Modular poly(ethylene glycol) scaffolds provide the ability to decouple the effects of stiffness and protein concentration on PC12 cells

Mimicking the neural stem cell niche: An engineer’s view of cell: material interactions

Neural stem cells have attracted attention in recent years to treat neurodegeneration. There are two neurogenic regions in the brain where neural stem cells reside, one of which is called the subventricular zone (SVZ). The SVZ niche is a complicated microenvironment providing cues to regulate self-renewal and differentiation while maintaining the neural stem cell’s pool. Many scientists have spent years understanding the cellular and structural characteristics of the SVZ niche, both in homeostasis and pathological conditions. On the other hand, engineers focus primarily on designing platforms using the knowledge they acquire to understand the effect of individual factors on neural stem cell fate decisions. This review provides a general overview of what we know about the components of the SVZ niche, including the residing cells, extracellular matrix (ECM), growth factors, their interactions, and SVZ niche changes during aging and neurodegenerative diseases. Furthermore, an overview will be given on the biomaterials used to mimic neurogenic niche microenvironments and the design considerations applied to add bioactivity while meeting the structural requirements. Finally, it will discuss the potential gaps in mimicking the microenvironment.

Bioinks-materials used in printing cells in designed 3D forms

Use of materials to activate non-functional or damaged organs and tissues goes back to early ages. The first materials used for this purpose were metals, and in time, novel materials such as ceramics, polymers and composites were introduced to the field to serve in medical applications. In the last decade, the advances in material sciences, cell biology, technology and engineering made 3D printing of living tissues or organ models in the designed structure and geometry possible by using cells alone or together with hydrogels through additive manufacturing. This review aims to give a brief information about the chemical structures and properties of bioink materials and their applications in the production of 3D tissue constructs.

Engineered Collagen Matrices

Collagen is the most abundant protein in mammals, accounting for approximately one-third of the total protein in the human body. Thus, it is a logical choice for the creation of biomimetic environments, and there is a long history of using collagen matrices for various tissue engineering applications. However, from a biomaterial perspective, the use of collagen-only scaffolds is associated with many challenges. Namely, the mechanical properties of collagen matrices can be difficult to tune across a wide range of values, and collagen itself is not highly amenable to direct chemical modification without affecting its architecture or bioactivity. Thus, many approaches have been pursued to design scaffold environments that display critical features of collagen but enable improved tunability of physical and biological characteristics. This paper provides a brief overview of approaches that have been employed to create such engineered collagen matrices. Specifically, these approaches include blending of collagen with other natural or synthetic polymers, chemical modifications of denatured collagen, de novo creation of collagen-mimetic chains, and reductionist methods to incorporate collagen moieties into other materials. These advancements in the creation of tunable, engineered collagen matrices will continue to enable the interrogation of novel and increasingly complex biological questions.

Fabrication techniques of biomimetic scaffolds in three-dimensional cell culture: A review

In the last four decades, several researchers worldwide have routinely and meticulously exercised cell culture experiments in two-dimensional (2D) platforms. Using traditionally existing 2D models, the therapeutic efficacy of drugs has been inappropriately validated due to the failure in generating the precise therapeutic response. Fortunately, a 3D model addresses the foregoing limitations by recapitulating the in vivo environment. In this context, one has to contemplate the design of an appropriate scaffold for favoring the organization of cell microenvironment. Instituting pertinent model on the platter will pave way for a precise mimicking of in vivo conditions. It is because animal cells in scaffolds oblige spontaneous formation of 3D colonies that molecularly, phenotypically, and histologically resemble the native environment. The 3D culture provides insight into the biochemical aspects of cell-cell communication, plasticity, cell division, cytoskeletal reorganization, signaling mechanisms, differentiation, and cell death. Focusing on these criteria, this paper discusses in detail, the diversification of polymeric scaffolds based on their available resources. The paper also reviews the well-founded and latest techniques of scaffold fabrication, and their applications pertaining to tissue engineering, drug screening, and tumor model development.

Microgels: Modular, tunable constructs for tissue regeneration

Biopolymer microgels are emerging as a versatile tool for aiding in the regeneration of damaged tissues due to their biocompatible nature, tunable microporous structure, ability to encapsulate bioactive factors, and tailorable properties such as stiffness and composition. These properties of microgels, along with their injectability, have allowed for their utilization in a multitude of different tissue engineering applications. Controlled release of growth factors, antibodies, and other bioactive factors from microgels have demonstrated their capabilities as transporters for essential bioactive molecules necessary for guiding tissue reconstruction. Additionally, recent in vitro studies of cellular interaction and proliferation within microgel structures have laid the initial groundwork for regenerative tissue engineering using these materials. Microgels have even been crosslinked together in various ways or 3D printed to form three-dimensional scaffolds to support cell growth. In vivo studies of microgels have pioneered the clinical relevance of these novel and innovative materials for regenerative tissue engineering. This review will cover recent developments and research of microgels as they pertain to bioactive factor release, cellular interaction and proliferation in vitro, and tissue regeneration in vivo. STATEMENT OF SIGNIFICANCE: This review is focused on state-of-the-art microgel technology and innovations within the tissue engineering field, focusing on the use of microgels in bioactive factor delivery and as cell-interactive scaffolds, both in vitro and in vivo. Microgels are hydrogel microparticles that can be tuned based on the biopolymer from which they are derived, the crosslinking chemistry used, and the fabrication method. The emergence of microgels for tissue regeneration applications in recent years illuminates their versatility and applicability in clinical settings.

Neural tissue engineering with structured hydrogels in CNS models and therapies

The development of techniques to create and use multiphase microstructured hydrogels (granular hydrogels or microgels) has enabled the generation of cultures with more biologically relevant architecture and use of structured hydrogels is especially pertinent to the development of new types of central nervous system (CNS) culture models and therapies. We review material choice and the customisation of hydrogel structure, as well as the use of hydrogels in developmental models. Combining the use of structured hydrogel techniques with developmentally relevant tissue culture approaches will enable the generation of more relevant models and treatments to repair damaged CNS tissue architecture.

Novel microgel-based scaffolds to study the effect of degradability on human dermal fibroblasts

For improved cell integration, tissue engineering scaffolds must be designed to degrade over time. Typically, the chemistry of scaffolds is modified to alter the degradation profile by using different hydrolytic or enzymatic sites within a material. It is more challenging, however, to fabricate self-assembling, injectable scaffolds that provide tunable degradation. Our laboratory has developed microgel-based scaffolds, where individual micron-sized hydrogels are crosslinked to make larger bulk scaffolds. The size of the individual microgels permits injection, and the microgels then self-assemble into a bulk structure and crosslink. We hypothesized that the microgel-based scaffolds can be used to tune degradability by mixing degradable and non-degradable microgels at various ratios within a self-assembling scaffold. Therefore, two types of microgels were fabricated, those composed of polyethylene glycol (PEG) and those composed of a PEG-lactic acid. Importantly, the microgels were similar in size and swelling and had a low polydispersity index due to their method of fabrication. Microgels were then mixed in four ratios to fabricate scaffolds and study how changes in scaffold composition altered the 3D proliferation and morphology of human dermal fibroblasts. Microgel-based scaffolds formed with 100% degradable microgels lost >60% of their mass over the 14 days of the study. Human dermal fibroblasts were mixed within the 3D scaffolds at the time of assembly and all scaffolds had cells with high viability and typical morphology. The scaffolds that had 25%-50% degradable microgels showed statistically increased proliferation of fibroblasts after 1 and 2 weeks over non-degradable scaffolds and those scaffolds with 75% or 100% degradable microgels. Overall, this work demonstrates the development and use of a tunable, self-assembled, microgel-based scaffold to investigate the effects of degradability on cellular response.

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

Endothelial to mesenchymal transformation is induced by altered extracellular matrix in aortic valve endothelial cells

Alterations in shear stress, mechanical deformation, extracellular matrix (ECM) composition and exposure to inflammatory conditions are known to cause endothelial to mesenchymal transformation (EndMT). This change in endothelial phenotype has only recently been linked to adult pathologies such as cancer progression, organ fibrosis, and calcific aortic valve disease; and its function in adult physiology, especially in response to tissue mechanics, has not been rigorously investigated. EndMT is a response to mechanical and biochemical signals that results in the remodeling of underlying tissues. In diseased aortic valves, glycosaminoglycans (GAGs) are present in the collagen-rich valve fibrosa, and are deposited near calcified nodules. In this study, in vitro models of early and late-stage valve disease were developed by incorporating the GAGs chondroitin sulfate (CS), hyaluronic acid, and dermatan sulfate into 3D collagen hydrogels with or without exposure to TGF-β1 to simulate EndMT in response to microenvironmental changes. High levels of CS induced the highest rate of EndMT and led to the most collagen I and GAG production by mesenchymally transformed cells, which indicates a cell phenotype most likely to promote fibrotic disease. Mesenchymal transformation due to altered ECM was found to depend on cell-ECM bond strength and extracellular signal-regulated protein kinases 1/2 signaling. Determining the environmental conditions that induce and promote EndMT, and the subsequent behavior of mesenchymally transformed cells, will advance understanding on the role of endothelial cells in tissue regeneration or disease progression. © 2017 Wiley Periodicals Inc. J Biomed Mater Res Part A: 105A: 2729-2741, 2017.

Tuning the Mechanical Properties of Poly(Ethylene Glycol) Microgel-Based Scaffolds to Increase 3D Schwann Cell Proliferation

2D in vitro studies have demonstrated that Schwann cells prefer scaffolds with mechanical modulus approximately 10× higher than the modulus preferred by nerves, limiting the ability of many scaffolds to promote both neuron extension and Schwann cell proliferation. Therefore, the goals of this work are to develop and characterize microgel-based scaffolds that are tuned over the stiffness range relevant to neural tissue engineering and investigate Schwann cell morphology, viability, and proliferation within 3D scaffolds. Using thiol-ene reaction, microgels with surface thiols are produced and crosslinked into hydrogels using a multiarm vinylsulfone (VS). By varying the concentration of VS, scaffold stiffness ranges from 0.13 to 0.76 kPa. Cell morphology in all groups demonstrates that cells are able to spread and interact with the scaffold through day 5. Although the viability in all groups is high, proliferation of Schwann cells within the scaffold of G* = 0.53 kPa is significantly higher than other groups. This result is ≈ 5× lower than previously reported optimal stiffnesses on 2D surfaces, demonstrating the need for correlation of 3D cell response to mechanical modulus. As proliferation is the first step in Schwann cell integration into peripheral nerve conduits, these scaffolds demonstrate that the stiffness is a critical parameter to optimizing the regenerative process.

A modular, plasmin-sensitive, clickable poly(ethylene glycol)-heparin-laminin microsphere system for establishing growth factor gradients in nerve guidance conduits

Peripheral nerve regeneration is a complex problem that, despite many advancements and innovations, still has sub-optimal outcomes. Compared to biologically derived acellular nerve grafts and autografts, completely synthetic nerve guidance conduits (NGC), which allow for precise engineering of their properties, are promising but still far from optimal. We have developed an almost entirely synthetic NGC that allows control of soluble growth factor delivery kinetics, cell-initiated degradability and cell attachment. We have focused on the spatial patterning of glial-cell derived human neurotrophic factor (GDNF), which promotes motor axon extension. The base scaffolds consisted of heparin-containing poly(ethylene glycol) (PEG) microspheres. The modular microsphere format greatly simplifies the formation of concentration gradients of reversibly bound GDNF. To facilitate axon extension, we engineered the microspheres with tunable plasmin degradability. 'Click' cross-linking chemistries were also added to allow scaffold formation without risk of covalently coupling the growth factor to the scaffold. Cell adhesion was promoted by covalently bound laminin. GDNF that was released from these microspheres was confirmed to retain its activity. Graded scaffolds were formed inside silicone conduits using 3D-printed holders. The fully formed NGC's contained plasmin-degradable PEG/heparin scaffolds that developed linear gradients in reversibly bound GDNF. The NGC's were implanted into rats with severed sciatic nerves to confirm in vivo degradability and lack of a major foreign body response. The NGC's also promoted robust axonal regeneration into the conduit.

Characteristics of precipitation-formed polyethylene glycol microgels are controlled by molecular weight of reactants

This work describes the formation of poly(ethylene glycol) (PEG) microgels via a photopolymerized precipitation reaction. Precipitation reactions offer several advantages over traditional microsphere fabrication techniques. Contrary to emulsion, suspension, and dispersion techniques, microgels formed by precipitation are of uniform shape and size, i.e. low polydispersity index, without the use of organic solvents or stabilizers. The mild conditions of the precipitation reaction, customizable properties of the microgels, and low viscosity for injections make them applicable for in vivo purposes. Unlike other fabrication techniques, microgel characteristics can be modified by changing the starting polymer molecular weight. Increasing the starting PEG molecular weight increased microgel diameter and swelling ratio. Further modifications are suggested such as encapsulating molecules during microgel crosslinking. Simple adaptations to the PEG microgel building blocks are explored for future applications of microgels as drug delivery vehicles and tissue engineering scaffolds.

Adaptable poly(ethylene glycol) microspheres capable of mixed-mode degradation

A simple, degradable poly(ethylene glycol) (PEG) microsphere system formed from a water-in-water emulsion process is presented. Microsphere network degradation and erosion were controlled by adjusting the number of hydrolytically labile sites, by varying the PEG molecular weight, and by adjusting the emulsion conditions. Microsphere size was also controllable by adjusting the polymer formulation. Furthermore, it is demonstrated that alternative degradation and erosion mechanisms, such as proteolytic degradation, can be incorporated into PEG microspheres, resulting in mixed-mode degradation. Owing to the adaptability of this approach, it may serve as an attractive option for emerging tissue engineering, drug delivery and gene delivery applications.

Engineering peripheral nerve repair

Current approaches for treating peripheral nerve injury have resulted in promising, yet insufficient functional recovery compared to the clinical standard of care, autologous nerve grafts. In order to design a construct that can match the regenerative potential of the autograft, all facets of nerve tissue must be incorporated in a combinatorial therapy. Engineered biomaterial scaffolds in the future will have to promote enhanced regeneration and appropriate reinnervation by targeting the highly sensitive response of regenerating nerves to their surrounding microenvironment.

Naturally and synthetic smart composite biomaterials for tissue regeneration

The development of smart biomaterials for tissue regeneration has become the focus of intense research interest. More opportunities are available by the composite approach of combining the biomaterials in the form of biopolymers and/or bioceramics either synthetic or natural. Strategies to provide smart capabilities to the composite biomaterials primarily seek to achieve matrices that are instructive/inductive to cells, or that stimulate/trigger target cell responses that are crucial in the tissue regeneration processes. Here, we review in-depth, recent developments concerning smart composite biomaterials available for delivery systems of biofactors and cells and scaffolding matrices in tissue engineering. Smart composite designs are possible by modulating the bulk and surface properties that mimic the native tissues, either in chemical (extracellular matrix molecules) or in physical properties (e.g. stiffness), or by introducing external therapeutic molecules (drugs, proteins and genes) within the structure in a way that allows sustainable and controllable delivery, even time-dependent and sequential delivery of multiple biofactors. Responsiveness to internal or external stimuli, including pH, temperature, ionic strength, and magnetism, is another promising means to improve the multifunctionality in smart scaffolds with on-demand delivery potential. These approaches will provide the next-generation platforms for designing three-dimensional matrices and delivery systems for tissue regenerative applications.

Fluorinated methacrylamide chitosan hydrogel systems as adaptable oxygen carriers for wound healing

In this study a series of novel, biocompatible hydrogels able to repeatedly takeup and deliver oxygen at beneficial levels have been developed by conjugating various perfluorocarbon (PFC) chains to methacrylamide chitosan via Schiff base nucleophilic substitution, followed by photopolymerization to form hydrogels. The synthesized fluorinated methacrylamide chitosan (MACF) hydrogels were confirmed by high resolution (19)F NMR. Synthesized MACF hydrogels were tested for their ability to takeup and then release oxygen for future use in dermal wound healing. Depending on the PFC substitution type maximum O(2) uptake was observed within 2-6h, followed by complete release to the surrounding environment (5% CO(2)) within 12-120h at oxygen partial pressures of 1-25mm Hg h(-1), providing outstanding system tuning for wound healing and regenerative medicine. MACFs with the most fluorines per substitution showed the greatest uptake and release of oxygen. Interestingly, adding PFC chains with a fluorinated aromatic group considerably enhanced oxygen uptake and extended release compared with a linear PFC chain with the same number of fluorine molecules. MACF hydrogels proved to be readily reloaded with oxygen once release was complete, and regeneration could be performed as long as the hydrogel was intact. Fibroblasts were cultured on MACFs and assays confirmed that materials containing more fluorines per substitution supported the most cells with the greatest metabolic activity. This result was true, even without oxygenation, suggesting PFC-facilitated oxygen diffusion from the culture medium. Finally, MACF gradient hydrogels were created, demonstrating that these materials can control oxygen levels on a spatial scale of millimeters and greatly enhance cellular proliferative and metabolic responses.

3D differentiation of neural stem cells in macroporous photopolymerizable hydrogel scaffolds

Neural stem/progenitor cells (NSPCs) are the stem cell of the adult central nervous system (CNS). These cells are able to differentiate into the major cell types found in the CNS (neurons, oligodendrocytes, astrocytes), thus NSPCs are the mechanism by which the adult CNS could potentially regenerate after injury or disorder. Microenviromental factors are critical for guiding NSPC differentiation and are thus important for neural tissue engineering. In this study, D-mannitol crystals were mixed with photocrosslinkable methacrylamide chitosan (MAC) as a porogen to enhance pore size during hydrogel formation. D-mannitol was admixed to MAC at 5, 10 and 20 wt% D-mannitol per total initial hydrogel weight. D-mannitol crystals were observed to dissolve and leave the scaffold within 1 hr. Quantification of resulting average pore sizes showed that D-mannitol addition resulted in larger average pore size (5 wt%, 4060±160 µm(2), 10 wt%, 6330±1160 µm(2), 20 wt%, 7600±1550 µm(2)) compared with controls (0 wt%, 3150±220 µm(2)). Oxygen diffusion studies demonstrated that larger average pore area resulted in enhanced oxygen diffusion through scaffolds. Finally, the differentiation responses of NSPCs to phenotypic differentiation conditions were studied for neurons, astrocytes and oligodendrocytes in hydrogels of varied porosity over 14 d. Quantification of total cell numbers at day 7 and 14, showed that cell numbers decreased with increased porosity and over the length of the culture. At day 14 immunohistochemistry quantification for primary cell types demonstrated significant differentiation to the desired cells types, and that total percentages of each cell type was greatest when scaffolds were more porous. These results suggest that larger pore sizes in MAC hydrogels effectively promote NSPC 3D differentiation.

Comparison of neurite growth in three dimensional natural and synthetic hydrogels

Extracellular matrix incorporated within a scaffold plays an important role in assisting cell behavior in neural tissue engineering. In this study, we investigated how the concentration of fibronectin (FN) affected neurite growth when incorporated within a synthetic polymer gel made of poly(ethylene glycol) (PEG) or a natural polymer gel of collagen I. Mechanical and chemical properties of the scaffold were varied by using a range of concentrations of gels and FN. Rheology was used to determine the mechanical stiffness of hydrogels and neurite length and viability were measured to evaluate cell response. In both types of gels, increasing the concentration of the base scaffold (PEG or collagen) increased the mechanical stiffness as denoted by G∗. Neurite lengths in PEG gels increased with increasing FN concentration and decreased with increasing G∗. In collagen gels, FN reduced neurite extension for the lowest concentrations of collagen (0.4-0.6 mg/mL) while FN increased neurite extension for mid and high collagen concentrations (1.0-2.0 mg/mL). The results from these two different scaffolds indicate that both stiffness and FN concentration impact the growth of the neurite and that the addition of small amounts of FN (100 μg/ml) permits PEG gels to perform on par with similar stiffness collagen gels.

Decoupling polymer properties to elucidate mechanisms governing cell behavior

Determining how a biomaterial interacts with cells ("structure-function relationship") reflects its eventual clinical applicability. Therefore, a fundamental understanding of how individual material properties modulate cell-biomaterial interactions is pivotal to improving the efficacy and safety of clinically translatable biomaterial systems. However, due to the coupled nature of material properties, their individual effects on cellular responses are difficult to understand. Structure-function relationships can be more clearly understood by the effective decoupling of each individual parameter. In this article, we discuss three basic decoupling strategies: (1) surface modification, (2) cross-linking, and (3) combinatorial approaches (i.e., copolymerization and polymer blending). Relevant examples of coupled material properties are briefly reviewed in each section to highlight the need for improved decoupling methods. This follows with examples of more effective decoupling techniques, mainly from the perspective of three primary classes of synthetic materials: polyesters, polyethylene glycol, and polyacrylamide. Recent strides in decoupling methodologies, especially surface-patterning and combinatorial techniques, offer much promise in further understanding the structure-function relationships that largely govern the success of future advancements in biomaterials, tissue engineering, and drug delivery.