Microarchitecture of three-dimensional scaffolds influences cell migration behavior via junction interactions

Cryogel micromechanics unraveled by atomic force microscopy-based nanoindentation

Cell-instructive physical characteristics of macroporous scaffolds, developed for tissue engineering applications, often remain difficult to assess. Here, an atomic force microscopy-based nanoindentation approach is adapted to quantify the local mechanical properties of biohybrid glycosaminoglycan-poly(ethylene glycol) cryogels. Resulting from cryoconcentration effects upon gel formation, cryogel struts are observed to feature a higher stiffness compared to the corresponding bulk hydrogel materials. Local Young's moduli, porosity, and integral moduli of the cryogel scaffolds are compared in dependence on gel formation parameters. The results provide valuable insights into the cryogelation process and a base for adjusting physical characteristics of the obtained cryogel scaffolds, which can critically influence the cellular response.

Photopatterning of vascular endothelial growth factor within collagen-glycosaminoglycan scaffolds can induce a spatially confined response in human umbilical vein endothelial cells

Biomolecular signals within the native extracellular matrix are complex, with bioactive factors found in both soluble and sequestered states. In the design of biomaterials for tissue engineering applications it is increasingly clear that new approaches are required to locally tailor the biomolecular environment surrounding cells within the matrix. One area of particular focus is strategies to improve the speed or quality of vascular ingrowth and remodeling. While the addition of soluble vascular endothelial growth factor (VEGF) has been shown to improve vascular response, strategies to immobilize such signals within a biomaterial offer the opportunity to optimize efficiency and to explore spatially defined patterning of such signals. Here we describe the use of benzophenone (BP) photolithography to decorate three-dimensional collagen-glycosaminoglycan (CG) scaffolds with VEGF in a spatially defined manner. In this effort we demonstrate functional patterning of a known agonist of vascular remodeling and directly observe phenotypic effects induced by this immobilized cue. VEGF was successfully patterned in both stripes and square motifs across the scaffold with high specificity (on:off pattern signal). The depth of patterning was determined to extend up to 500 μm into the scaffold microstructure. Notably, photopatterned VEGF retained native functionality as it was shown to induce morphological changes in human umbilical vein cells indicative of early vasculogenesis. Immobilized VEGF led to greater cell infiltration into the scaffold and the formation of immature vascular network structures. Ultimately, these results suggest that BP-mediated photolithography is a facile method to spatially control the presentation of instructive biological cues to cells within CG scaffolds.

Molecular mobility of scaffolds' biopolymers influences cell growth

Understanding biocompatibility of materials and scaffolds is one of the main challenges in the field of tissue engineering and regeneration. The complex nature of cell-biomaterial interaction requires extensive preclinical functionality testing by studying specific cell responses to different biomaterial properties, from morphology and mechanics to surface characteristics at the molecular level. Despite constant improvements, a more general picture of biocompatibility is still lacking and tailormade scaffolds are not yet available. The scope of our study was thus the investigation of the correlation of fibroblast cell growth on different gelatin scaffolds with their morphological, mechanical as well as surface molecular properties. The latter were thoroughly investigated via polymer molecular mobility studied by site-directed spin labeling and electron paramagnetic resonance spectroscopy (EPR) for the first time. Anisotropy of the rotational motion of the gelatin side chain mobility was identified as the most correlated quantity with cell growth in the first days after adhesion, while weaker correlations were found with scaffold viscoelasticity and no correlations with scaffold morphology. Namely, the scaffolds with highly mobile or unrestricted polymers identified with the cell growth being five times less efficient (N(cells) = 60 ± 25 mm(-2)) as compared to cell growth on the scaffolds with considerable part of polymers with the restricted rotational motion (N(cells) = 290 ± 25 mm(-2)). This suggests that molecular mobility of scaffold components could play an important role in cell response to medical devices, reflecting a new aspect of the biocompatibility concept.

Collagen-glycosaminoglycan matrix implantation promotes angiogenesis following surgical brain trauma

Surgical brain injury (SBI) is unavoidable during many neurosurgical procedures intrinsically linked to postoperative neurological deficits. We have previously demonstrated that implantation of collagen glycosaminoglycan (CG) following surgical brain injury could significantly promote functional recovery and neurogenesis. In this study we further hypothesized that this scaffold may provide a microenvironment by promoting angiogenesis to favor neurogenesis and subsequent functional recovery. Using the rodent model of surgical brain injury as we previously established, we divided Sprague-Dawley male rats (weighting 300-350 g) into three groups: (1) sham (2) surgical injury with a lesion (L), and (3) L with CG matrix implantation (L + CG). Our results demonstrated that L + CG group showed a statistically significant increase in the density of vascular endothelial cells and blood vessels over time. In addition, tissue concentrations of angiogenic growth factors (such as VEGF, FGF2, and PDGF) significantly increased in L + CG group. These results suggest that implantation of a CG scaffold can promote vascularization accompanied by neurogenesis. This opens prospects for use of CG scaffolds in conditions such as brain injury including trauma and ischemia.

Neural stem cell transplantation in a double-layer collagen membrane with unequal pore sizes for spinal cord injury repair

A novel double-layer collagen membrane with unequal pore sizes in each layer was designed and tested in this study. The inner, loose layer has about 100-μm-diameter pores, while the outer, compact layer has about 10-μm-diameter pores. In a rat model of incomplete spinal cord injury, a large number of neural stem cells were seeded into the loose layer, which was then adhered to the injured side, and the compact layer was placed against the lateral side. The results showed that the transplantation of neural stem cells in a double-layer collagen membrane with unequal pore sizes promoted the differentiation of neural stem cells, attenuated the pathological lesion, and significantly improved the motor function of the rats with incomplete spinal cord injuries. These experimental findings suggest that the transplantation of neural stem cells in a double-layer collagen membrane with unequal pore sizes is an effective therapeutic strategy to repair an injured spinal cord.

Strategies to balance covalent and non-covalent biomolecule attachment within collagen-GAG biomaterials

Strategies to integrate instructive biomolecular signals into a biomaterial are becoming increasingly complex and bioinspired. While a large majority of reports still use repeated treatments with soluble factors, this approach can be prohibitively costly and difficult to translate in vivo for applications where spatial control over signal presentation is necessary. Recent efforts have explored the use of covalent immobilization of biomolecules to the biomaterial, via both bulk (ubiquitous) as well as spatially-selective light-based crosslinking, as a means to both enhance stability and bioactivity. However, little is known about how processing conditions during immobilization impact the degree of unintended non-covalent interactions, or fouling, that takes place between the biomaterial and the biomolecule of interest. Here we demonstrate the impact of processing conditions for bulk carbodiimide (EDC) and photolithography-based benzophenone (BP) crosslinking on specific attachment vs. fouling of a model protein (Concanavalin A, ConA) within collagen-glycosaminoglycan (CG) scaffolds. Collagen source significantly impacts the selectivity of biomolecule immobilization. EDC crosslinking intensity and ligand concentration significantly impacted selective immobilization. For benzophenone photoimmobilization we observed that increased UV exposure time leads to increased ConA immobilization. Immobilization efficiency for both EDC and BP strategies was maximal at physiological pH. Increasing ligand concentration during immobilization process led to enhanced immobilization for EDC chemistry, no impact on BP immobilization, but significant increases in non-specific fouling. Given recent efforts to covalently immobilize biomolecules to a biomaterial surface to enhance bioactivity, improved understanding of the impact of crosslinking conditions on selective attachment versus non-specific fouling will inform the design of instructive biomaterials for applications across tissue engineering.

Aligned collagen-GAG matrix as a 3D substrate for Schwann cell migration and dendrimer-based gene delivery

The development of artificial off-the-shelf conduits that facilitate effective nerve regeneration and recovery after repair of traumatic nerve injury gaps is of fundamental importance. Collagen-glycosaminoglycan (GAG) matrix mimicking Schwann cell (SC) basal lamina has been proposed as a suitable and biologically rational substrate for nerve regeneration. In the present study, we have focused on the permissiveness of this matrix type for SC migration and repopulation, as these events play an essential role in nerve remodeling. We have also demonstrated that SCs cultured within collagen-GAG matrix are compatible with non-viral dendrimer-based gene delivery, that may allow conditioning of matrix-embedded cells for future gene therapy applications.

Spatial optimization in perfusion bioreactors improves bone tissue-engineered construct quality attributes

Perfusion bioreactors have shown great promise for tissue engineering applications providing a homogeneous and consistent distribution of nutrients and flow-induced shear stresses throughout tissue-engineered constructs. However, non-uniform fluid-flow profiles found in the perfusion chamber entrance region have been shown to affect tissue-engineered construct quality characteristics during culture. In this study a whole perfusion and construct, three dimensional (3D) computational fluid dynamics approach was used in order to optimize a critical design parameter such as the location of the regular pore scaffolds within the perfusion bioreactor chamber. Computational studies were coupled to bioreactor experiments for a case-study flow rate. Two cases were compared in the first instance seeded scaffolds were positioned immediately after the perfusion chamber inlet while a second group was positioned at the computationally determined optimum distance were a steady state flow profile had been reached. Experimental data showed that scaffold location affected significantly cell content and neo-tissue distribution, as determined and quantified by contrast enhanced nanoCT, within the constructs both at 14 and 21 days of culture. However, gene expression level of osteopontin and osteocalcin was not affected by the scaffold location. This study demonstrates that the bioreactor chamber environment, incorporating a scaffold and its location within it, affects the flow patterns within the pores throughout the scaffold requiring therefore dedicated optimization that can lead to bone tissue engineered constructs with improved quality attributes.

Structural and biochemical modification of a collagen scaffold to selectively enhance MSC tenogenic, chondrogenic, and osteogenic differentiation

Biomaterial approaches for engineering orthopedic interfaces such as the tendon-bone junction (TBJ) are limited by a lack of understanding of how insoluble (microstructure, composition) and soluble regulators of stem cell fate work in concert to promote bioactivity and differentiation. One strategy for regenerating the interface is to design biomaterials containing spatially graded structural properties sufficient to induce divergent mesenchymal stem cell (MSC) differentiation into multiple interface-specific phenotypes. This work explores the hypothesis that selective structural modification to a 3D collagen-glycosaminoglycan (CG) scaffold combined with biochemical supplementation can drive human bone-marrow-derived MSC differentiation down tenogenic, osteogenic, and chondrogenic lineages. Tenogenic differentiation is enhanced in geometrically anisotropic scaffolds versus a standard isotropic control. Notably, blebbistatin treatment abrogates this microstructurally driven effect. Further, enhanced osteogenic differentiation and new mineral synthesis are achieved by incorporation of a calcium phosphate mineral phase within the CG scaffold along with the use of osteogenic induction media. Finally, chondrogenic differentiation is optimally driven by combining chondrogenic induction media with a reduced density scaffold that promotes increased cellular condensation, significantly higher expression of chondrogenic genes, and increased GAG deposition. Together these data provide critical insight regarding design rules for elements of an integrated biomaterial platform for orthopedic interface regeneration.

Matrix-driven formation of mesenchymal stem cell-extracellular matrix microtissues on soft alginate hydrogels

Mesenchymal stem cells (MSCs) can be made to rearrange into microtissues in response to specific matrix cues, a process that depends on a balance between cell-matrix and cell-cell interactions. The effect of such cues, and especially their interplay, is still not fully understood, particularly in three-dimensional (3-D) systems. Here, the behaviour of human MSCs cultured within hydrogel matrices with tailored stiffness and composition was evaluated. MSC aggregation occurred only in more compliant matrices (G'≤ 120 Pa), when compared to stiffer ones, both in the presence and in the absence of matrix-bound arginine-glycine-aspartic acid cell-adhesion ligands (RGD; 0, 100 and 200 μM). Fibronectin assembly stabilized cell-cell contacts within aggregates, even in non-adhesive matrices. However, MSCs were able to substantially contract the artificial matrix only when RGD was present. Moreover, compliant matrices facilitated cell proliferation and provided an environment conducive for MSC osteogenic differentiation, even without RGD. Cell interactions with the original matrix became less important as time progressed, while the de novo-produced extracellular matrix became a more critical determinant of cell fate. These data provide further insights into the mechanisms by which MSCs sense their microenvironment to organize into tissues, and provide new clues to the design of cell-instructive 3-D matrices.

Three-dimensional cancer models mimic cell-matrix interactions in the tumour microenvironment

Basic in vitro systems can be used to model and assess complex diseases, such as cancer. Recent advances in this field include the incorporation of multiple cell types and extracellular matrix proteins into three-dimensional (3D) models to recapitulate the structure, organization and functionality of live tissue in situ. Cells within such a 3D environment behave very differently from cells on two-dimensional (2D) substrates, as cell-matrix interactions trigger signalling pathways and cellular responses in 3D, which may not be observed in 2D. Thus, the use of 3D systems can be advantageous for the assessment of disease progression over 2D set-ups alone. Here, we highlight the current advantages and challenges of employing 3D systems in the study of cancer and provide an overview to guide the appropriate use of distinct models in cancer research.

Collagen-GAG scaffold biophysical properties bias MSC lineage choice in the presence of mixed soluble signals

Biomaterial strategies for regenerating multitissue structures require unique approaches. One strategy is to design scaffolds so that their local biophysical properties can enhance site-specific effects of an otherwise heterogeneous biomolecular environment. This investigation examined the role of biomaterial physical properties (relative density, mineral content) on the human mesenchymal stem cell phenotype in the presence of mixed soluble signals to drive osteogenesis or chondrogenesis. We tested a series of three-dimensional collagen-glycosaminoglycan scaffolds with properties inspired by extracellular matrix characteristics across the osteotendinous interface (tendon, cartilage, and bone). We found that selective scaffold mineralization induced a depressed chondrogenic response compared with nonmineralized groups as demonstrated by gene expression and histological analyses. Interestingly, the greatest chondrogenic response was found in a higher density, nonmineralized scaffold variant despite increased contraction and cellular condensation in lower density nonmineralized scaffolds. In fact, the lower density scaffolds demonstrated a significantly higher expression of osteogenic transcripts as well as ample mineralization after 21 days of culture. This effect may be due to local stiffening of the scaffold microenvironment as the scaffold contracts, leading to increased cell density, accelerated differentiation, and possible endochondral ossification as evidenced by a transition from a glycosaminoglycan (GAG)-rich milieu to higher mineralization at later culture times. These findings will help shape the design rules for graded biomaterials to regenerate distinct fibrillar, fibrocartilagenous, and mineralized regions of orthopedic interfaces.

Microfluidic generation of gradient hydrogels to modulate hematopoietic stem cell culture environment

The bone marrow provides spatially and temporally variable signals that impact the behavior of hematopoietic stem cells (HSCs). While multiple biomolecular signals and bone marrow cell populations have been proposed as key regulators of HSC fate, new tools are required to probe their importance and mechanisms of action. Here, a novel method based on a microfluidic mixing platform to create small volume, 3D hydrogel constructs containing overlapping patterns of cell and matrix constituents inspired by the HSC niche is described. This approach is used to generate hydrogels containing opposing gradients of fluorescent microspheres, MC3T3-E1 osteoblasts, primary murine hematopoietic stem and progenitor cells (HSPCs), and combinations thereof in a manner independent of hydrogel density and cell/particle size. Three different analytical methods are described to characterize local properties of these hydrogels at multiple scales: 1) whole construct fluorescent analysis; 2) multi-photon imaging of individual cells within the construct; 3) retrieval of discrete sub-regions from the hydrogel post-culture. The approach reported here allows the creation of stable gradients of cell and material cues within a single, optically translucent 3D biomaterial to enable a range of investigations regarding how microenvironmental signals impact cell fate.

25th anniversary article: Designer hydrogels for cell cultures: a materials selection guide

Cell culturing, whether for tissue engineering or cell biology studies, always involves placing cells in a non-natural environment and no material currently exist that can mimic the entire complexity of natural tissues and variety of cell-matrix interactions that is found in vivo. Here, we review the vast range of hydrogels, composed of natural or synthetic polymers that provide a route to tailored microenvironments.

Innovations in preclinical biology: ex vivo engineering of a human kidney tissue microperfusion system

Kidney disease is a public health problem that affects more than 20 million people in the US adult population, yet little is understood about the impact of kidney disease on drug disposition. Consequently there is a critical need to be able to model the human kidney and other organ systems, to improve our understanding of drug efficacy, safety, and toxicity, especially during drug development. The kidneys in general, and the proximal tubule specifically, play a central role in the elimination of xenobiotics. With recent advances in molecular investigation, considerable information has been gathered regarding the substrate profiles of the individual transporters expressed in the proximal tubule. However, we have little knowledge of how these transporters coupled with intracellular enzymes and influenced by metabolic pathways form an efficient secretory and reabsorptive mechanism in the renal tubule. Proximal tubular secretion and reabsorption of xenobiotics is critically dependent on interactions with peritubular capillaries and the interstitium. We plan to robustly model the human kidney tubule interstitium, utilizing an ex vivo three-dimensional modular microphysiological system with human kidney-derived cells. The microphysiological system should accurately reflect human physiology, be usable to predict renal handling of xenobiotics, and should assess mechanisms of kidney injury, and the biological response to injury, from endogenous and exogenous intoxicants.

Complete horizontal skin cell resurfacing and delayed vertical cell infiltration into porcine reconstructive tissue matrix compared to bovine collagen matrix and human dermis

BACKGROUND

Xenogenous dermal matrices are used for hernia repair and breast reconstruction. Full-thickness skin replacement is needed after burn or degloving injuries with exposure of tendons or bones. The authors used a human skin organ culture model to study whether porcine reconstructive tissue matrix (Strattice) is effective as a dermal tissue replacement.

METHODS

Skin cells or split-thickness skin grafts were seeded onto human deepidermized dermis, Strattice, and Matriderm. Cellular resurfacing and matrix infiltration were monitored by live fluorescence imaging, histology, and electron microscopy. Proliferation, apoptosis, cell differentiation, and adhesion were analyzed by immunohistochemistry.

RESULTS

Epithelial resurfacing and vertical proliferation were reduced and delayed with both bioartificial matrices compared with deepidermized dermis; however, no differences in apoptosis, cell differentiation, or basement membrane formation were found. Vertical penetration was greatest on Matriderm, whereas no matrix infiltration was found on Strattice in the first 12 days. Uncompromised horizontal resurfacing was greatest with Strattice but was absent with Matriderm. Strattice showed no stimulatory effect on cellular inflammation.

CONCLUSIONS

Matrix texture and surface properties governed cellular performance on tissues. Although dense dermal compaction delayed vertical cellular ingrowth for Strattice, it allowed uncompromised horizontal resurfacing. Dense dermal compaction may slow matrix decomposition and result in prolonged biomechanical stability of the graft. Reconstructive surgeons should choose the adequate matrix substitute depending on biomechanical requirements at the recipient site. Strattice may be suitable as a dermal replacement at recipient sites with high mechanical load requirements.

The use of bioinspired alterations in the glycosaminoglycan content of collagen-GAG scaffolds to regulate cell activity

The design of biomaterials for regenerative medicine can require biomolecular cues such as growth factors to induce a desired cell activity. Signal molecules are often incorporated into the biomaterial in either freely-diffusible or covalently-bound forms. However, biomolecular environments in vivo are often complex and dynamic. Notably, glycosaminoglycans (GAGs), linear polysaccharides found in the extracellular matrix, are involved in transient sequestration of growth factors via charge interactions. Biomaterials mimicking this phenomenon may offer the potential to amplify local biomolecular signals, both endogenously produced and exogenously added. GAGs of increasing sulfation (hyaluronic acid, chondroitin sulfate, heparin) were incorporated into a collagen-GAG (CG) scaffold under development for tendon tissue engineering. Manipulating the degree of GAG sulfation significantly impacts sequestration of growth factors from the media. Increasing GAG sulfation improved equine tenocyte metabolic activity in normal serum (10% FBS), low serum (1% FBS), and IGF-1 supplemented media conditions. Notably, previously reported dose-dependent changes in tenocyte bioactivity to soluble IGF-1 within the CG scaffold were replicated by using a single dose of soluble IGF-1 in scaffolds containing increasingly sulfated GAGs. Collectively, these results suggest that CG scaffold GAG content can be systematically manipulated to regulate the sequestration and resultant enhanced bioactivity of growth factor signals on cell behavior within the matrix.

The roles of knitted mesh-reinforced collagen-chitosan hybrid scaffold in the one-step repair of full-thickness skin defects in rats

Full-thickness skin defects represent a significant and urgent clinical problem. Dermal substitutes serving as a regenerative template to induce dermal reconstruction provide a promising method to treat serious skin defects. Although collagen-chitosan dermal scaffolds display good biocompatibility and a suitable porous structure for angiogenesis and tissue regeneration, their poor mechanical properties compromise their application. To develop a well-supported dermal substitute, a poly(l-lactide-co-glycolide) (PLGA) knitted mesh was fabricated and integrated with collagen-chitosan scaffold (CCS) to obtain a PLGA knitted mesh-reinforced CCS (PLGAm/CCS). The morphology of this PLGAm/CCS was investigated in vitro. To characterize the tissue response, specifically angiogenesis and tissue regeneration, the PLGAm/CCS was transplanted in combination with thin split-thickness autografts to repair full-thickness skin wounds using a one-step surgical procedure in Sprague-Dawley rats. These results were then compared with CCSs. At weeks 2, 4 and 8 after the operation, the healing wounds were imaged to analyse wound changes, and tissue specimens were harvested for histology, immunohistochemistry, real-time quantitative polymerase chain reaction and Western blot analysis. The results demonstrated that collagen-chitosan sponge in the PLGAm/CCS remained porous, interconnected and occupied the openings of PLGA mesh, and the incorporation of the PLGA knitted mesh into CCS improved the mechanical strength with little influence on its mean pore size and porosity. Following transplantation, PLGAm/CCS inhibited wound contraction, and effectively promoted neotissue formation and blood vessel ingrowth. In conclusion, the mechanical strength of the scaffolds plays an important role in the process of tissue regeneration and vascularization. The ability of PLGAm/CCS to promote angiogenesis and induce in situ tissue regeneration demonstrates its potential in skin tissue engineering.

The impact of discrete compartments of a multi-compartment collagen-GAG scaffold on overall construct biophysical properties

Orthopedic interfaces such as the tendon-bone junction (TBJ) present unique challenges for biomaterials development. Here we describe a multi-compartment collagen-GAG scaffold fabricated via lyophilization that contains discrete mineralized (CGCaP) and non-mineralized (CG) regions joined by a continuous interface. Modifying CGCaP preparation approaches, we demonstrated scaffold variants of increasing mineral content (40 vs. 80wt% CaP). We report the impact of fabrication parameters on microstructure, composition, elastic modulus, and permeability of the entire multi-compartment scaffold as well as discrete mineralized and non-mineralized compartments. Notably, individual mineralized and non-mineralized compartments differentially impacted the global properties of the multi-compartment composite. Of particular interest for the development of mechanically-loaded multi-compartment composites, the elastic modulus and permeability of the entire construct were governed primarily by the non-mineralized and mineralized compartments, respectively. Based on these results we hypothesize spatial variations in scaffold structural, compositional, and mechanical properties may be an important design parameter in orthopedic interface repair.

Influence of nucleus deformability on cell entry into cylindrical structures

The mechanical properties of cell nuclei have been demonstrated to play a fundamental role in cell movement across extracellular networks and micro-channels. In this work, we focus on a mathematical description of a cell entering a cylindrical channel composed of extracellular matrix. An energetic approach is derived in order to obtain a necessary condition for which cells enter cylindrical structures. The nucleus of the cell is treated either (i) as an elastic membrane surrounding a liquid droplet or (ii) as an incompressible elastic material with Neo-Hookean constitutive equation. The results obtained highlight the importance of the interplay between mechanical deformability of the nucleus and the capability of the cell to establish adhesive bonds and generate active forces in the cytoskeleton due to myosin action.

Hyaluronic acid click hydrogels emulate the extracellular matrix

Hydrogels are used to create 3D microenvironments with properties that direct cell function. The current study demonstrates the versatility of hyaluronic acid (HA)-based hydrogels with independent control over hydrogel properties such as mechanics, architecture, and the spatial distribution of biological factors. Hydrogels were prepared by reacting furan-modified HA with bis-maleimide-poly(ethylene glycol) in a Diels-Alder click reaction. Biomolecules were photopatterned into the hydrogel by two-photon laser processing, resulting in spatially defined growth factor gradients. The Young's modulus was controlled by either changing the hydrogel concentration or the furan substitution on the HA backbone, thereby decoupling the hydrogel concentration from mechanical properties. Porosity was controlled by cryogelation, and the pore size distribution, by the thaw temperature. The addition of galactose further influenced the porosity, pore size, and Young's modulus of the cryogels. These HA-based hydrogels offer a tunable platform with a diversity of properties for directing cell function, with applications in tissue engineering and regenerative medicine.

Tissue engineering and regenerative medicine as applied to the gastrointestinal tract

The gastrointestinal (GI) tract is a complex system characterized by multiple cell types with a determined architectural arrangement. Tissue engineering of the GI tract aims to reinstate the architecture and function of all structural layers. The key point for successful tissue regeneration includes the use of cells/biomaterials that elucidate minimal immune response after implantation. Different biomaterial choices and cell sources have been proposed to engineer the GI tract. This review summarizes the recent advances in bioengineering the GI tract with emphasis on cell sources and scaffolding biomaterials.

A Cellular Potts Model simulating cell migration on and in matrix environments

Cell migration on and through extracellular matrix is fundamental in a wide variety of physiological and pathological phenomena, and is exploited in scaffold-based tissue engineering. Migration is regulated by a number of extracellular matrix- or cell-derived biophysical parameters, such as matrix fiber orientation, pore size, and elasticity, or cell deformation, proteolysis, and adhesion. We here present an extended Cellular Potts Model (CPM) able to qualitatively and quantitatively describe cell migration efficiencies and phenotypes both on two-dimensional substrates and within three-dimensional matrices, close to experimental evidence. As distinct features of our approach, cells are modeled as compartmentalized discrete objects, differentiated into nucleus and cytosolic region, while the extracellular matrix is composed of a fibrous mesh and a homogeneous fluid. Our model provides a strong correlation of the directionality of migration with the topological extracellular matrix distribution and a biphasic dependence of migration on the matrix structure, density, adhesion, and stiffness, and, moreover, simulates that cell locomotion in highly constrained fibrillar obstacles requires the deformation of the cell's nucleus and/or the activity of cell-derived proteolysis. In conclusion, we here propose a mathematical modeling approach that serves to characterize cell migration as a biological phenomenon in healthy and diseased tissues and in engineering applications.

Microenvironment and tumor cell plasticity: an easy way out

Cancer cells undergo genetic changes allowing their adaptation to environmental changes, thereby obtaining an advantage during the long metastatic route, disseminated of several changes in the surrounding environment. In particular, plasticity in cell motility, mainly due to epigenetic regulation of cancer cells by environmental insults, engage adaptive strategies aimed essentially to survive in hostile milieu, thereby escaping adverse sites. This review is focused on tumor microenvironment as a collection of structural and cellular elements promoting plasticity and adaptive programs. We analyze the role of extracellular matrix stiffness, hypoxia, nutrient deprivation, acidity, as well as different cell populations of tumor microenvironment.

Composite growth factor supplementation strategies to enhance tenocyte bioactivity in aligned collagen-GAG scaffolds

Biomolecular environments encountered in vivo are complex and dynamic, with combinations of biomolecules presented in both freely diffusible (liquid-phase) and sequestered (bound to the extracellular matrix) states. Strategies for integrating multiple biomolecular signals into a biomimetic scaffold provide a platform to simultaneously control multiple cell activities, such as motility, proliferation, phenotype, and regenerative potential. Here we describe an investigation elucidating the influence of the dose and mode of presentation (soluble, sequestered) of five biomolecules (stromal cell-derived factor 1α [SDF-1α], platelet-derived growth factor BB [PDGF-BB], insulin-like growth factor 1 [IGF-1], basic fibroblast growth factor [bFGF], and growth/differentiation factor 5 [GDF-5]) on the recruitment, proliferation, collagen synthesis, and genomic stability of equine tenocytes within an anisotropic collagen-GAG scaffold for tendon regeneration applications. Critically, we found that single factors led to a dose-dependent trade-off between driving tenocyte proliferation (PDGF-BB, IGF-1) versus maintenance of a tenocyte phenotype (GDF-5, bFGF). We identified supplementation schemes using factor pairs (IGF-1, GDF-5) to rescue the tenocyte phenotype and gene expression profiles while simultaneously driving proliferation. These results suggest coincident application of multi-biomolecule cocktails has a significant value in regenerative medicine applications where control of cell proliferation and phenotype are required. Finally, we demonstrated an immobilization strategy that allows efficient sequestration of bioactive levels of these factors within the scaffold network. We showed sequestration can lead to a greater sustained bioactivity than soluble supplementation, making this approach particularly amenable to in vivo translation where diffusive loss is a concern and continuous biomolecule supplementation is not feasible.

Neovascularization in biodegradable inverse opal scaffolds with uniform and precisely controlled pore sizes

The formation of a stable vascular network in a scaffold is one of the most challenging tasks in tissue engineering and regenerative medicine. Despite the common use of porous scaffolds in these applications, little is known about the effect of pore size on the neovascularization in these scaffolds. Herein is fabricated poly(D, L-lactide-co-glycolide) inverse opal scaffolds with uniform pore sizes of 79, 147, 224, and 312 μm in diameter and which are then used to systematically study neovascularization in vivo. Histology analyses reveal that scaffolds with small pores (<200 μm) favor the formation of vascular networks with small vessels at high densities and poor penetration depth. By contrast, scaffolds with large pores (>200 μm) favor the formation of vascular networks with large blood vessels at low densities and deep penetration depth. Based on the different patterns of vessel ingrowth as regulated by the pore size, a model is proposed to describe vascularization in a 3D porous scaffold, which can potentially serve as a guideline for future design of porous scaffolds.

Modeling the influence of nucleus elasticity on cell invasion in fiber networks and microchannels

Cell migration in highly constrained extracellular matrices is exploited in scaffold-based tissue engineering and is fundamental in a wide variety of physiological and pathological phenomena, among others in cancer invasion and development. Research into the critical processes involved in cell migration has mainly focused on cell adhesion and proteolytic degradation of the external environment. However, rising evidence has recently shown that a number of cell-derived biophysical and mechanical parameters, among others nucleus stiffness and cell deformability, plays a major role in cell motility, especially in the ameboid-like migration mode in 3D confined tissue structures. We here present an extended cellular Potts model (CPM) first used to simulate a micro-fabricated migration chip, which tests the active invasive behavior of cancer cells into narrow channels. As distinct features of our approach, cells are modeled as compartmentalized discrete objects, differentiated in the nucleus and in the cytosolic region, while the migration chamber is composed of channels of different widths. We find that cell motile phenotype and velocity in open spaces (i.e., 2D flat surfaces or large channels) are not significantly influenced by cell elastic properties. On the contrary, the migratory behavior of cells within subcellular and subnuclear structures strongly relies on the deformability of the cytosol and of the nuclear cluster, respectively. Further, we characterize two migration dynamics: a stepwise way, characterized by fluctuations in cell length, within channels smaller than nucleus dimensions and a smooth sliding (i.e., maintaining constant cell length) behavior within channels larger than the nuclear cluster. These resulting observations are then extended looking at cell migration in an artificial fiber network, which mimics cell invasion in a 3D extracellular matrix. In particular, in this case, we analyze the effect of variations in elasticity of the nucleus on cell movement. In order to summarize, with our simulated migration assays, we demonstrate that the dimensionality of the environment strongly affects the migration phenotype and we suggest that the cytoskeletal and nuclear elastic characteristics correlate with the tumor cell's invasive potential.

Chondrogenesis of Adipose Stem Cells in a Porous Polymer Scaffold: Influence of the Pore Size

This study examined how the difference in pore size of porous scaffolds affected the in vitro chondrogenic differentiation of seeded adipose stem cells (ASCs) and the in vivo cartilage repair of ASC/scaffold construct. ASCs were isolated from 18 rabbits and seeded in a porous poly (ε-caprolactone) (PCL) scaffold with different pore sizes (100, 200, 400 μm). The ASCs underwent in vitro chondrogenic induction under TGF-β2 and BMP-7 for 21 days before analysis. The ASC/scaffold construct was also implanted on the osteochondral defect created on the distal femur of the same rabbits, and the quality of cartilage regeneration was analyzed after 8 weeks. At day 21, the ASCs proliferated and spread on the surface of the scaffolds with a pore size 100 and 200 μm, whereas there were many lumps of conglomerated ASCs on those with a pore size of 400 μm. The DNA content was significantly lower in the scaffold with a pore size of 400 μm than in that with a pore size of 100 or 200 μm. Proteoglycan production was significantly greater in the scaffold with a pore size of 400 and 200 μm than in that with a pore size of 100 μm. The chondrogenic marker gene expression including SOX9 and COL2A1 was greatest in the scaffold with a pore size of 400 μm followed by 200 μm. Immunofluorescent imaging showed that, while SOX9 was localized to nucleus, type II collagen was observed on the cytoplasm and secreted matrix around the cells most abundantly in the scaffold with a pore size of 400 μm followed by 200 μm. The gross and histological findings from the osteochondral defects showed that the cartilage repair was better in the scaffold with a pore size of 400 and 200 μm than in that with a pore size of 100 μm.

3D polylactide-based scaffolds for studying human hepatocarcinoma processes in vitro

We evaluated the combination of leaching techniques and melt blending of polymers and particles for the preparation of highly interconnected three-dimensional polymeric porous scaffolds for in vitro studies of human hepatocarcinoma processes. More specifically, sodium chloride and poly(ethylene glycol) (PEG) were used as water-soluble porogens to form porous and solvent-free poly(L,D-lactide) (PLA)-based scaffolds. Several characterization techniques, including porosimetry, image analysis and thermogravimetry, were combined to improve the reliability of measurements and mapping of the size, distribution and microarchitecture of pores. We also investigated the effect of processing, in PLA-based blends, on the simultaneous bulk/surface modifications and pore architectures in the scaffolds, and assessed the effects on human hepatocarcinoma viability and cell adhesion. The influence of PEG molecular weight on the scaffold morphology and cell viability and adhesion were also investigated. Morphological studies indicated that it was possible to obtain scaffolds with well-interconnected pores of assorted sizes. The analysis confirmed that SK-Hep1 cells adhered well to the polymeric support and emitted surface protrusions necessary to grow and differentiate three-dimensional systems. PEGs with higher molecular weight showed the best results in terms of cell adhesion and viability.

In vitro microvessels for the study of angiogenesis and thrombosis

Microvascular networks support metabolic activity and define microenvironmental conditions within tissues in health and pathology. Recapitulation of functional microvascular structures in vitro could provide a platform for the study of complex vascular phenomena, including angiogenesis and thrombosis. We have engineered living microvascular networks in three-dimensional tissue scaffolds and demonstrated their biofunctionality in vitro. We describe the lithographic technique used to form endothelialized microfluidic vessels within a native collagen matrix; we characterize the morphology, mass transfer processes, and long-term stability of the endothelium; we elucidate the angiogenic activities of the endothelia and differential interactions with perivascular cells seeded in the collagen bulk; and we demonstrate the nonthrombotic nature of the vascular endothelium and its transition to a prothrombotic state during an inflammatory response. The success of these microvascular networks in recapitulating these phenomena points to the broad potential of this platform for the study of cardiovascular biology and pathophysiology.

Controlling matrix stiffness and topography for the study of tumor cell migration

Cellular studies have long been performed on the bench top, within Petri dishes and flasks that expose cells to surroundings that differ greatly from their native environment. The complexity of a human tissue is such that to truly replicate a cell's physiologic microenvironment in vitro is currently impossible. It is nevertheless important to determine how various factors of the microenvironment interact to drive cell behavior, particularly with regard to disease states, such as cancer. Here we focus on two key elements of the cellular microenvironment, matrix stiffness and architecture, in the context of tumor cell behavior. We discuss recent work focusing on the effects of these individual properties on cancer cell migration and describe one technique developed by our lab that could be applied to dissect the effects of specific structural and mechanical cues, and which may lead to useful insights into the potentially synergistic effects of these properties on tumor cell behavior.

Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography

The success of tissue engineering will rely on the ability to generate complex, cell seeded three-dimensional (3D) structures. Therefore, methods that can be used to precisely engineer the architecture and topography of scaffolding materials will represent a critical aspect of functional tissue engineering. Previous approaches for 3D scaffold fabrication based on top-down and process driven methods are often not adequate to produce complex structures due to the lack of control on scaffold architecture, porosity, and cellular interactions. The proposed projection stereolithography (PSL) platform can be used to design intricate 3D tissue scaffolds that can be engineered to mimic the microarchitecture of tissues, based on computer aided design (CAD). The PSL system was developed, programmed and optimized to fabricate 3D scaffolds using gelatin methacrylate (GelMA). Variation of the structure and prepolymer concentration enabled tailoring the mechanical properties of the scaffolds. A dynamic cell seeding method was utilized to improve the coverage of the scaffold throughout its thickness. The results demonstrated that the interconnectivity of pores allowed for uniform human umbilical vein endothelial cells (HUVECs) distribution and proliferation in the scaffolds, leading to high cell density and confluency at the end of the culture period. Moreover, immunohistochemistry results showed that cells seeded on the scaffold maintained their endothelial phenotype, demonstrating the biological functionality of the microfabricated GelMA scaffolds.

The biomechanical properties of 3d extracellular matrices and embedded cells regulate the invasiveness of cancer cells

The malignancy of tumors depends on the biomechanical properties of cancer cells and their microenvironment, which enable cancer cells to migrate through the connective tissue, transmigrate through basement membranes and endothelial monolayers and form metastases in targeted organs. The current focus of cancer research is still based on biological capabilities such as molecular genetics and gene signaling, but these approaches ignore the mechanical nature of the invasion process of cancer cells. This review will focus on how structural, biochemical and mechanical properties of extracellular matrices (ECMs), and adjacent cells regulate the invasiveness of cancer cells. In addition, it presents how cancer cells create their own microenvironment by restructuring of the ECM and by interaction with stromal cells, which then further contribute to the progression of cancer disease. Finally, this review will point out that mechanical properties are a critical determinant for the efficiency of cancer cell invasion and the progression of cancer which might affect the future development of new cancer treatments.

Influence of flow rate and scaffold pore size on cell behavior during mechanical stimulation in a flow perfusion bioreactor

Mechanically stimulating cell-seeded scaffolds by flow-perfusion is one approach utilized for developing clinically applicable bone graft substitutes. A key challenge is determining the magnitude of stimuli to apply that enhances cell differentiation but minimizes cell detachment from the scaffold. In this study, we employed a combined computational modeling and experimental approach to examine how the scaffold mean pore size influences cell attachment morphology and subsequently impacts upon cell deformation and detachment when subjected to fluid-flow. Cell detachment from osteoblast-seeded collagen-GAG scaffolds was evaluated experimentally across a range of scaffold pore sizes subjected to different flow rates and exposure times in a perfusion bioreactor. Cell detachment was found to be proportional to flow rate and inversely proportional to pore size. Using this data, a theoretical model was derived that accurately predicted cell detachment as a function of mean shear stress, mean pore size, and time. Computational modeling of cell deformation in response to fluid flow showed the percentage of cells exceeding a critical threshold of deformation correlated with cell detachment experimentally and the majority of these cells were of a bridging morphology (cells stretched across pores). These findings will help researchers optimize the mean pore size of scaffolds and perfusion bioreactor operating conditions to manage cell detachment when mechanically simulating cells via flow perfusion.

Generation of mechanical and biofunctional gradients in PEG diacrylate hydrogels by perfusion-based frontal photopolymerization

The spatial presentation of soluble growth factors, immobilized extracellular matrix molecules, as well as matrix rigidity, plays an important role in directed and guided cell migration. Synthetic hydrogel scaffolds offer the ability to systematically introduce gradients of these factors contributing to our understanding of how the 3D arrangement of biochemical and mechanical cues influence cell behavior. Using a novel photopolymerization technique, perfusion-based frontal photopolymerization (PBFP), we have engineered poly(ethylene glycol) diacrylate (PEGDA) hydrogel scaffolds with gradients of mechanical properties and immobilized biofunctionality. The controlled delivery of a buoyant photoinitiator, eosin Y, through a glass frit filter results in the formation and subsequent propagation of a polymer reaction front that is self-sustained and able to propagate through the monomeric mixture. Propagation of this front results in monomer depletion, leading to variations in cross-linking, as well as spatial gradients of elastic modulus and immobilized concentrations of the YRGDS cell adhesion ligand within PEGDA hydrogels. Furthermore, the magnitudes of the resulting gradients are controlled through alterations in polymerization conditions. Preliminary in vitro cell-culture studies demonstrate that the gradients generated stimulate directed 2D cell growth on the surface of PEGDA hydrogels. By day 14, fibroblast aggregates spread roughly twice as far in the direction parallel to the slope of the gradient as compared to the perpendicular direction. The presented technique has great potential in controlling gradients of mechanical properties and immobilized biofunctionality for directing and guiding 3D cell behavior within tissue-engineered scaffolds.

The influence of collagen-glycosaminoglycan scaffold relative density and microstructural anisotropy on tenocyte bioactivity and transcriptomic stability

Biomaterials for orthopedic tissue engineering must balance mechanical and bioactivity concerns. This work describes the fabrication of a homologous series of anisotropic collagen-GAG (CG) scaffolds with aligned tracks of ellipsoidal pores but increasing relative densities (ρ(∗)/ρ(s)), and we report the role scaffold relative density plays in directing tenocyte bioactivity. Scaffold permeability and mechanical properties, both in tension and compression, were significantly influenced by relative density in a manner predicted by cellular solids models. Equine tenocytes showed greater levels of attachment, metabolic activity, soluble collagen synthesis, and alignment as well as less cell-mediated scaffold contraction in anisotropic CG scaffolds of increasing relative density. Notably, the lowest density scaffolds experienced significant cell-mediated contraction with associated decreases in tenocyte number as well as loss of microstructural integrity, aligned contact guidance cues, and preferential tenocyte orientation over a 14 day culture period. Gene expression analyses suggested tenocyte de-differentiation in the lowest density scaffold while indicating that the highest density scaffold supported significant increases in COMP (4-fold), tenascin-C (3-fold), and scleraxis (15-fold) expression as well as significant decreases in MMP-1 (9-fold) and MMP-13 (13-fold) expression on day 14. These results suggest that anisotropic scaffold relative density can help to modulate the maintenance of a more tendon-like microenvironment and aid long-term tenocyte transcriptomic stability. Overall, this work demonstrates that relative density is a critical scaffold parameter, not only for insuring mechanical competence, but also for directing cell transcriptomic stability and behavior.