2.3 Classification of Smart Materials in Tissue Engineering
2.3.3 Biologic Materials
2.3.3.3 ECM Materials
the eCM is Mother nature’s version of a smart material. resident cells of the tissues secrete the eCM components, forming a network of more than 300 molecules, which includes collagens, glycoproteins, glycosaminogly- cans, proteoglycans, growth factors, and cryptic peptides, among others,98 as shown in Figure 2.3. this naturally occurring complex accounts for both the biomechanical (e.g. stiffness, elasticity, etc.) and biochemical (e.g. growth factor and cryptic signaling regulations) properties of the tissue.123,124 the eCM plays an essential role in cell homeostasis, tissue development, and maintenance of cell and tissue health.125 the main molecules that consti- tute the eCM are very well preserved among different species,1,126 therefore, allogeneic and xenogeneic eCM scaffolds have shown great utility as smart materials for tissue engineering applications.
the manufacture of eCM scaffolds is accomplished through decellular- ization of a source tissue, and subsequent disinfection and sterilization of the resultant matrix material.127 decellularization processes include the use of physical (e.g. freeze–thawing cycles, mechanical pressure), chemical (e.g. detergents, ionic solutions), and biochemical (e.g. enzymes) methods.
Published on 03 May 2017 on http://pubs.rsc.org | doi:10.1039/9781788010542-00039
Source tissues vary widely and include the urinary bladder, small intestinal submucosa, dermis, skeletal muscle, esophagus, liver, and heart, among others.128,129 all methods of tissue decellularization inevitably involve disruption of the matrix relative to its native structure/composition. Failure of adequate decellularization promotes an adverse, pro-inflammatory host response and poor functional outcomes.130,131 For this reason, a critical balance between decellularization and preservation of native structural and functional compounds of the eCM should be systematically evaluated for each tissue.132–134 although there is a lack of consensus regarding metrics to determine sufficient decellularization, criteria have been proposed to guide the threshold above which an eCM scaffold material is prone to promote a pro-inflammatory response.135
eCM materials are versatile and can be applied in different configurations (Figure 2.4). these scaffolds have been modified to facilitate their applica- tions in three-dimensional filling spaces. Solubilization of eCM materials into gels is becoming a common practice of the post-processing modifica- tions of these scaffolds. Gels produced from eCM can acquire any desired Figure 2.3 Structure of the extracellular matrix (eCM). the eCM is composed of different molecules forming a complex network that provides mechan- ical and biochemical properties to the tissues. Collagen forms long fibers from which the three-dimensional eCM structure is formed. Gly- cosaminoglycans store water, growth factors, and cytokines, regulating their availability to the cells. the active binding domains of glycopro- teins, such as fibronectin, provide binding sites to connect the eCM components to the cells.
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three-dimensional structure after physiological temperature-induced polym- erization.136 the self-assembly of the eCM components at physiological con- ditions provides advantages in the delivery methods for clinical applications, facilitating their use as injectable materials with minimally invasive tech- niques. eCM gels can be applied as a sole material or conjugated with other polymers to produce surface modifications on them (Figure 2.5).
eCM scaffolds that are properly prepared provide structural and microen- vironmental cues that can induce functional repair of the damaged tissue, a process described as “constructive remodeling” and which results in site- appropriate organized tissue deposited at the implanted site.71,137 the mecha- nisms by which functional remodeling mediated by eCM scaffolds occur are being increasingly understood, and at least three different mechanisms have been identified (Figure 2.6) and will be discussed below.
2.3.3.3.1 Modulation of the Immune Response. Macrophages play an essential role in tissue remodeling following implantation of an eCM scaf- fold.138 Circulating and tissue resident mononuclear cells are recruited to the injured site and differentiate into macrophages within the first 24 to 48 hours. these cells show a predominantly M1-like (pro-inflammatory, cyto- toxic) phenotype.139 the long-term presence of the M1-like macrophage phe- notype has been associated with chronic inflammation and FBr, whereas the presence of an M2-like (anti-inflammatory, immuno-regulatory) phenotype of macrophages has been associated with constructive tissue remodeling outcomes.140 implanted eCM scaffolds modulate the immune host response by promoting a transition of macrophage phenotype to an M2-like behavior within 3–7 days after implantation.141 the mechanisms responsible for Figure 2.4 Configurations of acellular biomaterials for diverse tissue engineering applications. (a) the immediate product of decellularization is a two-di- mensional sheet with defined layers of tissue. SeM image of a vascular eCM scaffold with cross-sections of the abluminal (upper) and lumi- nal sections (lower); scale bar 100 µm. (b) eCM scaffold materials can be further processed by milling the eCM scaffold to generate a powder material for space-filling applications. (c) eCM gels are formed by solu- bilization of the eCM scaffold; the polymerization process is controlled by temperature, facilitating their use as injectable materials.
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modulating macrophage phenotypes include factors released during the degradation of the scaffold material and the availability and presentation of cryptic peptides to the resident cells.142,143
2.3.3.3.2 Degradation of ECM Scaffold Materials. normal tissue exists in a dynamic state and includes processes of degradation and remodeling of the eCM as mechanisms for development, homeostasis, or response to injury.144 Such a dynamic environment may be replicated by the implanted eCM scaffold material through mechanisms involving cellular and enzy- matic pathways.137,145 the degradation of the implanted eCM scaffold facili- tates the cellular infiltration and the deposition of new site-appropriate tissue. Furthermore, and as previously discussed, the enzymatic degradation of the polymeric components of the eCM scaffold releases and/or exposes peptide motifs, aMps, growth factors, GaGs, and other bioactive molecules, Figure 2.5 Functionalization of synthetic polymer materials with eCM gels.
Coating of biomaterials with eCM gels is known to modulate the immune host response. (a) and (c) SeM images showing the topo- graphical structure of non-coated biomaterials used in vascular tissue engineering (eptFe) and hernial repair applications (polypropylene), respectively. (b) and (d) the same materials were coated with eCM gel, 8%, using a physical method of functionalization. the eCM gel is adsorbed onto the biomaterials modifying the topographical struc- ture. Scale bar 100 µm.
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Chapter 256
Figure 2.6 Mechanisms involved in constructive remodeling of tissues mediated by eCM scaffolds.
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all of which have the potential to regulate cell migration, adhesion, differen- tiation, and angiogenesis.146–148
2.3.3.3.3 Cell Infiltration on ECM Bioscaffolds. the role of the eCM scaf- folds and their degradation products is not limited to the immunomodula- tory effect upon macrophages at the site of injury. the release of matricryptic peptides from the degraded eCM scaffold also facilitates the recruitment of local and systemic stem/progenitor cells, which in turn participate in the scaffold remodeling process.149,150
a large number of pre-clinical and clinical studies have been conducted using eCM scaffold materials for esophageal, musculoskeletal, vascular, and thoracic repair applications, among others. Variability in decellularization and manufacturing methods, and surgical technique, has resulted in dispa- rate clinical outcomes. For example, Sicari et al. (2014) successfully showed the implantation of eCM scaffold materials in sites of volumetric muscle loss (VMl) in both pre-clinical and clinical studies with resultant constructive remodeling, characterized by cell mobilization and de novo skeletal muscle formation.151 in contrast, aurora et al. (2015) utilized the same eCM scaf- fold material but did not show the same functional outcome.152 the studies differ markedly in the associated protocol for post-surgical physical therapy.
Whereas the former group initiated the physical/mechanical load of the affected region within 48 hours after implantation of the eCM scaffold, the latter group delayed the initiation of rehabilitation and lacked rigorous con- trol in the rodent model. it is important to emphasize that immediate phys- ical therapy is required to promote cell infiltration and differentiation, align the deposited eCM fibers, and promote angiogenesis, among other factors, which finally have an effect on the final functional outcome.153,154
Many studies have shown the potential of using eCM scaffolds derived from non-homologous tissues in a set of clinical applications. remlinger et al.
(2013) investigated the role of urinary bladder matrix (UBM) and cardiac eCM scaffolds in the repair of a full thickness defect in the right ventricular outflow tract in rats for the treatment of congenital heart defects. the results showed a robust cell mobilization from the bone marrow, cell infiltration into the eCM scaffold accompanied by site-appropriate tissue remodeling and the presence of cardiomyocytes after 16 weeks of implantation when UBM scaffolds were employed, whereas less degrees of functional outcome were obtained when the cardiac eCM was employed.155 an additional example of utilization of heterol- ogous eCM scaffolds is the study performed by nieponice et al. (2014). in the pilot clinical study, a commercially available UBM scaffold was employed for the functional repair of damaged esophageal tissue. in all the evaluated cases, the implanted UBM scaffold was replaced by site-appropriate functional tissue with complete epithelialization.156 Whether or not heterologous tissues can be employed for specific applications is still a topic of discussion and requires further investigation. Factors such as anatomic localization,157 complexity of the tissue, and mechanical loads, among others, might affect the functionality of the repaired tissue. the large body of studies to date clearly shows the ability
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of eCM scaffolds to behave as smart materials, in which common molecules present within tissues regulate events such as cell infiltration, angiogenesis, and functional tissue repair.