The interplay between tissue growth and scaffold degradation in engineered tissue constructs

Review on Porous Scaffolds Generation Process: A Tissue Engineering Approach

Porous scaffolds have been widely explored for tissue regeneration and engineering in vitro three-dimensional models. In this review, a comprehensive literature analysis is conducted to identify the steps involved in their generation. The advantages and disadvantages of the available techniques are discussed, highlighting the importance of considering pore geometrical parameters such as curvature and size, and summarizing the requirements to generate the porous scaffold according to the desired application. This paper considers the available design tools, mathematical models, materials, fabrication techniques, cell seeding methodologies, assessment methods, and the status of pore scaffolds in clinical applications. This review compiles the relevant research in the field in the past years. The trends, challenges, and future research directions are discussed in the search for the generation of a porous scaffold with improved mechanical and biological properties that can be reproducible, viable for long-term studies, and closer to being used in the clinical field.

Poly-l-Lactic Acid (PLLA)-Based Biomaterials for Regenerative Medicine: A Review on Processing and Applications

Synthetic biopolymers are effective cues to replace damaged tissue in the tissue engineering (TE) field, both for in vitro and in vivo application. Among them, poly-l-lactic acid (PLLA) has been highlighted as a biomaterial with tunable mechanical properties and biodegradability that allows for the fabrication of porous scaffolds with different micro/nanostructures via various approaches. In this review, we discuss the structure of PLLA, its main properties, and the most recent advances in overcoming its hydrophobic, synthetic nature, which limits biological signaling and protein absorption. With this aim, PLLA-based scaffolds can be exposed to surface modification or combined with other biomaterials, such as natural or synthetic polymers and bioceramics. Further, various fabrication technologies, such as phase separation, electrospinning, and 3D printing, of PLLA-based scaffolds are scrutinized along with the in vitro and in vivo applications employed in various tissue repair strategies. Overall, this review focuses on the properties and applications of PLLA in the TE field, finally affording an insight into future directions and challenges to address an effective improvement of scaffold properties.

Digital Twins for Tissue Culture Techniques—Concepts, Expectations, and State of the Art

Techniques to provide in vitro tissue culture have undergone significant changes during the last decades, and current applications involve interactions of cells and organoids, three-dimensional cell co-cultures, and organ/body-on-chip tools. Efficient computer-aided and mathematical model-based methods are required for efficient and knowledge-driven characterization, optimization, and routine manufacturing of tissue culture systems. As an alternative to purely experimental-driven research, the usage of comprehensive mathematical models as a virtual in silico representation of the tissue culture, namely a digital twin, can be advantageous. Digital twins include the mechanistic of the biological system in the form of diverse mathematical models, which describe the interaction between tissue culture techniques and cell growth, metabolism, and the quality of the tissue. In this review, current concepts, expectations, and the state of the art of digital twins for tissue culture concepts will be highlighted. In general, DT’s can be applied along the full process chain and along the product life cycle. Due to the complexity, the focus of this review will be especially on the design, characterization, and operation of the tissue culture techniques.

A multiphase model of growth factor-regulated atherosclerotic cap formation

Atherosclerosis is characterised by the growth of fatty plaques in the inner artery wall. In mature plaques, vascular smooth muscle cells (SMCs) are recruited from adjacent tissue to deposit a collagenous cap over the fatty plaque core. This cap isolates the thrombogenic plaque content from the bloodstream and prevents the clotting cascade that leads to myocardial infarction or stroke. Despite the protective role of the cap, the mechanisms that regulate cap formation and maintenance are not well understood. It remains unclear why some caps become stable, while others become vulnerable to rupture. We develop a multiphase PDE model with non-standard boundary conditions to investigate collagen cap formation by SMCs in response to diffusible growth factor signals from the endothelium. Platelet-derived growth factor stimulates SMC migration, proliferation and collagen degradation, while transforming growth factor (TGF)-[Formula: see text] stimulates SMC collagen synthesis and inhibits collagen degradation. The model SMCs respond haptotactically to gradients in the collagen phase and have reduced rates of migration and proliferation in dense collagenous tissue. The model, which is parameterised using in vivo and in vitro experimental data, reproduces several observations from plaque growth in mice. Numerical and analytical results demonstrate that a stable cap can be formed by a relatively small SMC population and emphasise the critical role of TGF-[Formula: see text] in effective cap formation. These findings provide unique insight into the mechanisms that may lead to plaque destabilisation and rupture. This work represents an important step towards the development of a comprehensive in silico plaque model.

A Hybrid Model of Cartilage Regeneration Capturing the Interactions Between Cellular Dynamics and Porosity

To accelerate the development of strategies for cartilage tissue engineering, models are necessary to investigate the interactions between cellular dynamics and the local microenvironment. We use a discrete framework to capture the individual behavior of cells, modeling experiments where cells are seeded in a porous scaffold or hydrogel and over the time course of a month, the scaffold slowly degrades while cells divide and synthesize extracellular matrix constituents. The movement of cells and the ability to proliferate is a function of the local porosity, defined as the volume fraction of fluid in the surrounding region. A phenomenological approach is used to capture a continuous profile for the degrading scaffold and accumulating matrix, which will then change the local porosity throughout the construct. We parameterize the model by first matching total cell counts in the construct to chondrocytes seeded in a polyglycolic acid scaffold (Freed et al. in Biotechnol Bioeng 43:597-604, 1994). We investigate the influence of initial scaffold porosity on the total cell count and spatial profiles of cell and ECM in the construct. Cell counts were higher at day 30 in scaffolds of lower initial porosity, and similar cell counts were obtained using different models of scaffold degradation and matrix accumulation (either uniform or cell-specific). Using this modeling framework, we study the interplay between a phenomenological representation of scaffold architecture and porosity as well as the potential continuous application of growth factors. We determine parameter regimes where large cellular aggregates occur, which can hinder matrix accumulation and cellular proliferation. The developed modeling framework can easily be extended and can be used to identify optimal scaffolds and culture conditions that lead to a desired distribution of extracellular matrix and cell counts throughout the construct.

Hybrid manufacturing strategies for tissue engineering scaffolds using methacrylate functionalised poly(glycerol sebacate)

Poly(glycerol sebacate) is an attractive biomaterial for tissue engineering due to its biocompatibility, elasticity and rapid degradation rate. However, poly(glycerol sebacate) requires harsh processing conditions, involving high temperatures and vacuum for extended periods, to produce an insoluble polymer matrix. These conditions make generating accurate and intricate geometries from poly(glycerol sebacate), such as those required for tissue engineering scaffolds, difficult. Functionalising poly(glycerol sebacate) with methacrylate groups produces a photocurable polymer, poly(glycerol sebacate)-methacrylate, which can be rapidly crosslinked into an insoluble matrix. Capitalising on these improved processing capabilities, here, we present a variety of approaches for fabricating porous tissue engineering scaffolds from poly(glycerol sebacate)-methacrylate using sucrose porogen leaching combined with other manufacturing methods. Mould-based techniques were used to produce porous disk-shaped and tubular scaffolds. Porogen size was shown to influence scaffold porosity and mechanical performance, and the porous poly(glycerol sebacate)-methacrylate scaffolds supported the proliferation of primary fibroblasts in vitro. Additionally, scaffolds with spatially variable mechanical properties were generated by combining variants of poly(glycerol sebacate)-methacrylate with different stiffness. Finally, subtractive and additive manufacturing methods were developed with the capabilities to generate porous poly(glycerol sebacate)-methacrylate scaffolds from digital designs. These hybrid manufacturing strategies offer the ability to produce accurate macroscale poly(glycerol sebacate)-methacrylate scaffolds with tailored microscale porosity and spatially resolved mechanical properties suitable for a broad range of applications across tissue engineering.

Multi-dimensional models for functional testing of keloid scars: In silico, in vitro, organoid, organotypic, ex vivo organ culture, and in vivo models

Keloid scars are described as benign fibro-proliferative dermal outgrowths that commonly occur in pigmented skin post cutaneous injury, and continue to grow beyond the boundary of the original wound margin. There is a lack of thorough understanding of keloid pathogenesis and thus keloid therapeutic options remain ill-defined. In view of the poor response to current therapy and high recurrence rates, there is an unmet need in improving our knowledge and therefore in identifying targeted and effective treatment strategies in management of keloids. Keloid research however, is hampered by a lack of relevant animal models as keloids do not spontaneously occur in animals and are unique to human skin. Therefore, developing novel animal models and nonanimal models for functional evaluation of keloid cells and tissue for better understanding their pathobiology and response to putative candidate therapies are essential. Here, we present the key concepts and relevant emerging research on two-dimensional and three-dimensional cell and tissue models for functional testing of keloid scars. We will describe in detail current models including in vitro mono- and co-cultures, multi-cellular spheroids (organoids) and organotyopic cultures, ex vivo whole skin keloid tissue organ culture models as well as in vivo human patient models. Finally, we discuss the role played by time as the fourth dimension in a novel model that involves sequential temporal biopsies of human patients with keloids (a so called 4D in vivo human model). The use of these unique models will no doubt prove pivotal in identification of new drug targets as well as biomarkers, in functional testing of emerging novel therapeutics, and in enhancing our understanding of keloid disease biology.

Pattern formation in multiphase models of chemotactic cell aggregation

We develop a continuum model for the aggregation of cells cultured in a nutrient-rich medium in a culture well. We consider a 2D geometry, representing a vertical slice through the culture well, and assume that the cell layer depth is small compared with the typical lengthscale of the culture well. We adopt a continuum mechanics approach, treating the cells and culture medium as a two-phase mixture. Specifically, the cells and culture medium are treated as fluids. Additionally, the cell phase can generate forces in response to environmental cues, which include the concentration of a chemoattractant that is produced by the cells within the culture medium. The model leads to a system of coupled nonlinear partial differential equations for the volume fraction and velocity of the cell phase, the culture medium pressure and the chemoattractant concentration, which must be solved subject to appropriate boundary and initial conditions. To gain insight into the system, we consider two model reductions, appropriate when the cell layer depth is thin compared to the typical length scale of the culture well: a (simple) 1D and a (more involved) thin-film extensional flow reduction. By investigating the resulting systems of equations analytically and numerically, we identify conditions under which small amplitude perturbations to a homogeneous steady state (corresponding to a spatially uniform cell distribution) can lead to a spatially varying steady state (pattern formation). Our analysis reveals that the simpler 1D reduction has the same qualitative features as the thin-film extensional flow reduction in the linear and weakly nonlinear regimes, motivating the use of the simpler 1D modelling approach when a qualitative understanding of the system is required. However, the thin-film extensional flow reduction may be more appropriate when detailed quantitative agreement between modelling predictions and experimental data is desired. Furthermore, full numerical simulations of the two model reductions in regions of parameter space when the system is not close to marginal stability reveal significant differences in the evolution of the volume fraction and velocity of the cell phase, and chemoattractant concentration.

Blocking of matrix metalloproteinases-13 responsive peptide in poly(urethane urea) for potential cartilage tissue engineering applications

The matching of scaffold degradation rate with neotissue growth is required for tissue engineering applications. Timely provision of proper spaces especially for cartilage tissue engineering plays a pivotal role in chondrocyte cluster formation. In this study, poly(urethane urea) was synthesized using conventional two-stage method by extending the isocyanate group terminated prepolymers with different amounts of GPLGLWARK peptide, which responses the degrading induced by matrix metalloproteinase 13, the main proteinase for cartilage matrix degradation. The Fourier transform infrared spectrometer with the attenuated total reflection and 1H nuclear magnetic resonance spectra revealed that the peptides were introduced to poly(urethane urea) according to the characteristic absorption bands of the peptide and the newly formed urea bonds. The ultraviolet-visible spectroscopy spectra showed that the weight percentages of the peptide in the three poly(urethane urea) were 25%, 32%, and 35%. Atomic force microscopy images revealed that phase separation occurred in all poly(urethane urea) samples and became increasingly apparent with increasing amount of peptides introduced. Mechanical tests showed that the poly(urethane urea) strength increased with increasing amount of peptides in poly(urethane urea). Poly(urethane urea) proteolysis in matrix metalloproteinase 13 solution was more rapid than hydrolysis in aqueous buffer, and proteolysis rate was dependent on the amount of peptides in poly(urethane urea). Cell proliferation on the material surface in vitro displayed nontoxicity for all synthesized poly(urethane urea). In vivo subcutaneous implantation evaluation revealed the presence of local foreign body reactions triggered by poly(urethane urea) but was not due to peptide in poly(urethane urea). Moreover, the synthesized poly(urethane urea) with significant phase separation did not degrade under the matrix metalloproteinase 13 free subcutaneous environment, but poly(urethane urea) with minimal phase separation was degraded by attacking of the enzymes adsorbed on the hydrophobic surface through non-specific adsorption.

Adult Stem Cell Therapies for Wound Healing: Biomaterials and Computational Models

The increased incidence of diabetes and tumors, associated with global demographic issues (aging and life styles), has pointed out the importance to develop new strategies for the effective management of skin wounds. Individuals affected by these diseases are in fact highly exposed to the risk of delayed healing of the injured tissue that typically leads to a pathological inflammatory state and consequently to chronic wounds. Therapies based on stem cells (SCs) have been proposed for the treatment of these wounds, thanks to the ability of SCs to self-renew and specifically differentiate in response to the target bimolecular environment. Here, we discuss how advanced biomedical devices can be developed by combining SCs with properly engineered biomaterials and computational models. Examples include composite skin substitutes and bioactive dressings with controlled porosity and surface topography for controlling the infiltration and differentiation of the cells. In this scenario, mathematical frameworks for the simulation of cell population growth can provide support for the design of bioconstructs, reducing the need of expensive, time-consuming, and ethically controversial animal experimentation.

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.

Multiphase modelling of the effect of fluid shear stress on cell yield and distribution in a hollow fibre membrane bioreactor

We present a simplified two-dimensional model of fluid flow, nutrient transport and cell distribution in a hollow fibre membrane bioreactor, with the aim of exploring how fluid flow can be used to control the distribution and yield of a cell population which is sensitive to both fluid shear stress and nutrient concentration. The cells are seeded in a scaffold in a layer on top of the hollow fibre, only partially occupying the extracapillary space. Above this layer is a region of free-flowing fluid which we refer to as the upper fluid layer. The flow in the lumen and upper fluid layer is described by the Stokes equations, whilst the flow in the porous fibre membrane is assumed to follow Darcy’s law. Porous mixture theory is used to model the dynamics of and interactions between the cells, scaffold and fluid in the cell–scaffold construct. The concentration of a limiting nutrient (e.g. oxygen) is governed by an advection–reaction–diffusion equation in each region. Through exploitation of the small aspect ratio of each region and asymptotic analysis, we derive a coupled system of partial differential equations for the cell volume fraction and nutrient concentration. We use this model to investigate the effect of mechanotransduction on the distribution and yield of the cell population, by considering cases in which cell proliferation is either enhanced or limited by fluid shear stress and by varying experimentally controllable parameters such as flow rate and cell–scaffold construct thickness.

Control of scaffold degradation in tissue engineering: a review

Tissue engineering has shown a great promise as a solution to the high demand for tissue and organ transplantations. Biomaterial scaffolds serve to house and direct cells to grow, exposing them to an adequate perfusion of nutrients, oxygen, metabolic products, and appropriate growth factors to enhance their differentiation and function. The degradation of biomaterial scaffolds is a key factor to successful tissue regeneration. In this article, the existing degradation control approaches in the context of scaffold tissue engineering were reviewed and a new paradigm of thinking called active control of scaffold degradation, proposed elsewhere by us, was also revisited and discussed in light of its benefit and requirement of this new technology.

Geometry-driven cell organization determines tissue growths in scaffold pores: consequences for fibronectin organization

To heal tissue defects, cells have to bridge gaps and generate new extracellular matrix (ECM). Macroporous scaffolds are frequently used to support the process of defect filling and thus foster tissue regeneration. Such biomaterials contain micro-voids (pores) that the cells fill with their own ECM over time. There is only limited knowledge on how pore geometry influences cell organization and matrix production, even though it is highly relevant for scaffold design. This study hypothesized that 1) a simple geometric description predicts cellular organization during pore filling at the cell level and that 2) pore closure results in a reorganization of ECM. Scaffolds with a broad distribution of pore sizes (macroporous starPEG-heparin cryogel) were used as a model system and seeded with primary fibroblasts. The strategies of cells to fill pores could be explained by a simple geometrical model considering cells as tensioned chords. The model matched qualitatively as well as quantitatively by means of cell number vs. open cross-sectional area for all pore sizes. The correlation between ECM location and cell position was higher when the pores were not filled with tissue (Pearson's coefficient ρ = 0.45±0.01) and reduced once the pores were closed (ρ = 0.26±0.04) indicating a reorganization of the cell/ECM network. Scaffold pore size directed the time required for pore closure and furthermore impacted the organization of the fibronectin matrix. Understanding how cells fill micro-voids will help to design biomaterial scaffolds that support the endogenous healing process and thus allow a fast filling of tissue defects.