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Peptide amphiphiles (PAs) are peptide-based molecules that contain a peptide sequence as a head group covalently conjugated to a hydrophobic segment, such as lipid tails. They can self-assemble into well-ordered supramolecular nanostructures such as micelles, vesicles, twisted ribbons and nanofibers. In addition, the diversity of natural amino acids gives the possibility to produce PAs with different sequences. These properties along with their biocompatibility, biodegradability and a high resemblance to native extracellular matrix (ECM) have resulted in PAs being considered as ideal scaffold materials for tissue engineering (TE) applications. This review introduces the 20 natural canonical amino acids as building blocks followed by highlighting the three categories of PAs: amphiphilic peptides, lipidated peptide amphiphiles and supramolecular peptide amphiphile conjugates, as well as their design rules that dictate the peptide self-assembly process. Furthermore, 3D bio-fabrication strategies of PAs hydrogels are discussed and the recent advances of PA-based scaffolds in TE with the emphasis on bone, cartilage and neural tissue regeneration both in vitro and in vivo are considered. Finally, future prospects and challenges are discussed.
Tissue engineering (TE) is a highly multidisciplinary field aiming to regenerate damaged tissues or organs through developing biological substitutes that maintain, restore or improve the function of the target tissues or organs. TE relies extensively on scaffolds which are typically seeded with cells. The scaffolds can be either cultured in vitro to synthesize tissue that is subsequently implanted into the injured site or implanted directly into injured site exploiting the body's own systems for the regeneration of tissue [1]. These scaffolds can be divided into natural [2] or synthetic categories [3]. The natural category is comprised of natural polymers such as silk fibroin, collagen, hyaluronic acids and Matrigel, while the synthetic category employs polymers such as polylactic acid (PLA), polycaprolactone (PCL), polyglycolic acid (PGA) and poly (lactic-co-glycolic acid) (PLGA) etc. [2,3] There are several basic requirements for these biomaterials to be used as scaffolds: 1) excellent biocompatibility to facilitate cell attachment and proliferation; 2) appropriate biodegradability allowing degradation and resorption to match the tissue regeneration; 3) appropriate mechanical properties to enable the tissue remodeling (different tissues need different mechanical properties); 4) appropriate porosity for cells to deposit extracellular matrix (ECM), migrate and proliferate as well as transport of nutrients, metabolites and waste products [[4], [5], [6], [7]]. However, there are several disadvantages and limitations exist with the aforementioned scaffolds. For example, Matrigel and collagen hydrogel have high batch to batch variations, and the synthetic scaffolds are normally lack of cell adhesion sites. In addition, the degraded products of these polymers contain acidic compounds that are harmful to human body [[8], [9], [10]]. On the other hand, the precise control of protein/peptide-based scaffolds give rise to many benefits for TE which include structural tunability, excellent biocompatibility and an abundance of sites for easy functionalisation and modification [11]. However, disadvantages of such scaffolds are also exist such as weak mechanical properties and fast biodegradability.
Proteins, made from amino acid building blocks, are ubiquitous in nature and can perform specific biological functions, including DNA replication, catalysis, transporting desired molecules and supporting cells in the extracellular matrix (ECM) [7,12]. In the early 1990s, the first self-assembling peptide EAK16 (Ac-(AEAEAKAK)2-CONH 2 where the N terminal is blocked by an acetyl group and the C terminal carboxylic group is blocked by an amine group was designed according to a repeating segment in the yeast protein Zuotin. This discovery led to the concept of self-assembling peptides as scaffold materials for TE, fuelling increased interest by many researchers to date. Subsequently, numerous de novo or biomimetically derived peptides mimicking the peptide EAK16 have been designed and synthesised [8,13,14]. The “Bottom-up” approach has been used as the strategy for the fabrication of self-assembled peptides, which exploits amino acid building blocks to form various nanoarchitectures, including: nanotubes, nanovesicles, nanobelts, nanofibers, nanorods, nanoparticles. These ordered aggregates are formed spontaneously by non-covalent bonds, such as electrostatic interaction and Van der Waals force, hydrogen bonding, hydrophobic interaction and π-π stacking. Based on the inherent biocompatibility and biodegradability of self-assembling peptides, they can be applied to many biomedical applications, including: drug and gene delivery, skincare and cosmetics as well as the stabilization of membrane proteins [[15], [16], [17]].
Peptide amphiphiles (PAs) are one important class of self-assembling peptides that have been extensively studied over the past three decades [[18], [19], [20], [21], [22]]. They can self-assemble into various nanostructures under certain solution conditions, which can be achieved through controlling the parameters such as pH, ionic strength and temperature [23]. This allowed PAs have potential applications in a wide range of fields, such as TE, drug and gene delivery [14,[24], [25], [26], [27]]. For example, PAs have been used to form peptide/siRNA nanocomplexes to improve the RNAi efficacy. Recent research suggests that the group of PAs can be classified into amphiphilic peptides, lipidated peptide amphiphiles and supramolecular peptide amphiphile conjugates [20]. In general, PAs contain a hydrophobic tail that can be designed via conjugating hydrophobic segments (e.g., lipid tails) at the ends of the peptide sequence or on specific residues. The hydrophobic tail of PAs is responsible for driving self-assembly and exposes the functional peptide sequences on the surface of the self-assembled nanostructures [28]. One example of PA molecule was designed in 2001 and the molecule was able to self-assemble into long nanofibers and produce a nanostructured fibrous scaffold with biological activities giving its great potential to mimic the ECM [25,29]. Since then, the use of PAs as functional biomaterials have been extensively studied and have been recognized as excellent scaffold materials [[30], [31], [32]] because of their unique advantages including: 1) excellent biocompatibility [33], 2) design flexibility, 3) self-assembling into highly stabilized structures [34]. 4) incorporation of bioactive segments [35], such as RGD [36]. The fragments of protein can also be included. For example, the C 12-GAGAGAGY peptide sequence is designed based on the structure of silk fibroin, and can be self-assembled into robust and plastic hydrogels induced by pH change [37].
This review focuses on the design and applications of PAs molecules as cell culture scaffolds for 2-dimensional (2D) and 3-dimensional (3D) TE applications. We outline recent advances in the design of PAs, in light of their composition and types as well as introducing the fabrication technologies of PA-based scaffolds. The applications of these scaffolds in TE were also discussed, with an emphasis on bone, cartilage and neural tissue regeneration.
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There are some 20 different natural amino acids which can serve as building blocks for peptide biosynthesis (Fig. 1). Out of these proline (P), glycine (G) and cysteine (C) are special amino acids due to their unique structures and play important roles in driving peptide folding [14]. Apart from proline, where the side chain (R group) is covalently linked with the amino terminus, all other amino acids possess the same 2 HN-CH-COOH motif, but bearing different R groups attached to the centre
As previously introduced PAs are an essential category of amphiphiles that can self-assemble into different aggregated structures controlled via their hydrophilic−lipophilic balance (HLB) within their molecular structures [51]. The self-assembly process is similar to that of lipids with a critical micelle concentration (CMC). Here, when the concentration is lower than CAC, it appears as monomers in aqueous solution, while for concentrations above their CAC, the process of aggregation starts and
Nowadays, the gelation of PAs is no longer a random event but can be achieved by designing the PA sequences. In addition to this, researchers are able to exploit external factors, such as temperature, pH, solvent, ionic strength, light and enzymes, to stimulate solubility or help with the self-aggregation of PAs [17,21,[126], [127], [128]]. This is be
