Spatially Assembled Bilayer Cell Sheets of Stem Cells and Endothelial Cells Using Thermosensitive Hydrogels for Therapeutic Angiogenesis

Although the coculture of multiple cell types has been widely employed in regenerative medicine, in vivo transplantation of cocultured cells while maintaining the hierarchical structure remains challenging. Here, a spatially assembled bilayer cell sheet of human mesenchymal stem cells and human umbilical vein endothelial cells on a thermally expandable hydrogel containing fibronectin is prepared and its effect on in vitro proangiogenic functions and in vivo ischemic injury is investigated. The expansion of hydrogels in response to a temperature change from 37 to 4 °C allows rapid harvest and delivery of the bilayer cell sheet to two different targets (an in vitro model glass surface and in vivo tissue). The in vitro study confirms that the bilayer sheet significantly increases proangiogenic functions such as the release of nitric oxide and expression of vascular endothelial cell genes. In addition, transplantation of the cell sheet from the hydrogels into a hindlimb ischemia mice model demonstrates significant retardation of necrosis particularly in the group transplated with the bilayer sheet. Collectively, the bilayer cell sheet is readily transferrable from the thermally expandable hydrogel and represents an alternative approach for recovery from ischemic injury, potentially via improved cell-cell communication.

Retention of poly(N-isopropylacrylamide) on 3-aminopropyltriethoxysilane

Silane coupling agents are commonly employed to link an organic polymer to an inorganic substrate. One of the widely utilized coupling agents is 3-aminopropyltriethoxy silane (APTES). In this study, the authors investigated the ability of APTES to retain thermo-responsive poly(N-isopropylacrylamide) (pNIPAAm) on hydroxylated surfaces such as glass. For comparison purposes, the authors also evaluated the retention behaviors of (1) polystyrene, which likely has weaker van der Waals interactions and acid-base interactions (contributed by hydrogen-bonding) with APTES, on APTES as well as (2) pNIPAAm on two other silane coupling agents, which have similar structures to APTES, but exhibit less interaction with pNIPAAm. Under our processing conditions, the stronger interactions, particularly hydrogen bonding, between pNIPAAm and APTES were found to contribute substantially to the retention of pNIPAAm on the APTES modified surface, especially on the cured APTES layer when the interpenetration was minimal or nonexistent. On the noncured APTES layer, the formation of an APTES-pNIPAAm interpenetrating network resulted in the retention of thicker pNIPAAm films. As demonstrated by water contact angles [i.e., 7°-15° higher at 40 °C, the temperature above the lower critical solution temperature (LCST) of 32 °C for pNIPAAm, as compared to those at 25 °C] and cell attachment and detachment behaviors (i.e., attached/spread at 37 °C, above LCST; detached at 20 °C, below LCST), the retained pNIPAAm layer (6-15 nm), on both noncured and cured APTES, exhibited thermo-responsive behavior. The results in this study illustrate the simplicity of using the coupling/adhesion promoting ability of APTES to retain pNIPAAm films on hydroxylated substrates, which exhibit faster cell sheet detachment (≤30 min) as compared to pNIPAAm brushes (in hours) prepared using tedious and costly grafting approaches. The use of adhesion promoters to retain pNIPAAm provides an affordable alternative to current thermo-responsive supports for cell sheet engineering and stem cell therapy applications.

Hydrogels with Modulated Ionic Load for Mammalian Cell Harvesting with Reduced Bacterial Adhesion

In this manuscript, we describe the fabrication of hydrogel supports for mammalian cell handling that can simultaneously prevent materials from microbial contamination and therefore allow storage in aqueous media. For that purpose, hydrogels based on the antifouling polymer polyvinylpyrrolidone (PVP) were functionalized with different ionic groups (anionic, cationic, or two types of zwitterions). In order to prevent bacterial adhesion in the long-term, we took advantage of the synergistic effect of inherently antifouling PVP and additional antifouling moieties incorporated within the hydrogel structure. We evaluated, in a separated series of experiments, both the capability of the materials to act as supports for the growth of mammalian cell monolayers for transplantation (using C-166-GFP endothelial cell line), as well their antifouling properties against Staphylococcus aureus, were studied. All of the hydrogels are structurally pseudodouble networks with high swelling (around 90%) and similar mechanical properties (in the low range for hydrogel materials with Young modulus below 1250 kPa). With some differences, all the charged hydrogels were capable of hosting mouse endothelial cell line C166-GFP to confluence, as well as a monolayer detachment and transplantation through simple mechanical agitation. On the contrary, the uncharged hydrogel was not capable to detach a full monolayer for transplantation. Bacterial adhesion and proliferation was highly sensitive to the functionality (type of charge and density). In particular, we evidenced that monomers bearing zwitterionic sulfobetaine groups, those negatively charged as well as "electro neutral" hydrogels fabricated from stoichiometric amounts of positive and negative units, exhibit excellent antifouling properties both at initial adhesion times and during longer periods up to 72 h.

Poly(glycidyl ether)-Based Monolayers on Gold Surfaces: Control of Grafting Density and Chain Conformation by Grafting Procedure, Surface Anchor, and Molecular Weight

For a meaningful correlation of surface coatings with their respective biological response reproducible coating procedures, well-defined surface coatings, and thorough surface characterization with respect to layer thickness and grafting density are indispensable. The same applies to polymeric monolayer coatings which are intended to be used for, e.g., fundamental studies on the volume phase transition of surface end-tethered thermoresponsive polymer chains. Planar gold surfaces are frequently used as model substrates, since they allow a variety of straightforward surface characterization methods. Herein we present reproducible grafting-to procedures performed with thermoresponsive poly(glycidyl ether) copolymers composed of glycidyl methyl ether (GME) and ethyl glycidyl ether (EGE). The copolymers feature different molecular weights (2 kDa, 9 kDa, 24 kDa) and are equipped with varying sulfur-containing anchor groups in order to achieve adjustable grafting densities on gold surfaces and hence control the tethered polymers' chain conformation. We determined "wet" and "dry" thicknesses of these coatings by QCM-D and ellipsometry measurements and deduced anchor distances and degrees of chain overlap of the polymer chains assembled on gold. Grafting under cloud point conditions allowed for higher degrees of chain overlap compared to grafting from a good solvent like ethanol, independent of the used sulfur-containing anchor group for polymers with low (2 kDa) and medium (9 kDa) molecular weights. By contrast, the achieved grafting densities and thus chain overlaps of surface-tethered polymers with high (24 kDa) molecular weights were identical for both grafting methods. Monolayers prepared from an ethanolic solution of poly(glycidyl ether)s equipped with sterically demanding disulfide-containing anchors revealed the lowest degrees of chain overlap. The ratio of the radius of gyration to the anchor distance (2 R_g/l) of the latter coating was found to be lower than 1.4, indicating that the assembly was rather in the mushroom-like than in the brush regime. Polymer chains with thiol-containing anchors of different alkyl chain lengths (C₁₁SH vs C₄SH) formed assemblies with comparable degrees of chain overlap with 2 R_g/l values above 1.4 and are thus in the brush regime. Molecular weights influenced the achievable degree of chain overlap on the surface. Coatings prepared with the medium molecular weight polymer (9 kDa) resulted in the highest chain packing density. Control of grafting density and thus chain overlap in different regimes (brush vs mushroom) on planar gold substrates are attainable for monolayer coatings with poly(GME-ran-EGE) by adjusting the polymer's molecular weight and anchor group as well as the conditions for the grafting-to procedure.

Stiffness-Controlled Thermoresponsive Hydrogels for Cell Harvesting with Sustained Mechanical Memory

Most mechanobiological investigations focused on in situ mechanical regulation of cells on stiffness-controlled substrates with few downstream applications, as it is still challenging to harvest and expand mechanically primed cells by enzymatic digestion (e.g., trypsin) without interrupting cellular mechanical memory between passages. This study develops thermoresponsive hydrogels with controllable stiffness to generate mechanically primed cells with intact mechanical memory for augmented wound healing. No significant cellular property alteration of the fibroblasts primed on thermoresponsive hydrogels with varied stiffness has been observed through thermoresponsive harvesting. When reseeding the harvested cells for further evaluation, softer hydrogels are proven to better sustain the mechanical priming effects compared to rigid tissue culture plate, which indicates that both the stiffness-controlled substrate and thermoresponsive harvesting are required to sustain cellular mechanical memory between passages. Moreover, epigenetics analysis reveals that thermoresponsive harvesting could reduce the rearrangement and loss of chromatin proteins compared to that of trypsinization. In vivo wound healing using mechanically primed fibroblasts shows featured epithelium and sebaceous glands, which indicates augmented skin recovery compared with trypsinized fibroblasts. Thus, the thermoresponsive hydrogel-based cell harvesting system offers a powerful tool to investigate mechanobiology between cell passages and produces abundant cells with tailored mechanical priming properties for cell-based applications.

Film Thickness Determines Cell Growth and Cell Sheet Detachment from Spin-Coated Poly(N-Isopropylacrylamide) Substrates

Poly(N-isopropylacrylamide) (pNIPAm) is widely used to fabricate thermoresponsive surfaces for cell sheet detachment. Many complex and expensive techniques have been employed to produce pNIPAm substrates for cell culture. The spin-coating technique allows rapid fabrication of pNIPAm substrates with high reproducibility and uniformity. In this study, the dynamics of cell attachment, proliferation, and function on non-cross-linked spin-coated pNIPAm films of different thicknesses were investigated. The measurements of advancing contact angle revealed increasing contact angles with increasing film thickness. Results suggest that more hydrophilic 50 and 80 nm thin pNIPAm films are more preferable for cell sheet fabrication, whereas more hydrophobic 300 and 900 nm thick spin-coated pNIPAm films impede cell attachment. These changes in cell behavior were correlated with changes in thickness and hydration of pNIPAm films. The control of pNIPAm film thickness using the spin-coating technique offers an effective tool for cell sheet-based tissue engineering.

Fabrication and Application of Photocrosslinked, Nanometer-Scale, Physically Adsorbed Films for Tissue Culture Regeneration

This study describes the development and cell culture application of nanometer thick photocrosslinkable thermoresponsive polymer films prepared by physical adsorption. Two thermoresponsive polymers, poly(N-isopropylacrylamide (NIPAm)-co-acrylamidebenzophenone (AcBzPh)) and poly(NIPAm-co-AcBzPh-co-N-tertbutylacrylamide) are investigated. Films are prepared both above and below the polymers' lower critical solution temperatures (LCSTs) and cross-linked, to determine the effect, adsorption preparation temperature has on the resultant film. The films prepared at temperatures below the LCST are smoother, thinner, and more hydrophilic than those prepared above. Human pulmonary microvascular endothelial cell (HPMEC) adhesion and proliferation are superior on the films produced below the polymers LCST compared to those produced above. Cells sheets are detached by simply lowering the ambient temperature to below the LCST. Transmission electron, scanning electron, and light microscopies indicate that the detached HPMEC sheets maintain their integrity.

A practical guide to hydrogels for cell culture

There is growing appreciation of the role that the extracellular environment plays in regulating cell behavior. Mechanical, structural, and compositional cues, either alone or in concert, can drastically alter cell function. Biomaterials, and particularly hydrogels, have been developed and implemented to present defined subsets of these cues for investigating countless cellular processes as a means of understanding morphogenesis, aging, and disease. Although most scientists concede that standard cell culture materials (tissue culture plastic and glass) do a poor job of recapitulating native cellular milieus, there is currently a knowledge barrier for many researchers in regard to the application of hydrogels for cell culture. Here, we introduce hydrogels to those who may be unfamiliar with procedures to culture and study cells with these systems, with a particular focus on commercially available hydrogels.

Magnetically Triggered Monodispersed Nanocomposite Fabricated by Microfluidic Approach for Drug Delivery

Responsive microgel poly(N-isopropylacrylamide) or PNIPAM is a gel that can swell or shrink in response to external stimuli (temperature, pH, etc.). In this work, a nanocomposite gel is developed consisting of PNIPAM and magnetic iron oxide nanobeads for controlled release of liquids (like drugs) upon exposure to an alternating magnetic field. Microparticles of the nanocomposite are fabricated efficiently with a monodisperse size distribution and a diameter ranging from 20 to 500 µm at a rate of up to 1 kHz using a simple and inexpensive microfluidic system. The nanocomposite is heated through magnetic losses, which is exploited for a remotely stimulated liquid release. The efficiency of the microparticles for controlled drug release applications is tested with a solution of Rhodamine B as a liquid drug model. In continuous and pulsatile mode, a release of 7% and 80% was achieved, respectively. Compared to external thermal actuation that heats the entire surrounding or embedded heaters that need complex fabrication steps, the magnetic actuation provides localized heating and is easy to implement with our microfluidic fabrication method.

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.

A thermoresponsive, micro-roughened cell culture surface

Surface topography has been shown to play a major role in cell behavior, but has yet to be seriously exploited in the field of cell surface engineering. In the present work, surface roughness has been used in combination with the thermoresponsive polymer polyisopropylacrylamide (PIPAAm) to generate cell sheets with tailored biochemical properties. Micro-roughened polystyrene (PS) with 1.5-5.5 μm features was derivatized with PIPAAm to form a cell culture surface for the growth of human fibroblast cell sheets that exhibit a modified cytoskeleton and extracellular matrix. Fibroblasts cell sheets cultured on the rough surfaces had fewer actin stress fibers and twice the average fibronectin (FN) fibril formation when compared to cell sheets on flat substrates. The cell sheets harvested from the roughened PS were collected after only 2 days of culture and detached from the PIPAAm grafted surface in <1h after cooling the culture system. The simple and rapid method for generating cell sheets with increased FN fibril formation has applications in tissue grafts or wound repair and has demonstrated that the thermoresponsive surface can be used for reliable cell sheet formation.

Injectable poly(oligoethylene glycol methacrylate)-based hydrogels with tunable phase transition behaviours: physicochemical and biological responses

The potential of poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels with varying thermosensitivities as soft materials for biomedical applications is demonstrated. Hydrogels are prepared from hydrazide and aldehyde functionalized POEGMA precursors, yielding POEGMA hydrogels with a volume phase transition temperature (VPTT) below (PO0), close to (PO10) and well above (PO100) physiological temperature. Hydrogels with VPTTs close to and above physiological temperature exhibit biological properties similar to those typically observed for poly(ethylene glycol) hydrogels (i.e. low protein adsorption, low cell adhesion and minimal inflammatory responses in vivo) while hydrogels with VPTTs lower than physiological temperature exhibit biological properties more analogous to poly(N-isopropylacrylamide) above its phase transition temperature (temperature-switchable cell adhesion, higher protein adsorption and somewhat more acute inflammation in vivo). As such, the use of POEGMA precursors with varying chain lengths of ethylene oxide grafts offers a versatile platform for the design of hydrogels with tunable physiological properties via simple copolymerization.

An integrated microrobotic platform for on-demand, targeted therapeutic interventions

The presented microrobotic platform combines together the advantages of self-folding NIR light sensitive polymer bilayers, magnetic alginate microbeads, and a 3D manipulation system, to propose a solution for targeted, on-demand drug and cell delivery. First feasibility studies are presented together with the potential of the full design.

Smart moisture management and thermoregulation properties of stimuli-responsive cotton modified with polymer brushes

Thermoresponsive PNIPAM polymer brushes are grafted onto the surface of cotton fabrics to construct a smart hierarchical system.

Pseudo-double network hydrogels with unique properties as supports for cell manipulation

Introducing new hydrogels for the support of confluent cell growth and from which cell sheets can be easily detached or transplanted.

Thermoresponsive substrates used for the expansion of human mesenchymal stem cells and the preservation of immunophenotype

The facile regeneration of undifferentiated human mesenchymal stem cells (hMSCs) from thermoresponsive surfaces facilitates the collection of stem cells avoiding the use of animal derived cell detachment agents commonly used in cell culture. This communication proposes a procedure to fabricate coatings from commercially available pNIPAm which is both affordable and a significant simplification on alternative approaches used elsewhere. Solvent casting was used to produce films in the micrometer range and successful cell adhesion and proliferation was highly dependent on the thickness of the coating produced with 1 μm thick coatings supporting cells to confluence. 3T3 cell sheets and hMSCs were successfully detached from the cast coatings upon temperature reduction. Furthermore, results indicate that the hMSCs remained undifferentiated as the surface receptor profile of hMSCs was not altered when cells were detached in this manner.

Responsive systems for cell sheet detachment

Cell sheet engineering has been progressing rapidly during the past few years and has emerged as a novel approach for cell based therapy. Cell sheet harvest technology enables fabrication of viable, transplantable cell sheets for various tissue engineering applications. Currently, the majority of cell sheet studies use thermo-responsive systems for cell sheet detachment. However, other responsive systems began showing their potentials for cell sheet harvest. This review provides an overview of current techniques in creating cell sheets using different types of responsive systems including thermo-responsive, electro-responsive, photo-responsive, pH-responsive and magnetic systems. Their mechanism, approach, as well as applications for cell detachment have been introduced. Further development of these responsive systems will allow efficient cell sheet harvesting and patterning of cells to reconstruct complex tissue for broad clinical applications.