Switching of cell growth/detachment on heparin-functionalized thermoresponsive surface for rapid cell sheet fabrication and manipulation

Molecular design of dynamically thermoresponsive biomaterials

Dynamically thermoresponsive biomaterials, particularly those utilizing poly(N-isopropylacrylamide) (PNIPAAm), have attracted much attention in biomedical applications due to their reversible phase transition near body temperature. These biomaterials provide innovations across drug delivery system, chromatography, and tissue engineering. Molecular designs, such as the incorporation of hydrophilic comonomers or graft copolymers in PNIPAAm hydrogels, enhance rapid kinetics of the gels when jumping the temperature across the phase transition temperature, because of avoiding 'skin layer' formation on the surface of the gels. Nanocarriers possessing PNIPAAm coronas facilitate spatial drug delivery and temporally on-demand drug release to targeted cancers in combination with hyperthermic therapy. Downsizing of PNIPAAm hydrogels accelerates the kinetics of shrinkage/swelling, leading to applications as thermoresponsive chromatographic matrices and cell cultureware. PNIPAAm-modified surfaces support thermoresponsive cell culture systems for the non-invasive recovery of intact cell sheets, enabling advanced regenerative therapies and layered 3D tissue formation. Recent developments also integrate growth factor delivery for sustained cell stimulation on culturewares. Newly developed biomaterials, including dynamically thermoresponsive PNIPAAm, are expected to expand the opportunity for novel treatment technologies such as targeted therapies and regenerative medicine.

BMP-Binding Polysulfonate Brushes to Control Growth Factor Presentation and Regulate Matrix Remodelling

Bone morphogenetic proteins (BMPs) are important targets to incorporate in biomaterial scaffolds to orchestrate tissue repair. Glycosaminoglycans (GAGs) such as heparin allow the capture of BMPs and their retention at the surface of biomaterials at safe concentrations. Although heparin has strong affinities for BMP2 and BMP4, two important types of growth factors regulating bone and tissue repair, it remains difficult to embed stably at the surface of a broad range of biomaterials and degrades rapidly in vitro and in vivo. In this report, biomimetic poly(sulfopropyl methacrylate) (PSPMA) brushes are proposed as sulfated GAG mimetic interfaces for the stable capture of BMPs. The growth of PSPMA brushes via a surface-initiated activator regenerated by electron transfer polymerization is investigated via ellipsometry, prior to characterization of swelling and surface chemistry via X-ray photoelectron spectroscopy and Fourier transform infrared. The capacity of PSPMA brushes to bind BMP2 and BMP4 is then characterized via surface plasmon resonance. BMP2 is found to anchor particularly stably and at high density at the surface of PSPMA brushes, and a strong impact of the brush architecture on binding capacity is observed. These results are further confirmed using a quartz crystal microbalance with dissipation monitoring, providing some insights into the mode of adsorption of BMPs at the surface of PSPMA brushes. Primary adsorption of BMP2, with relatively little infiltration, is observed on thick dense brushes, implying that this growth factor should be accessible for further binding of corresponding cell membrane receptors. Finally, to demonstrate the impact of PSPMA brushes for BMP2 capture, dermal fibroblasts were then cultured at the surface of functionalized PSPMA brushes. The presence of BMP2 and the architecture of the brush are found to have a significant impact on matrix deposition at the corresponding interfaces. Therefore, PSPMA brushes emerge as attractive coatings for scaffold engineering and stable capture of BMP2 for regenerative medicine applications.

Bioanalytical technologies using temperature-responsive polymers

In recent decades, various bioanalytical technologies have been investigated for appropriate medical treatment and effective therapy. Temperature-responsive chromatography is a promising bioanalytical technology owing to its functional properties. Temperature-responsive chromatography uses a poly(N-isopropylacrylamide)(PNIPAAm) modified stationary phase as the column packing material. The hydrophobic interactions between PNIPAAm and the analyte could be modulated by changing the column temperature because of the temperature-responsive hydrophobicity of PNIPAAm. Thus, the chromatography system does not require organic solvents in the mobile phase, making it suitable for therapeutic drug monitoring in medical settings such as hospitals. This review summarizes recent developments in temperature-responsive chromatography systems for therapeutic drug monitoring applications. In addition, separation methods for antibody drugs using PNIPAAm are also summarized because these methods apply to the therapeutic drug monitoring of biopharmaceutics. The temperature-responsive chromatography systems can also be utilized for clinical diagnosis, as they can assess multiple medicines simultaneously. This highlights the significant potential of temperature-responsive chromatography in medicine and healthcare.

Terminus-Selective Covalent Immobilization of Heparin on a Thermoresponsive Surface Using Click Chemistry for Efficient Binding of Basic Fibroblast Growth Factor

Cell therapy using endothelial cells (ECs) has great potential for the treatment of congenital disorders, such as hemophilia A. Cell sheet technology utilizing a thermoresponsive culture dish is a promising approach to efficiently transplant donor cells. In this study, a new method to prepare terminus-selective heparin-immobilized thermoresponsive culture surfaces is developed to facilitate the preparation of EC sheets. Alkynes are introduced to the reducing terminus of heparin via reductive amination. Cu-catalyzed azide-alkyne cycloaddition (CuAAC) facilitates efficient immobilization of the terminus of heparin on a thermoresponsive surface, resulting in a higher amount of immobilized heparin while preserving its function. Heparin-immobilized thermoresponsive surfaces prepared using CuAAC exhibit good adhesion to human endothelial colony-forming cells (ECFCs). In addition, upon further binding to basic fibroblast growth factor (bFGF) on heparin-immobilized surfaces, increased proliferation of ECFCs on the surface is observed. The confluent ECFC monolayer cultured on bFGF-bound heparin-immobilized thermoresponsive surfaces exhibits relatively high fibronectin accumulation and cell number and detaches at 22 °C while maintaining the sheet-like structure. Because heparin has an affinity for several types of bioactive molecules, the proposed method can be applied to facilitate efficient cultures and sheet formations of various cell types.

Cell Sheet Technology as an Engineering-Based Approach to Bone Regeneration

Bone defects that are congenital or the result of infection, malignancy, or trauma represent a challenge to the global healthcare system. To address this issue, multiple research groups have been developing novel cell sheet technology (CST)-based approaches to promote bone regeneration. These methods hold promise for use in regenerative medicine because they preserve cell-cell contacts, cell-extracellular matrix interactions, and the protein makeup of cell membranes. This review introduces the concept and preparation system of the cell sheet (CS), explores the application of CST in bone regeneration, highlights the current states of the bone regeneration via CST, and offers perspectives on the challenges and future research direction of translating current knowledge from the lab to the clinic.

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.

"Fuzzy hair" promotes cell sheet detachment from thermoresponsive brushes already above their volume phase transition temperature

Thermoresponsive poly(glycidyl ether) (PGE) brushes have shown to be viable substrates for the culture and temperature-triggered detachment of confluent cell sheets. Surface-tethered PGEs with a cloud point temperature (T_CP) around ~30 °C exhibit phase transitions well-centered within the physiological range (20-37 °C), which makes them ideal candidates for cell sheet fabrication. However, PGEs with T_CPs at ~20 °C also afford the detachment of various types of cell sheets, even at room temperature (20-23 °C), i.e., above the polymers' T_CPs. In this study, we investigate the phase transition of PGE brushes tethered to polystyrene (PS) culture substrates with varying grafting density and T_CP to arrive at a mechanistic understanding of their functionality in cell sheet fabrication. Using quartz crystal microbalance with dissipation (QCM-D) monitoring, we demonstrate that brushes fabricated from PGEs with T_CPs at ~20 °C display volume phase transition temperatures (VPTTs) well below room temperature. Although the investigated coatings obviously do not exhibit marked thermal switching in terms of brush hydration and layer thickness, their physical properties at the brush-water interface, as ascertained by QCM-D and AFM measurements, undergo subtle changes upon cooling from 37 °C to room temperature which is sufficient to promote cell sheet detachment. Thus, it appears that discreet rehydration of the outmost brush layer, resembling "fuzzy hair" at the brush-water interface, renders the surfaces less protein- and cell-adhesive at room temperature. This minor structural change of the interface allows for the reliable detachment of human dermal fibroblast sheets already at 20 °C well above the VPTT of the brushes.

Microfluidic assembly of small-molecule prodrug cocktail nanoparticles with high reproducibility for synergistic combination of cancer therapy

Therapeutic nanoparticles (NPs) self-assembled from small molecular (pro)drug entities, opens up novel avenues for the generation of a wide range of drug delivery systems. Particularly, cocktail NPs created by co-assembly of multiple therapeutics often show profound efficacy beyond their individual agents. However, fabrication of synergistic NPs with high reproducibility and capability to deliver multiple therapeutics in a predefined ratio remains a challenge, which deters NP therapeutics from further clinical translation. In this work, a simple but versatile strategy has been developed to combine drug reconstitution and supramolecular nanoassembly to prodrug cocktail nanoparticle fabrication with microfluidics. Prodrugs reconstructed by PUFAylation were self-assembled into hybrid nanoparticles via microfluidic chip to synergistically deliver two chemotherapeutic drugs, 7-ethyl-10-hydroxy camptothecin (SN38) and paclitaxel (PTX), in a single nanoparticle container. In vitro cell-based assays demonstrate that the combinatorial chemotherapy is superior to each prodrug used alone while reduces the dosage of both drugs at the same time. Furthermore, the double-drug combination suppresses colon tumors by 86% at a total dosage of 16.7 mg/kg through synergy, and histological analysis indicates the safety of the hybrid nanoparticles. In general, this work shows that the nanomedicine synthesized by microfluidics provides considerable advantages including better size control and reproducibility, and great potential in effective combination therapy. It is expected to be applied to the fabrication of more chemical agent combination for other cancer types.

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.

Versatile Mitogenic and Differentiation-Inducible Layer Formation by Underwater Adhesive Polypeptides

Artificial materials have no biological functions, but they are important for medical devices such as artificial organs and matrices for regenerative medicine. In this study, mitogenic and differentiation-inducible materials are devised via the simple coating of polypeptides, which contain the sequence of epidermal growth factor or insulin-like growth factor with a key amino acid (3,4-dihydroxyphenylalanine) of underwater adhesive proteins. The adhesive polypeptides prepared via solid-phase synthesis form layers on various substrates involving organic and inorganic materials to provide biological surfaces. Through the direct activation of cognate receptors on interactive surfaces, the materials enable increased cell growth and differentiation compared to that achieved by soluble growth factors. This superior growth and differentiation are attributed to the long-lasting signal transduction (triggered by the bound growth factors), which do not cause receptor internalization and subsequent downregulation.

Rapid harvesting of stem cell sheets by thermoresponsive bulk poly(N-isopropylacrylamide) (PNIPAAm) nanotopography

To date, cell sheet engineering-based technologies have actualized diverse scaffold-free bio-products to revitalize unintentionally damaged tissues/organs, including cardiomyopathy, corneal defects, and periodontal damage. Although substantial interest is now centered on the practical utilization of these bio-products for patients, the long harvest period of stem cells- or other primary cell-sheets has become a huge hurdle. Here, we dramatically reduce the total harvest period of a cell sheet (from cell layer formation to cell sheet detachment) composed of human bone marrow mesenchymal stem cells (hBMSCs) down to 2 d with the help of bulk thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) substrate nanotopography, which is not achievable via the previous grafting methods using PNIPAAm. We directly replicated an isotropic 400 nm-nanopore-array pattern on a bulk PNIPAAm substrate through UV polymerization of highly concentrated NIPAAm monomers, which was achieved using a remarkably increased Young's modulus of bulk PNIPAAm that was 1500 times higher than conventional PNIPAAm. The rapid harvesting of the hBMSC sheet on the bulk PNIPAAm substrate nanotopography was not only based on the accelerated formation and maturation of the hBMSC layer, but also the easy detachment of the hBMSC sheet induced by the abrupt change in the surface roughness of the substrate below the lower critical solution temperature (LCST) owing to the enlarged surface area of the substrate. Our findings may contribute to reverse presumptions about the limitations regarding the grafting methods for the cell sheet harvest and could broaden the practical utilization of cell sheets for patients in the near future.

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.

Preservation of heparin-binding EGF-like growth factor activity on heparin-modified poly(N-isopropylacrylamide)-grafted surfaces

A heparin-modified poly(N-isopropylacrylamide) (PIPAAm)-grafted surface bound with heparin-binding epidermal growth factor-like growth factor (HB-EGF) was able to culture hepatocytes maintaining high albumin secretion and high expression of hepatocyte-specific genes. However, the activity of HB-EGF on the surface and its binding effects on hepatocytes remain unclear. In this study, we investigated the temperature-dependent interactions of HB-EGF and EGF receptor (EGFR) with heparin-modified PIPAAm to evaluate the activity of HB-EGF on the surface. Quartz crystal microbalance (QCM) measurements revealed that the amounts of adsorbed HB-EGF on either the heparin-modified PIPAAm-grafted surface (heparin-IC1) or PIPAAm-grafted surfaces were almost the same regardless of swelling/deswelling of grafted PIPAAm chains. The heparin-IC1 surface bound to HB-EGF at 37 °C had the ability to bind to hepatocytes through specific affinity interaction with EGFR, whose activation was confirmed by western blotting. However, the physisorbed HB-EGF on the PIPAAm surface greatly diminished its activity. Taken together, the introduction of heparin into grafted PIPAAm chains on the surface plays a pivotal role in holding HB-EGF while preserving its activity. Hydration and swelling of surface-grafted PIPAAm chains at 20 °C greatly diminished the attachment of hepatocytes with HB-EGF bound to heparin-IC1, whereas hepatocytes were able to bind to HB-EGF bound to heparin-IC1 at 37 °C. Thus, the equilibrated affinity interaction between EGFRs and surface-bound HB-EGF was considered to be attenuated by steric hindrance due to hydration and/or swelling of grafted PIPAAm chains.

Activity of HB-EGF bound to a heparin-modified poly(N-isopropylacrylamide) (PIPAAm)-grafted surface was preserved through specific binding to heparin, whereas physisorbed HB-EGF on a PIPAAm-grafted surface greatly diminished its activity.

Trypsin-Free Cultivation of 3D Mini-Tissues in an Adaptive Membrane Bioreactor

The production of large scaffold-free tissues is a key challenge in regenerative medicine. Nowadays, temperature-responsive polymers allow intact tissue harvesting without needing proteolytic enzymes. This method is limited to tissue culture plastic with limited upscaling capacity and plain process control. Here, a thermoresponsive hollow fiber membrane bioreactor is presented to produce large scaffold-free tissues. Intact tissues, rich in cell-to-cell connections and ECM, are harvested from a poly(N-vinylcaprolactam) microgel functionalized poly(ether sulfone)/poly(vinylpyrrolidone) hollow fiber membrane by a temperature shift. The harvested 3D tissues adhere in successive cultivation and exhibit high vitality for several days. The facile adsorptive coating waives the need for extensive surface treatment. The research is anticipated to be a starting point for upscaling the production of interconnected tissues enabling new opportunities in regenerative medicine, large-scale drug screening on physiological relevant tissues, and potentially opening new chances in cell-based therapies.

Hierarchical Capillary Coating to Biofunctionlize Drug-Eluting Stent for Improving Endothelium Regeneration

The drug-eluting stent (DES) has become one of the most successful and important medical devices for coronary heart disease, but yet suffers from insufficient endothelial cell (EC) growth and intima repair, eventually leading to treatment failure. Although biomacromolecules such as vascular endothelial growth factor (VEGF) would be promising to promote the intima regeneration, combining hydrophilic and vulnerable biomacromolecules with hydrophobic drugs as well as preserving the bioactivity after harsh treatments pose a huge challenge. Here, we report on a design of hierarchical capillary coating, which composes a base solid region and a top microporous region for incorporating rapamycin and VEGF, respectively. The top spongy region can guarantee the efficient, safe, and controllable loading of VEGF up to 1 μg/cm2 in 1 minute, providing a distinctive real-time loading capacity for saving the bioactivity. Based on this, we demonstrate that our rapamycin-VEGF hierarchical coating impressively promoted the competitive growth of endothelial cells over smooth muscle cells (ratio of EC/SMC~25) while relieving the adverse impact of rapamycin to ECs. We further conducted the real-time loading of VEGF on stents and demonstrate that the hierarchical combination of rapamycin and VEGF showed remarkable endothelium regeneration while maintaining a very low level of in-stent restenosis. This work paves an avenue for the combination of both hydrophobic and hydrophilic functional molecules, which should benefit the next generation of DES and may extend applications to diversified combination medical devices.

Characteristic differences of cell sheets composed of mesenchymal stem cells with different tissue origins

INTRODUCTION

Stem cell therapy with mesenchymal stem cells (MSCs) has been widely used in many clinical trials, and therapy with MSC sheets shows promise for patients. However, there are few reports characterizing MSC sheets. In the present study, the properties of MSC sheets derived from bone marrow, adipose tissue, and umbilical cord were evaluated.

METHODS

Cell sheets were fabricated with MSCs from different tissue origins in temperature-responsive cell culture dishes with and without pre-coating of fetal bovine serum (FBS). MSC adhesion behavior in the culture dish was observed. Secretion of cytokines related to cell proliferation and immune regulation from MSC sheets was investigated by ELISA. The adhesion properties of the MSC sheets were investigated by time-lapse microscopy.

RESULTS

Different cell adhesion and proliferation rates in temperature-responsive cell culture dishes were observed among the three types of MSCs. FBS pre-coating of the dishes enhanced cell attachment and proliferation in all cell types. Harvested cell sheets showed high attachment capacity to tissue culture polystyrene dish surfaces.

CONCLUSIONS

MSC sheets can be fabricated from MSCs from different tissue origins using temperature-responsive cell culture dishes. The fabricated MSC sheets could be useful in cell transplantation therapies by choosing appropriate types of MSCs that secrete therapeutic cytokines for the targeted diseases.

Bioinert and Lubricious Surfaces by Macromolecular Design

The modification of a variety of biomaterials and medical devices often encompasses the generation of biopassive and lubricious layers on their exposed surfaces. This is valid when the synthetic supports are required to integrate within physiological media without altering their interfacial composition and when the minimization of shear stress prevents or reduces damage to the surrounding environment. In many of these cases, hydrophilic polymer brushes assembled from surface-interacting polymer adsorbates or directly grown by surface-initiated polymerizations (SIP) are chosen. Although growing efforts by polymer chemists have been focusing on varying the composition of polymer brushes in order to attain increasingly bioinert and lubricious surfaces, the precise modulation of polymer architecture has simultaneously enabled us to substantially broaden the tuning potential for the above-mentioned properties. This feature article concentrates on reviewing this latter strategy, comparatively analyzing how polymer brush parameters such as molecular weight and grafting density, the application of block copolymers, the introduction of branching and cross-links, or the variation of polymer topology beyond the simple, linear chains determine highly technologically relevant properties, such as biopassivity and lubrication.

Effect of Temperature Changes on Serum Protein Adsorption on Thermoresponsive Cell-Culture Surfaces Monitored by A Quartz Crystal Microbalance with Dissipation

Thermoresponsive cell-culture polystyrene (PS) surfaces that are grafted with poly(N-isopropylacrylamide) (PIPAAm) facilitate the cultivation of cells at 37 °C and the detachment of cultured cells as a sheet with an underlying extracellular matrix (ECM) by reducing the temperature. However, the ECM and cell detachment mechanisms are still unclear because the detachment of cells from thermoresponsive surfaces is governed by complex interactions among the cells/ECM/surface. To explore the dynamic behavior of serum protein adsorption/desorption, thermoresponsive surfaces that correspond to thermoresponsive tissue-culture PS dishes were formed on sensor chips for quartz crystal microbalance with dissipation (QCM-D) measurements. X-ray photoelectron spectroscopy (XPS) measurements and temperature-dependent frequency and dissipation shifts, Δf and ΔD, using QCM-D revealed that the thermoresponsive polymers were successfully grafted onto oxidized, thin PS films on the surfaces of the sensor chips. Increased amounts of adsorbed bovine serum albumin (BSA) and fibronectin (FN) were observed on the thermoresponsive polymer-grafted surfaces at 37 °C when compared with those at 20 °C because of enhanced hydrophobic interactions with the hydrophobic, thermoresponsive surface. While the calculated masses of adsorbed BSA and FN using QCM-D were 3⁻5 times more than those that were obtained from radiolabeling, the values were utilized for relative comparisons among the same substrate. More importantly, the thermoresponsive, dynamic behavior of serum protein adsorption/desorption was monitored using the QCM-D technique. Observations of this dynamic behavior revealed that the BSA and FN that were adsorbed at 37 °C remained on both surfaces after decreasing the temperature to 20 °C.

Design of Temperature-Responsive Cell Culture Surfaces for Cell Sheet-Based Regenerative Therapy and 3D Tissue Fabrication

This chapter describes the concept of "cell sheet engineering" for the creation of transplantable cellular tissues and organs. In contrast to scaffold-based tissue engineering, cell sheet engineering facilitates the reconstruction of scaffold-free, cell-dense tissues. Cell sheets were harvested by changing the temperature of thermoresponsive cell culture surfaces modified with poly(N-isopropylacrylamide) (PIPAAm) with a thickness on the nanometer scale. The transplantation of 2D cell sheet tissues has been used in clinical settings. Although 3D tissues were formed simply by layering 2D cell sheets, issues related to vascularization within 3D tissues and the large-scale production of cells must be addressed to create thick and large 3D tissues and organs.

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.

Dynamically Tunable Cell Culture Platforms for Tissue Engineering and Mechanobiology

Human tissues are sophisticated ensembles of many distinct cell types embedded in the complex, but well-defined, structures of the extracellular matrix (ECM). Dynamic biochemical, physicochemical, and mechano-structural changes in the ECM define and regulate tissue-specific cell behaviors. To recapitulate this complex environment in vitro, dynamic polymer-based biomaterials have emerged as powerful tools to probe and direct active changes in cell function. The rapid evolution of polymerization chemistries, structural modulation, and processing technologies, as well as the incorporation of stimuli-responsiveness, now permit synthetic microenvironments to capture much of the dynamic complexity of native tissue. These platforms are comprised not only of natural polymers chemically and molecularly similar to ECM, but those fully synthetic in origin. Here, we review recent in vitro efforts to mimic the dynamic microenvironment comprising native tissue ECM from the viewpoint of material design. We also discuss how these dynamic polymer-based biomaterials are being used in fundamental cell mechanobiology studies, as well as towards efforts in tissue engineering and regenerative medicine.

Smart Polymers with Special Wettability

Surface wettability plays a key role in addressing issues ranging from basic life activities to our daily life, and thus being able to control it is an attractive goal. Learning from nature, both of its structure and function, brings us much inspiration in designing smart polymers to tackle this major challenge. Life functions particularly depend on biomolecular recognition-induced interfacial properties from the aqueous phase onto either "soft" cell and tissue or "hard" inorganic bone and tooth surfaces. The driving force is noncovalent weak interactions rather than strong covalent combinations. An overview is provided of the weak interactions that perform vital actions in mediating biological processes, which serve as a basis for elaborating multi-component polymers with special wettabilities. The role of smart polymers from molecular recognitions to macroscopic properties are highlighted. The rationale is that highly selective weak interactions are capable of creating a dynamic synergetic communication in the building components of polymers. Biomolecules could selectively induce conformational transitions of polymer chains, and then drive a switching of physicochemical properties, e.g., roughness, stiffness and compositions, which are an integrated embodiment of macroscopic surface wettabilities.

A heparin-modified thermoresponsive surface with heparin-binding epidermal growth factor-like growth factor for maintaining hepatic functions in vitro and harvesting hepatocyte sheets

A heparin-modified thermoresponsive surface bound with heparin-binding epidermal growth factor-like growth factor (HB-EGF) was designed to allow creation of transferrable and functional hepatocyte sheets. A heparin-modified thermoresponsive surface was prepared by covalently tethering heparin onto poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide)-grafted tissue culture polystyrene surfaces (Heparin-IC). HB-EGFs were able to stably bind to heparin-IC via affinity interaction. The survival of primary rat hepatocytes was maintained through HB-EGF-bound heparin-IC (HB-EGF/heparin-IC). Moreover, cultured rat primary hepatocytes on HB-EGF/heparin-IC exhibited higher albumin-secretion than hepatocytes cultured on PIPAAm-grafted and collagen-coated surfaces with soluble HB-EGF in the culture medium, regardless of whether soluble EGF was added. These results suggested that HB-EGF/heparin-IC is able to effectively maintain hepatic function via continuous signaling of HB-EGF. After a 4-day cultivation, the cultured hepatocytes on HB-EGF/heparin-IC detached as a cell sheet with fibronectin and HB-EGF only after the temperature was lowered to 20 °C. In addition, higher expression of hepatocyte-specific genes (albumin, hepatocyte nuclear factor 4 alpha, coagulation factor VII, and coagulation factor IX) in hepatocyte sheets was detected on HB-EGF/heparin-IC than on a PIPAAm surface with soluble HB-EGF, indicating that HB-EGF/heparin-IC suppressed the dedifferentiation of cultured hepatocytes. Hence, heparin-modified thermoresponsive surfaces bound with HB-EGF facilitate the fabrication of transferrable hepatocyte sheets with intact hepatic functions and have the potential to provide an in vitro culture system using functional hepatocyte sheet tissues, which may serve as an effective hepatocyte-based tissue engineering platform for liver disease treatments.

Targeted grafting of thermoresponsive polymers from a penetrative honeycomb structure for cell sheet engineering

Responsive membranes have been used to construct smart biomaterial interfaces. We report a novel approach to fabricate honeycomb films with a pattern of thermoresponsive polymer, namely poly(N-isopropylacrylamide). The approach was based on a combination of the breath figure method and reversible addition-fragmentation chain transfer. The hybrid film had morphological and chemical patterns resulting in varied wettability and morphology at various stages, as well as high thermo-responsiveness. Enhanced cell adhesion was observed at an incubation temperature of 37 °C, which is above its lower critical solution temperature (LCST). Furthermore, cells could be harvested at temperatures below the LCST without trypsin treatment. The non-invasive characteristics give this membrane potential as a substrate for cell sheet engineering.

Modification of Bacterial Cellulose with Organosilanes to Improve Attachment and Spreading of Human Fibroblasts

Bacterial Cellulose (BC) synthesized by Acetobacter xylinum has been a promising candidate for medical applications. Modifying BC to possess the properties needed for specific applications has been reported. In this study, BCs functionalized by organosilanes were hypothesized to improve the attachment and spreading of Normal Human Dermal Fibroblast (NHDF). The BC gels obtained from biosynthesis were dried by either ambient-air drying or freeze drying. The surfaces of those dried BCs were chemically modified by grafting methyl terminated octadecyltrichlorosilane (OTS) or amine terminated 3-aminopropyltriethoxysilane (APTES) to expectedly increase hydrophobic or electrostatic interactions with NHDF cells, respectively. NHDF cells improved their attachment and spreading on the majority of APTES-modified BCs (∼70-80% of area coverage by cells) with more rapid growth (∼2.6-2.8× after incubations from 24 to 48h) than on tissue culture polystyrene (∼2×); while the inverse results (< 5% of area coverage and stationary growth) were observed on the OTS-modified BCs. For organosilane modified BCs, the drying method had no effect on in vitro cell attachment/spreading behaviors.

Cellular Responses Modulated by FGF-2 Adsorbed on Albumin/Heparin Layer-by-Layer Assemblies

In a typical cell culture system, growth factors immobilized on the cell culture surfaces can serve as a reservoir of bio-signaling molecules, without the need to supplement them additionally into the culture medium. In this paper, we report on the fabrication of albumin/heparin (Alb/Hep) assemblies for controlled binding of basic fibroblast growth factor (FGF-2). The surfaces were constructed by layer-by-layer adsorption of polyelectrolytes albumin and heparin and were subsequently stabilized by covalent crosslinking with glutaraldehyde. An analysis of the surface morphology by atomic force microscopy showed that two Alb/Hep bilayers are required to cover the surface of substrate. The formation of the Alb/Hep assemblies was monitored by the surface plasmon resonance (SPR), the infrared multiinternal reflection spectroscopy (FTIR MIRS) and UV/VIS spectroscopy. The adsorption of FGF-2 on the cross-linked Alb/Hep was followed by SPR. The results revealed that FGF-2 binds to the Alb/Hep assembly in a dose and time-dependent manner up to the surface concentration of 120 ng/cm². The bioactivity of the adsorbed FGF-2 was assessed in experiments in vitro, using calf pulmonary arterial endothelial cells (CPAE). CPAE cells could attach and proliferate on Alb/Hep surfaces. The adsorbed FGF-2 was bioactive and stimulated both the proliferation and the differentiation of CPAE cells. The improvement was more pronounced at a lower FGF-2 surface concentration (30 ng/cm²) than on surfaces with a higher concentration of FGF-2 (120 ng/cm²).

Enhanced Pulsatile Drug Release from Injectable Magnetic Hydrogels with Embedded Thermosensitive Microgels

Nanocomposite in situ-gelling hydrogels containing both superparamagnetic iron oxide nanoparticles (SPIONs) and thermoresponsive microgels are demonstrated to facilitate pulsatile, high-low release of a model drug (4 kDa fluorescein-labeled dextran). The materials can be injected through a minimally invasive route, facilitate a ∼4-fold enhancement of release when pulsed on relative to the off state, and, in contrast to previous gel-based systems, can maintain pulsatile release properties over multiple cycles and multiple days instead of only hours. Optimal pulsatile release is achieved when the microgel transition temperature is engineered to lie just above the (physiological) incubation temperature. Coupled with the demonstrated degradability of the nanocomposites and the cytocompatibility of all nanocomposite components, we anticipate these nanocomposites have potential to facilitate physiologically relevant, controlled pulsatile drug delivery.

Dynamic biointerfaces: from recognition to function

The transformation of recognition signals into regulating macroscopic behaviors of biological entities (e.g., biomolecules and cells) is an extraordinarily challenging task in engineering interfacial properties of artificial materials. Recently, there has been extensive research for dynamic biointerfaces driven by biomimetic techniques. Weak interactions and chirality are two crucial routes that nature uses to achieve its functions, including protein folding, the DNA double helix, phospholipid membranes, photosystems, and shell and tooth growths. Learning from nature inspires us to design dynamic biointerfaces, which usually take advantage of highly selective weak interactions (e.g., synergetic chiral H-bonding interactions) to tailor their molecular assemblies on external stimuli. Biomolecules can induce the conformational transitions of dynamic biointerfaces, then drive a switching of surface characteristics (topographic structure, wettability, etc.), and eventually achieve macroscopic functions. The emerging progresses of dynamic biointerfaces are reviewed and its role from molecular recognitions to biological functions highlighted. Finally, a discussion is presented of the integration of dynamic biointerfaces with the basic biochemical processes, possibly solving the big challenges in life science.

Stripe-patterned thermo-responsive cell culture dish for cell separation without cell labeling

A stripe-patterned thermo-responsive surface is prepared to enable cell separation without labeling. The thermo-responsive surface containing a 3 μm striped pattern exhibits various cell adhesion and detachment properties. A mixture of three cell types is separated on the patterned surface based on their distinct cell-adhesion properties, and the composition of the cells is analyzed by flow cytometry.

Advances in cell culture: anchorage dependence

Anchorage-dependent cells are of great interest for various biotechnological applications. (i) They represent a formidable production means of viruses for vaccination purposes at very large scales (in 1000-6000 l reactors) using microcarriers, and in the last decade many more novel viral vaccines have been developed using this production technology. (ii) With the advent of stem cells and their use/potential use in clinics for cell therapy and regenerative medicine purposes, the development of novel culture devices and technologies for adherent cells has accelerated greatly with a view to the large-scale expansion of these cells. Presently, the really scalable systems--microcarrier/microcarrier-clump cultures using stirred-tank reactors--for the expansion of stem cells are still in their infancy. Only laboratory scale reactors of maximally 2.5 l working volume have been evaluated because thorough knowledge and basic understanding of critical issues with respect to cell expansion while retaining pluripotency and differentiation potential, and the impact of the culture environment on stem cell fate, etc., are still lacking and require further studies. This article gives an overview on critical issues common to all cell culture systems for adherent cells as well as specifics for different types of stem cells in view of small- and large-scale cell expansion and production processes.

Recent development of temperature-responsive surfaces and their application for cell sheet engineering

Cell sheet engineering, which fabricates sheet-like tissues without biodegradable scaffolds, has been proposed as a novel approach for tissue engineering. Cells have been cultured and proliferate to confluence on a temperature-responsive cell culture surface at 37°C. By decreasing temperature to 20°C, an intact cell sheet can be harvested from the culture surface without enzymatic treatment. This new approach enables cells to keep their cell–cell junction, cell surface proteins and extracellular matrix. Therefore, recovered cell sheet can be easily not only transplanted to host tissue, but also constructed a three-dimensional (3D) tissue by layering cell sheets. Moreover, cell sheet manipulation technology and bioreactor have been combined with the cell sheet technology to fabricate a complex and functional 3D tissue in vitro. So far, cell sheet technology has been applied in regenerative medicine for several tissues, and a number of clinical studies have been performed. In this review, recent advances in the preparation of temperature-responsive cell culture surface, the fabrication of organ-like tissue and the clinical application of cell sheet engineering are summarized and discussed.

Polymer brush coatings regulating cell behavior: passive interfaces turn into active

Material technology platforms able to modulate the communication with cells at the interface of biomaterials are being increasingly experimented. Progress in the fabrication of supports is simultaneously introducing new surface modification strategies aimed at turning these supports from passive to active components in engineered preparations. Among these platforms, polymer brushes are arising not only as coatings determining the physical and (bio)chemical surface properties of biomaterials, but also as smart linkers between surfaces and biological cues. Their peculiar properties, especially when brushes are synthesized by "grafting-from" methods, enable closer mimicking of the complex and heterogeneous biological microenvironments. Inspired by the growing interest in this field of materials science, we summarize here the most prominent and recent advances in the synthesis of "grafted-from" polymer brush surfaces to modulate the response of adhering cells.

Surface design of antibody-immobilized thermoresponsive cell culture dishes for recovering intact cells by low-temperature treatment

Antibody-immobilized thermoresponsive poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) [poly(IPAAm-co-CIPAAm)]-grafted cell culture surfaces were designed to enhance both the initial adhesion of weakly adhering cells and the ability of cells to detach in response to low temperature through the regulation of affinity binding between immobilized antibodies and antigens on the cellular surface. Ty-82 cells and neonatal normal human dermal fibroblasts (NHDFs), which express CD90 on the cell surface, adhered to anti-CD90 antibody-immobilized thermoresponsive surfaces at 37°C, a condition at which the grafted thermoresponsive polymer chains shrank. Adherent Ty-82 cells were detached from the surfaces by lowering the temperature to 20°C and applying external forces, such as pipetting, whereas cultured NHDF sheets spontaneously detached themselves from the surface in response to reduced temperature alone. When the temperature was decreased to 20°C, the swelling of grafted thermoresponsive polymer chains weakened the affinity binding between immobilized antibody and antigen on the cells due to the increasing steric hindrance of the polymer chains around the antigen-recognition site of the immobilized antibodies. No contamination was detected on cells harvested from covalently immobilized antibodies on the culture surfaces by low-temperature treatment, whereas a carryover of the antibody and avidin from the avidin-biotin binding surface was observed. Furthermore, the initial adhesion of adipose tissue-derived cells, which adhere weakly to PIPAAm-grafted surfaces, was enhanced on the antibody-immobilized thermoresponsive surfaces.