2.3 Classification of Smart Materials in Tissue Engineering
2.3.1 Synthetic Materials
Synthetic materials include metals, alloys, ceramics, and polymers, among others, and have long been of interest in biomaterial sciences. Current manu facturing techniques, and the physico–chemical and mechanical properties of these materials, have been exhaustively described.14 Synthetic materials can be manufactured with extraordinary reproducibility and typically reasonable manufacturing costs.15 the in-depth understanding and extensive experience with synthetic materials, as described in subsequent chapters of this text, have provided a solid basis for the creation of next generation smart materials.
Bone tissue engineering has provided a fertile opportunity for smart mate- rials. new metallic biodegradable materials based on magnesium (Mg) and its alloys represent one such example of smart materials for orthopedic appli- cations.16 Current manufacturing techniques can be applied to generate a macroporous structure, where strength and stiffness of Mg-based materials are comparable with bone. in addition, Mg has the added advantage that it is a natural component of mammalian tissues. Mg-based scaffolds are con- sidered smart materials as they play an important role in bone tissue forma- tion17 and their degradation can facilitate the healing process, eliminating the need for implant removal.18
the degradation rates of Mg-based materials are an active topic of investiga- tion. In vitro studies imitating physiological conditions to the greatest extent possible have shown that Mg-based materials corrode very quickly, leading to a rapid change/loss of the mechanical properties.17 Several Mg-based alloys have been created to decrease the degradation rate.19 the corrosion of mag- nesium results in the formation of hydrogen gas (h2) that, once saturated in the tissue, creates gas cavities and increased mortality in preclinical rodent models.20 the non-lethal formation of hydrogen gas cavities has been shown in studies by Chaya et al. (2015)21 and by rössig et al. (2015)22 using pure and Mg-based alloy lae442, respectively. the results of these studies suggest that such materials still have limitations for effective clinical translation as a smart scaffold material.
Published on 03 May 2017 on http://pubs.rsc.org | doi:10.1039/9781788010542-00039
properties such as stiffness and degradation rate of magnesium can be modified by combination with other materials.4 the generation of surface modifications to the Mg-based materials is being investigated to improve the characteristics and the host response to the implanted scaffold. Mg-based scaffolds coated with polycaprolactone (pCl) and gelatin polymers reduce the degradation rate and control the formation of hydrogen gas. the slower degradation rate of pCl can mitigate the rapid degradation of the Mg-based scaffold when used in combination.23 Similarly, modifications with octacal- cium phosphate (oCp) and hydroxyapatite (hap) favorably modulate the degradation of Mg-based alloys and facilitate hydroxyapatite deposition.24 these promising results require further evaluation to determine translatability to clinical approaches.
as with metallic materials, ceramics have been applied to bone tissue engi- neering applications. the concept of smart ceramic materials, however, has evolved slowly. From the choices of ceramics used in orthopedic restorations, only hap and calcium metaphosphate (CMp) have been described as smart materials based upon their osteoinductive properties.25,26 the brittle nature of the hap limits its use as a material solely to repair hard tissue. alternative approaches have focused upon the generation of composites with polymeric materials to increase the strength of the scaffolds.27 lee et al. (2013) showed that hap coated with type i collagen resulted in a composite material with increased mechanical and structural properties that successfully promoted cell proliferation and osteogenesis when evaluated in a pre-clinical model.26 other techniques include the addition of growth factors, mainly bone mor- phogenetic proteins (BMps), with an active role in osteogenesis. the use of human recombinant BMp-2 has facilitated the clinical translation of com- posite scaffold materials for bone tissue engineering applications. although BMp-2 effectively induces osteogenesis, other tissues, including soft tissues, can be adversely affected by heterotopic osteogenesis. recent studies have documented the undesirable side effects associated with the high concentra- tions of BMp-2 administered to patients, raising questions about its safety for selected clinical applications.28
polymeric materials have great potential for the generation of smart materials. the diversity, malleability, and particular properties of synthetic polymers allow applications in tissue engineering ranging from hard to soft tissues. the first success toward the production of polymeric scaffolds is rep- resented by the ability to control the degradation rates of the materials by modifying the molecular weight.29 therefore, biodegradable polymers, like those derived from polyesters or α-hydroxyesters, are often preferred over permanent polymers like polypropylene.30 one advantage of implantable degradable polymers is that the monomeric compounds are often readily recognized by enzymes in the krebs cycle and can be further metabolized as naturally occurring substrates.27
as mentioned previously, the ability of a material to degrade under physio- logical conditions is not enough to ensure a functional repair of the tissue. it has been found, for example, that hydrophobic polyesters are unable to allow
Published on 03 May 2017 on http://pubs.rsc.org | doi:10.1039/9781788010542-00039
cell integration.31 the production of synthetic smart polymers by functionali- zation steps, by which the surface is modified with antibodies, peptides, or eCM components, is employed to improve their bioactivity.30 the strategies that are being explored include modifications ranging from complete pro- teins, usually an integrin ligand to facilitate the cellular attachment, to small peptides, containing only the active domain. Collagen, fibronectin, laminin, and elastin are among the most commonly used proteins for functionaliza- tion of synthetic scaffolds.32 For example, degradable polymeric stents coated with elastin increase endothelial cell adhesion and proliferation.33 although ideally the complete protein could provide the cues needed for cell fate regulation, disadvantages associated with the costs of purification, greater control over the functionalization, and biologic variability, have made short peptides, like the arg-Gly-asp (rGd) motif, the preferred option.34
a critical point during the functionalization of polymeric materials is the attachment of the target molecules to the substrate. Bioactive compounds are organic molecules, highly sensitive to temperature and ph, therefore, functionalization steps should avoid the use of conditions that can affect the structure and activity of these molecules.35 physical methods, based on the adsorption and generation of non-covalent interactions, are the less invasive approach to modify the properties of a polymeric material (Figure 2.1(a)).
Figure 2.1 Methods of functionalization of polymeric materials. physical methods of functionalization are based on adsorption and non-covalent bind- ing of the functional groups (a). Chemical methods covalently bind the functional groups (pendant structures) to the polymer (b).
Published on 03 May 2017 on http://pubs.rsc.org | doi:10.1039/9781788010542-00039
in a study performed by aguirre et al. (2012), composites of polylactic acid (pla) scaffold polymers coated with calcium phosphate glass provided an active microenvironment that mimicked the mechanotransductory and chemical signals of wound healing, promoting an increased vasculariza- tion.36 in studies conducted by Faulk et al. (2014), synthetic non-degradable surgical mesh materials (polypropylene) coated with dermal eCM hydrogel modified the host immune response, the pro-inflammatory response and the amount of fibrous tissue formation.37
an alternative method of functionalization of synthetic scaffolds is based on the covalent binding of the active molecule to the polymer (Figure 2.1(B)).
With this approach, the risk of delamination that exists with physical methods is eliminated.38 recent advances in “click chemistry”, a method that utilizes orthogonal modification of materials in a controlled fashion, have provided highly efficient functionalization processes to generate smart synthetic materials. Some derivatives of click chemistry can be made under environ- mental conditions and without the requirement of toxic catalysts, overcom- ing the limitations of other traditional chemical methods.39,40 Biodegradable poly(ester urethane)urea elastomers (peUUs) have been the target of click chemistry to generate surface-functionalized smart biomaterials for appli- cations in cardiac patches and soft tissue engineering.41 in particular, the covalent binding of small peptides able to promote endothelial progenitor cell adhesion has been successfully evaluated in vitro.42 Clinical translation of these smart synthetic materials has yet to occur, but the rapid advances of these technologies combined with the use of synthetic materials that are already in the biomedical market will facilitate their use in tissue engineering applications.
an additional surface functionalization method is based on self-assembly techniques to form hydrogels. the classical property associated with hydro- gels is their hydrophilic nature, which provides for a viscous, soft material, ideal for soft tissue engineering applications.43 the specific mechanical, physical, and chemical characteristics of hydrogels can be modulated with the use of different monomers or polymerization methods, like crosslink- ing modifications.44 Self-assembled smart hydrogels are generated by mim- icking the natural polymerization of the eCM components from a complex of polymeric materials interacting by covalent and non-covalent forces with the active molecules.45,46 the inclusion of cryptic peptides, zwitterionic molecules and/or growth factors has facilitated the modulation of cell behavior in an eCM-like environment.47
Wound healing after burn injuries is a good example of the potential clinical application of a smart hydrogel. extensive burns are usually associated with high morbidity and increased risk of infection. tissue engineering approaches in this area are investigating alternatives to reduce the healing time, combined with antimicrobial activities that protect the tissue from infection. For instance, the small peptide sequence (llkkk18), derived from the antimicrobial peptide (aMp) cathelicidin ll-37, conjugated to dextrin and indirectly linked to Carbopol® hydrogels (based on acrylic acid polymers), is
Published on 03 May 2017 on http://pubs.rsc.org | doi:10.1039/9781788010542-00039
being investigated for burn treatment.48 the aMp cathelicidin ll-37 is an active molecule, naturally produced by immune cells49 such as macrophages, that contributes not only to protection against bacterial infection, but also has chemotactic effects promoting vascularization and the wound healing process.50 Consistent with the properties attributed to cathelicidin ll-37, preclinical studies using these smart hydrogels showed increased angiogen- esis, a shorter healing time, and increased matrix deposition, compared to the non-conjugated hydrogels.48
the investigation of non-conventional characteristics of polymers, such as conductive properties, is another area of interest in the development of smart materials. Conductive polymers can be considered a hybrid, as they present mechanical properties of traditional degradable polymers as well as con- ductive properties of metallic materials.31,51 Conductive polymers are being widely studied in non-biomedical applications, but the finding that some of these materials have good biocompatibility has raised their biomedical pro- file. the polymers already investigated for tissue engineering applications belong to a group of polyheterocycles that includes polypyrrole, polyani- line, and polythiophene. the potential use that has been focused upon is the reconstruction of electro-active tissues such as the brain, heart, skeletal muscle, and bone.52,53 different molecules carry the charge to be mobilized, tailoring the conductive property of these polymers. Successful approaches involve the use of glycosaminoglycans (GaG) (e.g. chondroitin sulfate and hyaluronic acid) as such “doping” molecules, which also can improve cell integration.31
Currently, in vitro and animal studies have been performed with different modifications of these conductive materials, but to be translated to clinical applications, challenges in the mechanical properties and reproducibility of the manufacturing processes need to be addressed. these materials have been further modified to incorporate specific protein domains to provide cell cues and facilitate cell adhesion and differentiation.31
From the review presented herein, it is obvious that synthetic smart mate- rials are rapidly evolving to simulate the regulatory mechanisms that exist within naturally occurring eCM. in the following sections, the analysis of smart biomaterials derived from naturally occurring materials will provide an alternative approach to cell/tissue modulation applicable in tissue engi- neering approaches.