Surface engineering the cellular microenvironment via patterning and gradients

Nano-/micro-scaled hydroxyapatite ceramic construction and the regulation of immune-associated osteogenic differentiation

Hydroxyapatite (HA) bioceramic is a promising substitute for bone defects, and the surface properties are major factors that influence bioactivity and osteoinductivity. In this study, two kinds of HA bioceramics with nanoscale (n-HA) and microscale (m-HA) surface topography were designed to mimic the natural bone, thus enhancing the stimulation of osteogenic differentiation and revealing the potential mechanism. Compared to m-HA, n-HA owned a larger surface roughness, a stronger wettability, and reduced hardness and indentation modulus. Based on these properties, n-HA could maintain the conformation of vitronectin better than m-HA, which may contribute to higher cellular activities and a stronger promotion of osteogenic differentiation of mesenchymal stem cells (MSCs). Further RNA sequencing analysis compared the molecular expression between n-HA and m-HA. Six hundred twenty-seven differentially expressed genes were identified in MSCs, and 17 upregulated genes and 610 downregulated genes were included when n-HA compared to m-HA. The GO cluster analysis and enriched Kyoto encyclopedia of genes and genome signaling pathways revealed a close correlation with the immune process in both upregulated (chemokine signaling pathway and cytokine-cytokine receptor interaction) and downregulated pathways (osteoclasts differentiation). It suggested that the nanoscale surface topography of HA enhanced the osteoinductivity of MSCs and could not be separated from its regulation of immune function and the retention of adsorbed protein conformation.

Hydrolysis of Poly(fluoroacrylate) Thin Films Synthesized from the Vapor Phase

The post-synthesis surface reaction of vapor-deposited polymer thin films is a promising technique in engineering heterogeneous surface chemistry. Because the existing research has neglected marginally reactive precursor films in preference of their highly reactive counterparts, our knowledge of kinetics and loss of film integrity during the reaction are limited. To address these limitations, we characterize hydrolysis of two fluoroacrylates, poly(1H,1H,2H,2H-perfluorooctyl acrylate) (pPFOA) and poly(2,2,3,4,4,4-hexafluorobutyl acrylate) (pHFBA), with sodium hydroxide using X-ray photoelectron spectroscopy. Without crosslinking with di(ethylene glycol)divinyl ether (DEGDVE) and grafting with trichlorovinyl silane, the films degrade rapidly during hydrolysis. An SN2 mechanism describes hydrolysis well, with rate constants of 0.0029 ± 0.0004 and 0.011 ± 0.001 L mol-1s-1 at 30 °C for p(PFOA-co-DEGDVE) and p(HFBA-co-DEGDVE), respectively. Our detailed study of hydrolysis kinetics of marginally reactive fluoroacrylates demonstrates the full capability and limitations of the post-synthesis reaction. Importantly, copolymers are characterized using a density correction new to polymer chemical vapor deposition.

Grayscale surface patterning using electrophoretic motion through a heterogeneous hydrogel material

Chemical surface patterning can be an incredibly powerful tool in a variety of applications, as it enables precise spatial control over surface properties. But the equipment required to create functional surface patterns-especially "grayscale" patterns where independent control over species placement and density are needed-is often expensive and inaccessible. In this work, we leveraged equipment and methods readily available to many research labs, namely 3D printing and electroblotting, to generate controlled grayscale surface patterns. Three-dimensional-printed molds were used to cast polyacrylamide hydrogels with regions of variable polymer density; regions of low polymer density within the hydrogels served as reservoirs for proteins that were later driven onto a target surface using electrophoresis. This mechanism was used to deposit grayscale patterns of fluorescently labeled proteins, and the fluorescent intensity of these patterns was measured and compared to a theoretical analysis of the deposition mechanism.

Multifunctional Biomaterials: Combining Material Modification Strategies for Engineering of Cell-Contacting Surfaces

In the human body, cells in a tissue are exposed to signals derived from their specific extracellular matrix (ECM), such as architectural structure, mechanical properties, and chemical composition (proteins, growth factors). Research on biomaterials in tissue engineering and regenerative medicine aims to recreate such stimuli using engineered materials to induce a specific response of cells at the interface. Although traditional biomaterials design has been mostly limited to varying individual signals, increasing interest has arisen on combining several features in recent years to improve the mimicry of extracellular matrix properties. Tremendous progress in combinatorial surface modification exploiting, for example, topographical features or variations in mechanics combined with biochemical cues has enabled the identification of their key regulatory characteristics on various cell fate decisions. Gradients especially facilitated such research by enabling the investigation of combined continuous changes of different signals. Despite unravelling important synergies for cellular responses, challenges arise in terms of fabrication and characterization of multifunctional engineered materials. This review summarizes recent work on combinatorial surface modifications that aim to control biological responses. Modification and characterization methods for enhanced control over multifunctional material properties are highlighted and discussed. Thereby, this review deepens the understanding and knowledge of biomimetic combinatorial material modification, their challenges but especially their potential.

Nature-Inspired Hydrogels with Soft and Stiff Zones that Exhibit a 100-Fold Difference in Elastic Modulus

Many biological materials, such as the squid beak and the spinal disc, have a combination of stiff and soft parts with very different mechanical properties, for example, the elastic modulus (stiffness) of the stiffest part of the squid beak is about 100 times that of the softest part. Researchers have attempted to mimic such structures using hydrogels but have not succeeded in synthesizing bulk gels with such large variations in moduli. Here, we present a general approach that can be used to form hydrogels with two or more zones having appreciably different mechanical characters. For this purpose, we use a technique developed in our lab for creating hybrid hydrogels with distinct zones. For the soft zone of the gel, we form a polymer network using a conventional acrylic monomer [ N, N'-dimethylacrylamide (DMAA)] and with laponite (LAP) nanoparticles as the cross-linkers. For the stiff zone, we combine DMAA, LAP, and a methacrylated silica precursor ([3-(methacryloyloxy)-propyl]trimethoxy-silane). When this mixture is polymerized, nanoscale silica particles (∼300 nm in diameter) are formed, and these serve as additional cross-links between the polymer chains, making this network very stiff. The unique character of each zone is preserved in the hybrid gel, and different zones are covalently linked to each other, thereby ensuring robust interfaces. Rheological measurements show that the elastic modulus of the stiff zone can be more than 100 times that of the soft zone. This ratio of moduli is the highest reported to date in a single, continuous gel and is comparable to the ratio in the squid beak. We present different variations of our soft-stiff hybrid gels, including multizone cylinders and core-shell discs. Such soft-stiff gels could have utility in bioengineering, such as in interfacing stiff medical implants with soft tissues.

Polymer Pen Lithography with Lipids for Large-Area Gradient Patterns

Gradient patterns comprising bioactive compounds over comparably (in regard to a cell size) large areas are key for many applications in the biomedical sector, in particular, for cell screening assays, guidance, and migration experiments. Polymer pen lithography (PPL) as an inherent highly parallel and large area technique has a great potential to serve in the fabrication of such patterns. We present strategies for the printing of functional phospholipid patterns via PPL that provide tunable feature size and feature density gradients over surface areas of several square millimeters. By controlling the printing parameters, two transfer modes can be achieved. Each of these modes leads to different feature morphologies. By increasing the force applied to the elastomeric pens, which increases the tip-surface contact area and boosts the ink delivery rate, a switch between a dip-pen nanolithography (DPN) and a microcontact printing (μCP) transfer mode can be induced. A careful inking procedure ensuring a homogeneous and not-too-high ink-load on the PPL stamp ensures a membrane-spreading dominated transfer mode, which, used in combination with smooth and hydrophilic substrates, generates features with constant height, independently of the applied force of the pens. Ultimately, this allows us to obtain a gradient of feature sizes over a mm² substrate, all having the same height on the order of that of a biological cellular membrane. These strategies allow the construction of membrane structures by direct transfer of the lipid mixture to the substrate, without requiring previous substrate functionalization, in contrast to other molecular inks, where structure is directly determined by the printing process itself. The patterns are demonstrated to be viable for subsequent protein binding, therefore adding to a flexible feature library when gradients of protein presentation are desired.

Biological responses to immobilized microscale and nanoscale surface topographies

Cellular responses are highly influenced by biochemical and biomechanical interactions with the extracellular matrix (ECM). Due to the impact of ECM architecture on cellular responses, significant research has been dedicated towards developing biomaterials that mimic the physiological environment for design of improved medical devices and tissue engineering scaffolds. Surface topographies with microscale and nanoscale features have demonstrated an effect on numerous cellular responses, including cell adhesion, migration, proliferation, gene expression, protein production, and differentiation; however, relationships between biological responses and surface topographies are difficult to establish due to differences in cell types and biomaterial surface properties. Therefore, it is important to optimize implant surface feature characteristics to elicit desirable biological responses for specific applications. The goal of this work was to review studies investigating the effects of microstructured and nanostructured biomaterials on in vitro biological responses through fabrication of microscale and nanoscale surface topographies, physico-chemical characterization of material surface properties, investigation of protein adsorption dynamics, and evaluation of cellular responses in specific biomedical applications.

Micro- and nano-surface structures based on vapor-deposited polymers

Vapor-deposition processes and the resulting thin polymer films provide consistent coatings that decouple the underlying substrate surface properties and can be applied for surface modification regardless of the substrate material and geometry. Here, various ways to structure these vapor-deposited polymer thin films are described. Well-established and available photolithography and soft lithography techniques are widely performed for the creation of surface patterns and microstructures on coated substrates. However, because of the requirements for applying a photomask or an elastomeric stamp, these techniques are mostly limited to flat substrates. Attempts are also conducted to produce patterned structures on non-flat surfaces with various maskless methods such as light-directed patterning and direct-writing approaches. The limitations for patterning on non-flat surfaces are resolution and cost. With the requirement of chemical control and/or precise accessibility to the linkage with functional molecules, chemically and topographically defined interfaces have recently attracted considerable attention. The multifunctional, gradient, and/or synergistic activities of using such interfaces are also discussed. Finally, an emerging discovery of selective deposition of polymer coatings and the bottom-up patterning approach by using the selective deposition technology is demonstrated.

Effects of freezing, fixation and dehydration on surface roughness properties of porcine left anterior descending coronary arteries

BACKGROUND

To allow measurements of surface roughness to be made of coronary arteries using various imaging techniques, chemical processing, such as fixation and dehydration, is commonly used. Standard protocols suggest storing fresh biological tissue at -40°C. The aim of this study was to quantify the changes caused by freezing and chemical processing to the surface roughness measurements of coronary arteries, and to determine whether correction factors are needed for surface roughness measurements of coronary arteries following chemical processes typically used before imaging these arteries.

METHODS

Porcine left anterior descending coronary arteries were dissected ex vivo. Surface roughness was then calculated following three-dimensional reconstruction of surface images obtained using an optical microscope. Surface roughness was measured before and after a freeze cycle to assess changes during freezing, after chemical fixation, and again after dehydration, to determine changes during these steps of chemical processing.

RESULTS

No significant difference was caused due to the freeze cycle (p>0.05). There was no significant difference in the longitudinally measured surface roughness (Ra_L=0.99±0.39μm; p>0.05) of coronary arteries following fixation and dehydration either. However, the circumferentially measured surface roughness increased significantly following a combined method of processing (Ra_C=1.36±0.40, compared 1.98±0.27μm, respectively; p<0.05). A correction factor can compensate for the change Ra_Cβ=Ra_C1+0.46in Ra_C due to processing of tissue, Where Ra_Cβ, the corrected Ra_C, had a mean of 1.31±0.21μm.

CONCLUSIONS

Independently, freezing, fixation and dehydration do not alter the surface roughness of coronary arteries. Combined, however, fixation and dehydration significantly increase the circumferential, but not longitudinal, surface roughness of coronary arteries.

Dynamic Viscoelasticity and Surface Properties of Porcine Left Anterior Descending Coronary Arteries

The aim of this study was, for the first time, to measure and compare quantitatively the viscoelastic properties and surface roughness of coronary arteries. Porcine left anterior descending coronary arteries were dissected ex vivo. Viscoelastic properties were measured longitudinally using dynamic mechanical analysis, for a range of frequencies from 0.5 to 10 Hz. Surface roughness was calculated following three-dimensional reconstructed of surface images obtained using an optical microscope. Storage modulus ranged from 14.47 to 25.82 MPa, and was found to be frequency-dependent, decreasing as the frequency increased. Storage was greater than the loss modulus, with the latter found to be frequency-independent with a mean value of 2.10 ± 0.33 MPa. The circumferential surface roughness was significantly greater (p < 0.05) than the longitudinal surface roughness, ranging from 0.73 to 2.83 and 0.35 to 0.92 µm, respectively. However, if surface roughness values were corrected for shrinkage during processing, circumferential and longitudinal surface roughness were not significantly different (1.04 ± 0.47, 0.89 ± 0.27 µm, respectively; p > 0.05). No correlation was found between the viscoelastic properties and surface roughness. It is feasible to quantitatively measure the viscoelastic properties of coronary arteries and the roughness of their endothelial surface.

Multifunctional and Continuous Gradients of Biointerfaces Based on Dual Reverse Click Reactions

Chemical or biological gradients that are composed of multifunctional and/or multidirectional guidance cues are of fundamental importance for prospective biomaterials and biointerfaces. As a proof of concept, a general modification approach for generating multifunctional and continuous gradients was realized via two controlled and reversed click reactions, namely, thermo-activated thiol-yne and copper-free alkyne and azide click reactions. The cell adhesion property of fibroblasts was guided in a gradient with an enhancement, showing that the PEG molecule and RGD peptide were countercurrently immobilized to form such reversed gradients (with negating of the cell adhesion property). Using the gradient modification protocol to also create countercurrent distributions of FGF-2 and BMP-2 gradients, the demonstration of not only multifunctional but also gradient biointerfacial properties was resolved in time latencies on one surface by showing the manipulation in gradients toward proliferation and osteogenic differentiation for adipose-derived stem cells.

DNA orientation-specific adhesion and patterning of living mammalian cells on self-assembled DNA monolayers

To better understand cell behaviors on substrates, the precise control of density and orientation of cell-specific ligands remains a great challenge. In this study, we established an easy-to-use approach to manipulate the adhesion and patterning of mammalian cells on gold substrates. We prepared DNA self-assembled monolayers (DNA-SAMs) on gold substrates and found that the sequence-specific orientation of DNA-SAMs played an important role in modulating cell adhesion. We also found that the DNA-SAMs on gold substrates could be used as a potentially universal cell culture substrate, which showed properties similar to cationic polymers (e.g. poly-l lysine, PLL) substrates. Furthermore, we could manipulate cell adhesion by tuning the length of poly adenine (polyA) in the DNA sequence. We also prepared a DNA aptamer-based SAM to regulate cell adhesion by exploiting stimuli-responsive conformational change of the aptamer. By using the well-established DNA spotting technology, we patterned cells on DNA-SAMs to form a spot matrix and four English letters "CELL". Our findings suggest that DNA-SAMs on gold substrates are potentially useful for making smart surfaces for cell studies, thus introducing a new platform for cell/tissue engineering research.

Multi-color polymer pen lithography for oligonucleotide arrays

Multi-color patterning by polymer pen lithography (PPL) was used to fabricate covalently immobilized fluorophore and oligonucleotide arrays with up to five different components. These can easily be translated for presentation of multiple protein types to a single cell.

Functionalizing micro-3D-printed protein hydrogels for cell adhesion and patterning

A versatile and dynamic photoconjugation platform is introduced that provides high, 3D spatial resolution for functionalizing micro-3D-printed (μ-3DP) hydrogels. Schwann cells are patterned on μ-3DP hydrogels precisely labeled with RGD, a cell adhesive peptide, demonstrating utility of this platform for cell culture applications.

Osteogenic differentiation of adipose derived stem cells on high and low aspect ratio micropatterns

Adipose derived stem cells (ADSCs) were cultured on collagen-silk fibroin films with microchannel and micropillar patterns to investigate the effects of cell morphology changes on osteogenic differentiation. Channel and pillar micropatterned films were prepared from collagen type I and silk fibroin. While higher ADSC proliferation profiles were obtained on micropillar blend film, microchannel blend films, however, caused twice higher aspect ratio and effective orientation of cells. Alkaline phosphatase activity of ADSCs was several times higher on microchannel surface when the measured activities were normalized to cell number. Effective deposition of collagen type I and mineral by the cells were observed for patterned and unpatterned films, and these extracellular matrix components were oriented along the axis of the microchannels. In conclusion, the use of collagen-fibroin blend film with microchannel topography increased the aspect ratio and alignment of cells significantly, and was also effective in the differentiation of ADSCs into osteogenic lineage.

Functionalized PDMS with Versatile and Scalable Surface Roughness Gradients for Cell Culture

This manuscript describes a simple and versatile approach to engineering surface roughness gradients via combination of microfluidics and photopolymerization. Through UV-mediated polymerization, N-isopropylacrylamide with concentration gradients are successfully grafted onto PDMS surface, leading to diverse roughness degrees on the obtained PDMS substrate. Furthermore, the extent of surface roughness can be controllably regulated via tuning the flow rate ratio between the monomer solution and deionized water. Average roughness ranging from 2.6±0.7 nm to 163.6±11.7 nm has been well-achieved in this work. Such PDMS samples are also demonstrated to be capable of working as supporting substrates for controlling cell adhesion or detachment. Because of the different degrees of surface roughness on a single substrate, our method provides an effective approach for designing advanced surfaces for cell culture. Finally, the thermosensitive property of N-isopropylacrylamide makes our sample furnish as another means for controlling the cell detachment from the substrates with correspondence to the surrounding temperature.

Orthogonal Morphological Feature Size and Density Gradients for Exploring Synergistic Effects in Biology

Gradient surfaces enable rapid screening and high-throughput investigations in various fields, such as biology and tribology. A new method is described for the preparation of material-independent morphological gradients, in which the density and height of roughness features are varied along two orthogonal axes. A polystyrene-particle-density gradient was produced by a dip-coating process on titanium-oxide-coated silicon wafers. A controlled exposure to ultraviolet light enabled the generation of a particle-height gradient in the orthogonal direction. These gradients were replicated to generate material-independent morphology gradients. MC3T3 cell proliferation studies were performed on titanium-coated replicas and showed a higher cell density on the high-feature-density region of the gradient. The cell area coverage was found to increase with decreasing particle height.

Current Trends and Challenges in Biointerfaces Science and Engineering

The cellular microenvironment is extremely complex, and a plethora of materials and methods have been employed to mimic its properties in vitro. In particular, scientists and engineers have taken an interdisciplinary approach in their creation of synthetic biointerfaces that replicate chemical and physical aspects of the cellular microenvironment. Here the focus is on the use of synthetic materials or a combination of synthetic and biological ligands to recapitulate the defined surface chemistries, microstructure, and function of the cellular microenvironment for a myriad of biomedical applications. Specifically, strategies for altering the surface of these environments using self-assembled monolayers, polymer coatings, and their combination with patterned biological ligands are explored. Furthermore, methods for augmenting an important physical property of the cellular microenvironment, topography, are highlighted, and the advantages and disadvantages of these approaches are discussed. Finally, the progress of materials for prolonged stem cell culture, a key component in the translation of stem cell therapeutics for clinical use, is featured.

Designing ordered micropatterned hydroxyapatite bioceramics to promote the growth and osteogenic differentiation of bone marrow stromal cells

HAp bioceramics with micropatterned surfaces significantly enhance cell responses.

Large area micropatterning of cells on polydimethylsiloxane surfaces

BACKGROUND

Precise spatial control and patterning of cells is an important area of research with numerous applications in tissue engineering, as well as advancing an understanding of fundamental cellular processes. Poly (dimethyl siloxane) (PDMS) has long been used as a flexible, biocompatible substrate for cell culture with tunable mechanical characteristics. However, fabrication of suitable physico-chemical barriers for cells on PDMS substrates over large areas is still a challenge.

RESULTS

Here, we present an improved technique which integrates photolithography and cell culture on PDMS substrates wherein the barriers to cell adhesion are formed using the photo-activated graft polymerization of polyethylene glycol diacrylate (PEG-DA). PDMS substrates with varying stiffness were prepared by varying the base to crosslinker ratio from 5:1 to 20:1. All substrates show controlled cell attachment confined to fibronectin coated PDMS microchannels with a resistance to non-specific adhesion provided by the covalently immobilized, hydrophilic PEG-DA.

CONCLUSIONS

Using photolithography, it is possible to form patterns of high resolution stable at 37°C over 2 weeks, and microstructural complexity over large areas of a few cm(2). As a robust and scalable patterning method, this technique showing homogenous and stable cell adhesion and growth over macroscales can bring microfabrication a step closer to mass production for biomedical applications.

Engineering biomolecular microenvironments for cell instructive biomaterials

Engineered cell instructive microenvironments with the ability to stimulate specific cellular responses are a topic of high interest in the fabrication and development of biomaterials for application in tissue engineering. Cells are inherently sensitive to the in vivo microenvironment that is often designed as the cell "niche." The cell "niche" comprising the extracellular matrix and adjacent cells, influences not only cell architecture and mechanics, but also cell polarity and function. Extensive research has been performed to establish new tools to fabricate biomimetic advanced materials for tissue engineering that incorporate structural, mechanical, and biochemical signals that interact with cells in a controlled manner and to recapitulate the in vivo dynamic microenvironment. Bioactive tunable microenvironments using micro and nanofabrication have been successfully developed and proven to be extremely powerful to control intracellular signaling and cell function. This Review is focused in the assortment of biochemical signals that have been explored to fabricate bioactive cell microenvironments and the main technologies and chemical strategies to encode them in engineered biomaterials with biological information.

Chemically orthogonal three-patch microparticles

Compared to two-dimensional substrates, only a few methodologies exist for the spatially controlled decoration of three-dimensional objects, such as microparticles. Combining electrohydrodynamic co-jetting with synthetic polymer chemistry, we were able to create two- and three-patch microparticles displaying chemically orthogonal anchor groups on three distinct surface patches of the same particle. This approach takes advantage of a combination of novel chemically orthogonal polylactide-based polymers and their processing by electrohydrodynamic co-jetting to yield unprecedented multifunctional microparticles. Several micropatterned particles were fabricated displaying orthogonal click functionalities. Specifically, we demonstrate novel two- and three-patch particles. Multi-patch particles are highly sought after for their potential to present multiple distinct ligands in a directional manner. This work clearly establishes a viable route towards orthogonal reaction strategies on multivalent micropatterned particles.