers as promising systems for sustained drug delivery are discussed through selected examples from the literature. Finally, there is a brief overview of current progress in nano-sized systems incorporating thermoresponsive properties.

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.

Synthesis of poly(N-isopropylacrylamide) particles for metal affinity binding of peptides

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgel particles with metal affinity ligands were prepared for selective binding of peptides containing the His6-tag (six consecutive histidine residues). The PNIPAM particles were copolymerized with the functional ligand vinylbenzyl iminodiacetic acid (VBIDA) through a two-stage dispersion polymerization using poly(N-vinyl pyrrolidone) (PVP) as a steric stabilizer. The resulting particles were monodisperse in size and colloidally stable over a wide range of temperature and ionic strength due to chemically grafted PVP chains. The particle size was also found to be sensitive to ionic strength and pH of the aqueous environment, likely due to the electrostatic repulsion between ionized VBIDA groups. Divalent nickel ions were chelated to the VBIDA groups, allowing selective metal affinity attachment of a His6-Cys peptide. The peptide was released upon the addition of the competitive ligand imidazole, demonstrating that the peptide attachment to the particles is reversible and selective.

Tissue engineering of the corneal endothelium: a review of carrier materials

Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings.

Heparin-functionalized thermoresponsive surface: a versatile cell culture substrate for regulating multivalent affinity binding with heparin-binding proteins by temperature changes

Temperature-dependent regulation of affinity binding between bioactive ligands and their cell membrane receptors is an attractive approach for the dynamic control of cellular adhesion, proliferation, migration, differentiation, and signal transduction. Covalent conjugation of bioactive ligands onto thermoresponsive poly(N-isopropylacrylamide) (PIPAAm)-grafted surfaces facilitates the modulation of one-on-one affinity binding between bioactive ligands and cellular receptors by changing temperature. For the dynamic control of the multivalent affinity binding between heparin and heparin-binding proteins, thermoresponsive cell culture surface modified with heparin, which interacts with heparin-binding proteins such as basic fibroblast growth factor (bFGF), has been proposed. Heparin-functionalized thermoresponsive cell culture surface induces (1) the multivalent affinity binding of bFGF in active form and (2) accelerating cell sheet formation in the state of shrunken PIPAAm chains at 37°C. By lowering temperature to 20°C, the affinity binding between bFGF and immobilized heparin is reduced with increasing the mobility of heparin and the swollen PIPAAm chains, leading to the detachment of cultured cells. Therefore, heparin-functionalized thermoresponsive cell culture surface was able to enhance cell proliferation and detach confluent cells as a contiguous cell sheet by changing temperature. A cell cultivation system using heparin-functionalized thermoresponsive cell culture surface is versatile for immobilizing other heparin-binding proteins such as vascular endothelial growth factor, fibronectin, antithrombin III, and hepatocyte growth factor, etc. for tuning the adhesion, growth, and differentiation of various cell species.

Efficient cell and cell-sheet harvesting based on smart surfaces coated with a multifunctional and self-organizing elastin-like recombinamer

A wide range of smart surfaces with novel properties relevant for biomedical applications have been developed recently. Herein we focus on thermoresponsive surfaces that switch between cell-adherent and nonadherent states and their applications for cell harvesting. These smart surfaces are obtained by covalently coupling a tailored elastin-like recombinamer onto glass surfaces by means of the well-known and widely applied Click Chemistry methodology. The resulting recombinamer-functionalized surfaces have been characterized by means of water contact angle measurements, XPS and TOF-SIMS. A cell-based analysis of these surfaces with human fibroblasts showed a high degree of adhesion to the surface in its adherent state (37 °C), thus, promoting cell viability and proliferation. A temperature decrease triggers reorganization of the recombinamer, thus, markedly increasing the number of nonadherent domains and masking the adherent ones. This process allows a specific and efficient temporal control of cell adhesion and cell detachment. After determination of the properties required for a suitable cell-harvesting system, optimization of the process allows single cells or cell sheets from at least two types of cells (HFF-1 and ADSCs) to be rapidly harvested.

Dynamic manipulation of hydrogels to control cell behavior: a review

For many tissue engineering applications and studies to understand how materials fundamentally affect cellular functions, it is important to have the ability to synthesize biomaterials that can mimic elements of native cell-extracellular matrix interactions. Hydrogels possess many properties that are desirable for studying cell behavior. For example, hydrogels are biocompatible and can be biochemically and mechanically altered by exploiting the presentation of cell adhesive epitopes or by changing hydrogel crosslinking density. To establish physical and biochemical tunability, hydrogels can be engineered to alter their properties upon interaction with external driving forces such as pH, temperature, electric current, as well as exposure to cytocompatible irradiation. Additionally, hydrogels can be engineered to respond to enzymes secreted by cells, such as matrix metalloproteinases and hyaluronidases. This review details different strategies and mechanisms by which biomaterials, specifically hydrogels, can be manipulated dynamically to affect cell behavior. By employing the appropriate combination of stimuli and hydrogel composition and architecture, cell behavior such as adhesion, migration, proliferation, and differentiation can be controlled in real time. This three-dimensional control in cell behavior can help create programmable cell niches that can be useful for fundamental cell studies and in a variety of tissue engineering applications.

Biomimetic approaches to modulate cellular adhesion in biomaterials: A review

Natural extracellular matrix (ECM) proteins possess critical biological characteristics that provide a platform for cellular adhesion and activation of highly regulated signaling pathways. However, ECM-based biomaterials can have several limitations, including poor mechanical properties and risk of immunogenicity. Synthetic biomaterials alleviate the risks associated with natural biomaterials but often lack the robust biological activity necessary to direct cell function beyond initial adhesion. A thorough understanding of receptor-mediated cellular adhesion to the ECM and subsequent signaling activation has facilitated development of techniques that functionalize inert biomaterials to provide a biologically active surface. Here we review a range of approaches used to modify biomaterial surfaces for optimal receptor-mediated cell interactions, as well as provide insights into specific mechanisms of downstream signaling activation. In addition to a brief overview of integrin receptor-mediated cell function, so-called "biomimetic" techniques reviewed here include (i) surface modification of biomaterials with bioadhesive ECM macromolecules or specific binding motifs, (ii) nanoscale patterning of the materials and (iii) the use of "natural-like" biomaterials.

Thermoresponsive cell culture substrates based on PNIPAM brushes functionalized with adhesion peptides: theoretical considerations of mechanism and design

Thermoresponsive tissue culture substrates based on PNIPAM brushes are used to harvest confluent cell sheets for tissue engineering. The prospect of clinical use imposes the utilization of culture medium free of bovine serum, thus suggesting conjugation with adhesion peptides containing the RGD minimal recognition sequence. The optimum position of the RGD along the chain should ensure both cell adhesion at 37 °C and cell detachment at T(L) below the lower critical solution temperature of PNIPAM. Design guidelines are formulated from considerations of brush confinement by the cells: (i) Cell adhesion at 37 °C is controlled by the RGDs accessible without brush compression. (ii) Cell detachment at T(L) is driven by a disjoining force due to confinement of the swollen brush by cells retaining integrin-RGD bonds formed at 37 °C. These suggest placing the RGDs at the grafting surface or its vicinity. Randomly placed RGDs do not enable efficient detachment because a large fraction of the integrin-RGD bonds are not sufficiently tensioned at T(L), in line with experimental observations (Ebara, M.; Yamato, M.; Aoyagi, T.; Kikuchi, A.; Sakai, K.; Okano, T. Immobilization of celladhesive peptides to temperature-responsive surfaces facilitates both serum-free cell adhesion and noninvasive cell harvest. Tissue Eng. 2004, 10, 1125-1135). The theory framework enables analysis of culture media based on polymer brushes conjugated with adhesion peptides in general.

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.

Review paper: Critical Issues in Tissue Engineering: Biomaterials, Cell Sources, Angiogenesis, and Drug Delivery Systems

Tissue engineering is a newly emerging biomedical technology, which aids and increases the repair and regeneration of deficient and injured tissues. It employs the principles from the fields of materials science, cell biology, transplantation, and engineering in an effort to treat or replace damaged tissues. Tissue engineering and development of complex tissues or organs, such as heart, muscle, kidney, liver, and lung, are still a distant milestone in twenty-first century. Generally, there are four main challenges in tissue engineering which need optimization. These include biomaterials, cell sources, vascularization of engineered tissues, and design of drug delivery systems. Biomaterials and cell sources should be specific for the engineering of each tissue or organ. On the other hand, angiogenesis is required not only for the treatment of a variety of ischemic conditions, but it is also a critical component of virtually all tissue-engineering strategies. Therefore, controlling the dose, location, and duration of releasing angiogenic factors via polymeric delivery systems, in order to ultimately better mimic the stem cell niche through scaffolds, will dictate the utility of a variety of biomaterials in tissue regeneration. This review focuses on the use of polymeric vehicles that are made of synthetic and/or natural biomaterials as scaffolds for three-dimensional cell cultures and for locally delivering the inductive growth factors in various formats to provide a method of controlled, localized delivery for the desired time frame and for vascularized tissue-engineering therapies.

Dual responsive nanostructured surfaces for biomedical applications

In this paper, we describe the construction and characteristics of thermoresponsive, thin nanostructured films prepared by layer-by-layer sequential assembly of chitosan-graft-NIPAAm and alginate. FTIR and (1)H NMR spectra have confirmed the introduction of NIPAAm moieties onto the chitosan backbone. The LCST of the synthesized copolymer was found to be around 31-33 °C. The formation of the polyelectrolyte multilayers containing the copolymer and alginate was followed in situ by quartz crystal microbalance with dissipation monitoring technique and ex situ by UV-vis measurements. Our results revealed the linear increase of the multilayer film growth and the influence of the presence of salt. Moreover, AFM analysis has confirmed that PNIPAAm is able to reconform upon temperature swaps even when combined with other layers in a polyelectrolyte multilayer, demonstrating that the nanoassemblies are thermoresponsive. Preliminary results showed that, upon reducing culture temperature below PNIPAAm LCST, a gradual detachment of cell sheets from these PNIPAAm-based coatings has occurred.

N-isopropylacrylamide-based thermoresponsive polyelectrolyte multilayer films for human mesenchymal stem cell expansion

Human mesenchymal stem cells (hMSCs) are colony-forming unit fibroblasts (CFU-F) derived from adult bone marrow and have significant potential for many cell-based tissue-engineering applications. Their therapeutic potential, however, is restricted by their diminishing plasticity as they are expanded in culture. In this study, we used N-isopropylacrylamide (NIPAM)-based thermoresponsive polyelectrolyte multilayer (N-PEMU) films as culture substrates to support hMSC expansion and evaluated their effects on cell properties. The N-PEMU films were made via layer-by-layer adsorption of thermoresponsive monomers copolymerized with charged monomers, positively charged allylamine hydrochloride (PAH), or negatively charged styrene sulfonic acid (PSS) and compared to fetal bovine serum (FBS) coated surfaces. Surface charges were shown to alter the extracellular matrix (ECM) structure and subsequently regulate hMSC responses including adhesion, proliferation, integrin expression, detachment, and colony forming ability. The positively charged thermal responsive surfaces improved cell adhesion and growth in a range comparable to control surfaces while maintaining significantly higher CFU-F forming ability. Immunostaining and Western blot results indicate that the improved cell adhesion and growth on the positively charged surfaces resulted from the elevated adhesion of ECM proteins such as fibronectin on the positively charge surfaces. These results demonstrate that the layer-by-layer approach is an efficient way to form PNIPAM-based thermal responsive surfaces for hMSC growth and removal without enzymatic treatment. The results also show that surface charge regulates ECM adhesion, which in turn influences not only cell adhesion but also CFU-forming ability and their multi-lineage differentiation potential.

Engineering the extracellular environment: Strategies for building 2D and 3D cellular structures

Cell fate is regulated by extracellular environmental signals. Receptor specific interaction of the cell with proteins, glycans, soluble factors as well as neighboring cells can steer cells towards proliferation, differentiation, apoptosis or migration. In this review, approaches to build cellular structures by engineering aspects of the extracellular environment are described. These methods include non-specific modifications to control the wettability and stiffness of surfaces using self-assembled monolayers (SAMs) and polyelectrolyte multilayers (PEMs) as well as methods where the temporal activation and spatial distribution of adhesion ligands is controlled. Building on these techniques, construction of two-dimensional cell sheets using temperature sensitive polymers or electrochemical dissolution is described together with current applications of these grafts in the clinical arena. Finally, methods to pattern cells in three-dimensions as well as to functionalize the 3D environment with biologic motifs take us one step closer to being able to engineer multicellular tissues and organs.