Polyethylene Glycol Coatings on Plastic Substrates for Chemically Defined Stem Cell Culture

Single-Component Hydrophilic Terpolymer Thin Film Systems for Imparting Surface Chemical Versatility on Various Substrates

We demonstrate a single-component hydrophilic photocrosslinkable copolymer system that incorporates all critical functionalities into one chain. This design allows for the creation of uniform functional organic coatings on a variety of substrates. The copolymers were composed of a poly(ethylene oxide)-containing monomer, a monomer that can release a primary amine upon UV light, and a monomer with reactive epoxide or cyclic dithiocarbonate with a primary amine. These copolymers are easily incorporated into the solution-casting process using polar solvents. Furthermore, the resulting coating can be readily stabilized through UV light-induced crosslinking, providing an advantage for controlling the surface properties of various substrates. The photocrosslinking capability further enables us to photolithographically define stable polymer domains in a desirable region. The resulting copolymer coatings were chemically versatile in immobilizing complex molecules by (i) post-crosslinking functionalization with the reactive groups on the surface and (ii) the formation of a composite coating by mixing varying amounts of a protein of interest, i.e., fish skin gelatin, which can form a uniform dual crosslinked network. The number of functionalization sites in a thin film could be controlled by tuning the composition of the copolymers. In photocrosslinking and subsequent functionalizations, we assessed the reactivity of the epoxide and cyclic dithiocarbonate with the generated primary amine. Moreover, the orthogonality of the possible reactions of the presented reactive functionalities in the crosslinked thin films with complex molecules is assessed. The resulting copolymer coatings were further utilized to define a hydrophobic surface or an active surface for the adhesion of biological objects.

Aligned skeletal muscle assembly on a biofunctionalized plant leaf scaffold

Decellularized plant scaffolds have drawn attention as alternative tissue culture platforms due to their wide accessibility, biocompatibility, and diversity of innate microstructures. Particularly, in this work, monocot leaves with innate uniaxial micropatterned topography were utilized to promote cell alignment and elongation. The leaf scaffold was biofunctionalized with poly(PEGMEMA-r-VDM-r-GMA) copolymer that prevented non-specific protein adsorption and was modified with cell adhesive RGD peptide to enable cell adhesion and growth in serum-free media. The biofunctionalized leaf supported the adhesion, growth, and alignment of various human cells including embryonic stem cells (hESC) derived muscle cells. The hESC-derived myogenic progenitor cells cultured on the biofunctionalized leaf scaffold adopted a parallel orientation and were elongated along the leaf topography. These cells showed significant early myogenic differentiation and muscle-like bundled myotube formation. The aligned cells formed compact myotube assemblies and showed uniaxial muscle contraction under chemical stimulation, a critical requirement for developing functional skeletal muscle tissue. Polymer-functionalized plant leaf scaffolds offer a novel human cell culture platform and have potential in human tissue engineering applications that require parallel alignment of cells. STATEMENT OF SIGNIFICANCE: Plant scaffolds are plentiful sources in nature and present a prefabricated construct to present topographical cues to cells. Their feature width is ideal for human cell alignment and elongation, especially for muscle cells. However, plant scaffolds lack proteins that support mammalian cell culture. We have developed a polymer coated leaf scaffold that enables cell adhesion and growth in serum-free media. Human muscle cells cultured on the biofunctionalized leaf, aligned along the natural parallel micro-patterned leaf topography, and formed muscle-like bundled myotube assemblies. These assemblies showed uniaxial muscular contraction, a critical requirement for developing functional skeletal muscle tissue. The biodiversity of the plant materials offers a novel human cell culture platform with potential in human tissue engineering.

The Cyclophilin-Dependent Calcineurin Inhibitor Voclosporin Inhibits SARS-CoV-2 Replication in Cell Culture

Kidney transplant recipients (KTRs) are at increased risk for a more severe course of COVID-19, due to their pre-existing comorbidity and immunosuppression. Consensus protocols recommend lowering immunosuppression in KTRs with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, but the optimal combination remains unclear. Calcineurin inhibitors (CNIs) are cornerstone immunosuppressants used in KTRs and some have been reported to possess antiviral activity against RNA viruses, including coronaviruses. Here, we evaluated the effect of the CNIs tacrolimus, cyclosporin A, and voclosporin (VCS), as well as other immunosuppressants, on SARS-CoV-2 replication in cell-based assays. Unexpected, loss of compound due to plastic binding and interference of excipients in pharmaceutical formulations (false-positive results) complicated the determination of EC50 values of cyclophilin-dependent CNI’s in our antiviral assays. Some issues could be circumvented by using exclusively glass lab ware with pure compounds. In these experiments, VCS reduced viral progeny yields in human Calu-3 cells at low micromolar concentrations and did so more effectively than cyclosporin A, tacrolimus or other immunosuppressants. Although, we cannot recommend a particular immunosuppressive regimen in KTRs with COVID-19, our data suggest a potential benefit of cyclophilin-dependent CNIs, in particular VCS in reducing viral progeny, which warrants further clinical evaluation in SARS-CoV-2-infected KTRs.

Polymer-Coated Magnetic Microspheres Conjugated with Growth Factor Receptor Binding Peptides Enable Cell Sorting

The separation and sorting of human cells is an important step in the bioprocessing of cell-based therapeutics. Heterogeneous mixtures of cells must be sorted to isolate the desired cell type and purify the final product. This process is often achieved by antibody-based sorting techniques. In this work, we demonstrate that magnetic microspheres may be functionalized with peptides that selectively bind to cells on the basis of their relative concentration of specific surface proteins. Five-micrometer-magnetic microspheres were coated with the synthetic copolymer PVG (poly(poly(ethylene glycol)methyl ether methacrylate-ran-vinyl dimethyl azlactone-ran-glycidyl methacrylate) and functionalized with the vascular endothelial growth factor receptor binding peptide (VRBP), which binds to the vascular endothelial growth factor receptor (VEGFR). These microspheres exhibited low cytotoxicity and bind to cells depending on their relative surface protein expression. Finally, coated, magnetic microspheres were used to separate heterogeneous populations of cells dependent on their VEGFR expression through magnetic-assisted cell sorting (MACS), demonstrating that peptide-based cell sorting mechanisms may be useful in the bioprocessing of human-cell-based products.

Xeno-Free Bioreactor Culture of Human Mesenchymal Stromal Cells on Chemically Defined Microcarriers

Human mesenchymal stromal cells (hMSC), also called mesenchymal stem cells, are adult cells that have demonstrated their potential in therapeutic applications, highlighted by their ability to differentiate down different lineages, modulate the immune system, and produce biologics. There is a pressing need for scalable culture systems for hMSC due to the large number of cells needed for clinical applications. Most current methods for expanding hMSC fail to provide a reproducible cell product in clinically required cell numbers without the use of serum-containing media or harsh enzymes. In this work, we apply a tailorable, thin, synthetic polymer coating-poly(poly(ethylene glycol) methyl ether methacrylate-ran-vinyl dimethyl azlactone-ran-glycidyl methacrylate) (P(PEGMEMA-r-VDM-r-GMA), PVG)-to the surface of commercially available polystyrene (PS) microcarriers to create chemically defined three-dimensional (3D) surfaces for large-scale cell expansion. These chemically defined microcarriers provide a reproducible surface that does not rely on the adsorption of xenogeneic serum proteins to mediate cell adhesion, enabling their use in xeno-free culture systems. Specifically, this work demonstrates the improved adhesion of hMSC to coated microcarriers over PS microcarriers in xeno-free media and describes their use in a readily scalable, bioreactor-based culture system. Additionally, these surfaces resist the adsorption of media-borne and cell-produced proteins, which result in integrin-mediated cell adhesion throughout the culture period. This feature allows the cells to be efficiently passaged from the microcarrier using a chemical chelating agent (ethylenediaminetetraacetic acid (EDTA)) in the absence of cleavage enzymes, an improvement over other microcarrier products in the field. Bioreactor culture of hMSC on these microcarriers enabled the production of hMSC over 4 days from a scalable, xeno-free environment.

Identification of Critical Surface Parameters Driving Lectin-Mediated Capture of Bacteria from Solution

Lectin-functional interfaces are useful for isolation of bacteria from solution because they are low-cost and allow nondestructive, reversible capture. This study provides a systematic investigation of physical and chemical surface parameters that influence bacteria capture over lectin-functionalized polymer interfaces and then applies these findings to construct surfaces with significantly enhanced bacteria capture. The designer block copolymer poly(glycidyl methacrylate)- block-poly(vinyldimethyl azlactone) was used as a lectin attachment layer, and lectin coupling into the polymer film through azlactone-lectin coupling reactions was first characterized. Here, experimental parameters including polymer areal chain density, lectin molecular weight, and lectin coupling buffer were systematically varied to identify parameters driving highest azlactone conversions and corresponding lectin surface densities. To introduce physical nanostructures into the attachment layer, nanopillar arrays (NPAs) of varied heights (300 and 2100 nm) were then used to provide an underlying surface template for the functional polymer layer. Capture of Escherichia coli on lectin-polymer surfaces coated over both flat and NPA surfaces was then investigated. For flat polymer interfaces, bacteria were detected on the surface after incubation at a solution concentration of 10³ cfu/mL, and a corresponding detection limit of 1.7 × 10³ cfu/mL was quantified. This detection limit was 1 order of magnitude lower than control lectin surfaces functionalized with standard, carbodiimide coupling chemistry. NPA surfaces containing 300 nm tall pillars further improved the detection limit to 2.1 × 10² cfu/mL, but also reduced the viability of captured cells. Finally, to investigate the impact of cell surface parameters on capture, we used Agrobacterium tumefaciens cells genetically modified to allow manipulation of exopolysaccharide adhesin production levels. Statistical analysis of surface capture levels revealed that lectin surface density was the primary factor driving capture, as opposed to exopolysaccharide adhesin expression. These findings emphasize the critical importance of the synthetic interface and the development of surfaces that combine high lectin densities with tailored physical features to drive high levels of capture. These insights will aid in design of biofunctional interfaces with physicochemical surface properties favorable for capture and isolation of bacteria cells from solutions.

Synthetic, Chemically Defined Polymer-Coated Microcarriers for the Expansion of Human Mesenchymal Stem Cells

Mesenchymal stem cells (MSC), also called marrow stromal cells, are adult cells that have attracted interest for their potential uses in therapeutic applications. There is a pressing need for scalable culture systems due to the large number of cells needed for clinical treatments. Here, a tailorable thin polymer coating-poly(poly(ethylene glycol) methyl ether methacrylate-ran-vinyl dimethyl azlactone-ran-glycidyl methacrylate) [P(PEGMEMA-r-VDM-r-GMA); PVG]-to the surface of commercially available polystyrene and glass microcarriers to create chemically defined surfaces for large-scale cell expansion is applied. These chemically defined microcarriers create a reproducible surface that does not rely on the adsorption of xenogenic serum proteins to mediate cell adhesion. Specifically, this coating method anchors PVG copolymer through ring opening nucleophilic attack by amine residues on poly-l-lysine that is pre-adsorbed to the surface of microcarriers. Importantly, this anchoring reaction preserves the monomer VDM reactivity for subsequent functionalization with an integrin-specific Arg-Gly-Asp peptide to enable cell adhesion and expansion via a one-step reaction in aqueous media. MSCs cultured on PVG-coated microcarriers achieve sixfold expansion-similar to the expansion achieved on PS microcarriers-and retain their ability to differentiate after harvesting.

New substrates for stem cell control

The capacity to culture stem cells in a controllable, robust and scalable manner is necessary in order to develop successful strategies for the generation of cellular and tissue platforms for drug screening, toxicity testing, tissue engineering and regenerative medicine. Creating substrates that support the expansion, maintenance or directional differentiation of stem cells would greatly aid these efforts. Optimally, the substrates used should be chemically defined and synthetically scalable, allowing growth under defined, serum-free culture conditions. To achieve this, the chemical and physical attributes of the substrates should mimic the natural tissue environment and allow control of their biological properties. Herein, recent advances in the development of materials to study/manipulate stem cells, both in vitro and in vivo, are described with a focus on the novelty of the substrates' properties, and on application of substrates to direct stem cells.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Fabricating Reactive Surfaces with Brush-like and Crosslinked Films of Azlactone-Functionalized Block Co-Polymers

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl dimethyl azlactone) (PGMA-b-PVDMA), are presented. Due to the high reactivity of azlactone groups towards amine, thiol, and hydroxyl groups, PGMA-b-PVDMA surfaces can be modified with secondary molecules to create chemically or biologically functionalized interfaces for a variety of applications. Previous reports of patterned PGMA-b-PVDMA interfaces have used traditional top-down patterning techniques that generate non-uniform films and poorly controlled background chemistries. Here, we describe customized patterning techniques that enable precise deposition of highly uniform PGMA-b-PVDMA films in backgrounds that are chemically inert or that have biomolecule-repellent properties. Importantly, these methods are designed to deposit PGMA-b-PVDMA films in a manner that completely preserves azlactone functionality through each processing step. Patterned films show well-controlled thicknesses that correspond to polymer brushes (~90 nm) or to highly crosslinked structures (~1-10 μm). Brush patterns are generated using either the parylene lift-off or interface directed assembly methods described and are useful for precise modulation of overall chemical surface reactivity by adjusting either the PGMA-b-PVDMA pattern density or the length of the VDMA block. In contrast, the thick, crosslinked PGMA-b-PVDMA patterns are obtained using a customized micro-contact printing technique and offer the benefit of higher loading or capture of secondary material due to higher surface area to volume ratios. Detailed experimental steps, critical film characterizations, and trouble-shooting guides for each fabrication method are discussed.

Amine-Reactive Azlactone-Containing Nanofibers for the Immobilization and Patterning of New Functionality on Nanofiber-Based Scaffolds

We report the design of amine-reactive polymer nanofibers and nonwoven reactive nanofiber mats fabricated by the electrospinning of azlactone-functionalized polymers. We demonstrate that randomly oriented nanofibers fabricated using a random copolymer of methyl methacrylate and 2-vinyl-4,4-dimethylazlactone contain intact and reactive azlactone groups that can be used to introduce new chemical functionality and modulate important interfacial properties of these materials (e.g., wetting behaviors) by postfabrication treatment with primary amine-based nucleophiles. The facile and "click-like" nature of these reactions permits functionalization under mild conditions without substantial changes to nanofiber or mat morphologies. This approach also enables the patterning of new functionality on mat-coated surfaces by treatment with bulk solutions of primary amines or by using methods such as microcontact printing. Further, these reactive mats can also, themselves, be contact-transferred or "printed" onto secondary surfaces by pressing them into contact with other amine-functionalized objects. Finally, we demonstrate that functionalization with hydrophobic amines can increase the stability of these materials in aqueous environments and yield hydrophobic nanofiber scaffolds useful for the design of "slippery" liquid-infused materials. The approaches reported here enable the introduction of new properties to reactive polymer mats after fabrication and, thus, reduce the need to synthesize individual functional polymers prior to electrospinning to achieve new properties. The azlactone chemistry used here broadens the scope of reactions that can be used to functionalize polymer nanofibers and is likely to prove general. We anticipate that this approach can be used with a range of amines or other nucleophiles (e.g., alcohols or thiols) to design nanofibers and reactive nanofiber-based materials with new physical properties, surface features, and behaviors that may be difficult to achieve by the direct electrospinning of conventional materials or other functional polymers.

Degradable Amine-Reactive Coatings Fabricated by the Covalent Layer-by-Layer Assembly of Poly(2-vinyl-4,4-dimethylazlactone) with Degradable Polyamine Building Blocks

We report the fabrication of reactive and degradable cross-linked polymer multilayers by the reactive/covalent layer-by-layer assembly of a non-degradable azlactone-functionalized polymer [poly(2-vinyl-4,4-dimethylazlactone), PVDMA] with hydrolytically or enzymatically degradable polyamine building blocks. Fabrication of multilayers using PVDMA and a hydrolytically degradable poly(β-amino ester) (PBAE) containing primary amine side chains yielded multilayers (∼100 nm thick) that degraded over ∼12 days in physiologically relevant media. Physicochemical characterization and studies on stable films fabricated using PVDMA and an analogous non-degradable poly(amidoamine) suggested that erosion occurred by chemical hydrolysis of backbone esters in the PBAE components of these assemblies. These degradable assemblies also contained residual amine-reactive azlactone functionality that could be used to impart new functionality to the coatings post-fabrication. Cross-linked multilayers fabricated using PVDMA and the enzymatically degradable polymer poly(l-lysine) were structurally stable for prolonged periods in physiological media, but degraded over ∼24 h when the enzyme trypsin was added. Past studies demonstrate that multilayers fabricated using PVDMA and non-degradable polyamines [e.g., poly(ethylenimine)] enable the design and patterning of useful nano/biointerfaces and other materials that are structurally stable in physiological media. The introduction of degradable functionality into PVDMA-based multilayers creates opportunities to exploit the reactivity of azlactone groups for the design of reactive materials and functional coatings that degrade or erode in environments that are relevant in biomedical, biotechnological, and environmental contexts. This "degradable building block" strategy should be general; we anticipate that this approach can also be extended to the design of amine-reactive multilayers that degrade upon exposure to specific chemical triggers, selective enzymes, or contact with cells by judicious design of the degradable polyamine building blocks used to fabricate the coatings.

Surface functionalization and dynamics of polymeric cell culture substrates

The promise of growing tissues to replace or improve the function of failing ones, a practice often referred to as regenerative medicine, has been driven in recent years by the development of stem cells and cell lines. Stem cells are typically cultured outside the body to increase cell number or differentiate the cells into mature cell types. In order to maximize the regenerative potential of these cells, there is a need to understand cell-material interactions that direct cell behavior and cell-material dynamics. Most synthetic surfaces used for growth and differentiation of cells in the lab are impractical and cost prohibitive in clinical labs. This review focuses on the modification of low cost polymer substrates that are already widely used for cell culture so that they may be used to control and understand cell-material interactions. In addition, we discuss the ability of cells to exert dynamic control over the microenvironment leading to a more complex, less controlled surface.

Peptide Conjugation to a Polymer Coating via Native Chemical Ligation of Azlactones for Cell Culture

Conjugation of biomolecules for stable presentation is an essential step toward reliable chemically defined platforms for cell culture studies. In this work, we describe the formation of a stable and site-specific amide bond via the coupling of a cysteine terminated peptide at low concentration to an azlactone containing copolymer coating. A copolymer of polyethylene glycol methyl ether methacrylate-ran-vinyl azlactone-ran-glycidyl methacrylate P(PEGMEMA-r-VDM-r-GMA) was used to form a thin coating (20-30 nm) on silicon and polycarbonate substrates. The formation and stability of coating-peptide bonds for peptides containing free thiols and amines were quantified by X-ray photoelectron spectroscopy (XPS) after exposure to cell culture conditions. Peptides containing a thiol as the only nucleophile coupled via a thioester bond; however, the bond was labile under cell culture conditions and almost all the bound peptides were displaced from the surface over a period of 2 days. Coupling with N-terminal primary amine peptides resulted in the formation of an amide bond with low efficiency (<20%). In contrast, peptides containing an N-terminal cysteine, which contain both nucleophiles (free thiol and amine) in close proximity, bound with 67% efficiency under neutral pH, and were stable under the same conditions for 2 weeks. Control studies confirm that the stable amide formation was a result of an intramolecular rearrangement through a N-acyl intermediate that resembles native chemical ligation. Through a combination of XPS and cell culture studies, we show that the cysteine terminated peptides undergo a native chemical ligation process at low peptide concentration in aqueous media, short reaction time, and at room temperature resulting in the stable presentation of peptides beyond 2 weeks for cell culture studies.