Design and development of the hydrogel scaffolds with tunable mechanical, physical, and chemical properties, inspired from the naturally occurring nanomaterials which have been extensively exploited as alternative biomaterials to conventional metal-based/polymeric counterparts in the emerging field of biomedical engineering, explored from both structural and functional point of view, and investigated from diverse aspects of morphogenesis, functional modification, and biodegradation, have demonstrated the inestimable values in drug delivery and tissue engineering, especially the myocardial regenerative medicine.
In this dissertation, we first reviewed the recent advance in the naturally occurring organic nanomaterials and prospects of their applications in biomedicine, the current arduous challenges in myocardial regenerative medicine and the exceptional advantages of developing functional hydrogel scaffolds with tunable mechanical properties by implementing the principles governing the hierarchical assembly of these naturally occurring nanomaterials (Chapter 1). We predominantly discuss the uniqueness and inherent advantages of these naturally occurring nanomaterials, especially the naturally occurring nanoparticles, over other synthetic nanostructures. In addition, we describe multiple representative naturally occurring nanomaterials discovered and identified from various kingdoms. The physical, chemical, and biological cues of these naturally occurring nanomaterials desirable for myocardial tissue engineering and capable of being mimicked in the bioinspired design of hydrogel scaffolds is further elaborated. Furthermore, the discussion is expanded over the bioinspired construction of tunable hydrogel via elucidating the hierarchical self-assembly processes of several naturally occurring nano-networks.
Then we dissect the adhesive hydrogel exuded by sundew plants in Chapter 2. As a typical bioadhesive, sticky exudate observed on the stalked glands of sundew plants aids in the capture of insects and this viscoelastic adhesive has triggered extensive interests in revealing the implied adhesion mechanisms. Despite significant progress has been made, the structural traits of the sundew adhesive, especially the morphological characteristics in nanoscale, which may give rise to the viscous and elastic properties of this mucilage, remain unclear. Here we show that the sundew adhesive is a naturally occurring hydrogel, consisting of nano-network architectures assembled with polysaccharides. The assembly process of the polysaccharides in this hydrogel is proposed to be driven by electrostatic interactions mediated with divalent cations. Negatively charged nanoparticles, with an average diameter of 231.9 ± 14.8 nm, are also obtained from this hydrogel and these nanoparticles are presumed to exert vital roles in the assembly of the nano-networks. Further characterization via atomic force microscopy indicates that the stretching deformation of the sundew adhesive is associated with the flexibility of its fibrous architectures. It is also observed that the adhesion strength of the sundew adhesive is susceptible to low temperatures. Both elasticity and adhesion strength of the sundew adhesive reduce in response to the descending of ambient temperatures. The feasibility of applying sundew adhesive for tissue engineering is subsequently explored in the present study. Results show that the fibrous scaffolds obtained from sundew adhesive are capable of increasing the adhesion of multiple types of cells, including fibroblast cells and smooth muscle cells, a property that results from the enhanced adsorption of serum proteins. In addition, in light of the weak cytotoxic activity exhibited by these scaffolds toward a variety of mammal cells, evidence is sufficient to propose that sundew adhesive is a promising nanomaterial worth further exploitation in the field of tissue engineering.
Another typical naturally occurring nanomaterial derived from the adventitious roots of Hedera helix (English ivy) is then discussed in Chapter 3. These glycoprotein-rich nanoparticles obtained from the sticky exudates of English ivy, have also shown promising potential to be used in nanomedicine owing to their excellent aqueous solubility, low intrinsic viscosity, biocompatibility, and biodegradability. Herein, the feasibilities of utilizing ivy nanoparticles (INPs) as nano-carriers for delivering chemotherapeutic drugs in cancer therapy and as nano-fillers to develop novel scaffolds for tissue engineering in regenerative medicine are evaluated. Via electrostatic and hydrophobic interactions, pH-responsive nanoconjugates are formed between the INPs and the doxorubicin (DOX) with an entrapment ratio of 77.9% ± 3.9%. While the INPs show minimal cytotoxicity, the formed INP-DOX conjugates exhibit substantially stronger cytotoxic activity than free DOX against multiple cancer cell lines, suggesting a synergistic effect is established upon conjugation. The anti-cancer effects of the INP-DOX conjugates are further evaluated via in vivo xenograft assays by subcutaneously implanting DOX resistant cell line, SW620/Ad-300, into nude mice. The tumor volumes in mice treated with the INP-DOX conjugates are significantly less than those of the mice treated with free DOX. In addition, the INPs are further exploited as nano-fillers to develop fibrous scaffolds with collagen, via mimicking the porous matrix where the INPs are embedded under natural condition. Enhanced adhesion of smooth muscle cells (SMCs) and accelerated proliferation of mouse aortic SMCs are observed in this newly constructed scaffold. Overall, the results obtained from the present study suggest great potential of the INPs to be used as biocompatible nanomaterials in nanomedicine.
In addition to the drug delivery and tissue engineering, the ivy nanoparticles are also explored for their potential application in cosmetics (Chapter 4). This study focused on analysis of the physicochemical properties of the ivy nanoparticles, specifically, those parameters which are crucial for use as sunscreen fillers, such as pH, temperature, and UV irradiation. The visual transparency and cytotoxicity of ivy nanoparticles were also investigated comparing them with other metal oxide nanoparticles. Results from this study demonstrated that, after treatment at 100 °C, there was a clear increase in the UV extinction spectra of the ivy nanoparticles caused by the partial decomposition. In addition, the UVA extinction spectra of the ivy nanoparticles gradually reduced slightly with the decrease of pH values in solvents. Prolonged UV irradiation indicated that the influence of UV light on the stability of the ivy nanoparticle was limited and time-independent. Compared to TiO2 and ZnO nanoparticles, ivy nanoparticles showed better visual transparency. Methylthiazol tetrazolium assay demonstrated that ivy nanoparticles exhibited lower cytotoxicity than the other two types of nanoparticles. Results also suggested that protein played an important role in modulating the three-dimensional structure of the ivy nanoparticles. Based on the results from this study it can be concluded that the ivy nanoparticles are capable of maintaining their UV protective capability at wide range of temperature and pH values, further demonstrating their potential as an alternative to metal oxide nanoparticles present in the current commercial sunscreens.
Eventually, the tunable hydrogel scaffold developed for myocardial regenerative medicine, inspired from the hierarchically assembling processes of aforementioned naturally occurring nano-networks, is discussed in detail in Chapter 5. Development of the ideal engineered scaffolds resembling the native extracellular matrices of heart tissue for myocardial regeneration is still regarded as one of the arduous and predominant challenges in cardiac tissue engineering. Given that the stiffness of the heart tissue is pathologically elevated in response to the myocardial infarction, to construct a biomaterial functionalized with tunable mechanical properties is thought of to be a potent strategy to adjust the rigidity of the infarct myocardium and reactivate the cardiac functions upon the implantation. Here we show that a hydrogel scaffold constituted with self-assembled peptides is designed and developed via the implementation of the principles underlying the self-assembly of several types of naturally occurring nano-networks derived from the mucilage exuded by sundew and English ivy, with tunable viscoelastic properties and rigidities. The elastic moduli of such bioinspired hydrogel scaffold are modulated in response to the variation in the ionic strength within the microenvironment and this peptide-assembled hydrogel exhibits exceptional capacities to favor the adhesion and proliferation of human induced pluripotent stem cells (HPSCs), as a result of the incorporation of RGD moieties. In addition, the HPSCs were arrayed within the peptide hydrogel in three distinct manners and an accelerated differentiation of HPSCs toward cardiac myocytes was observed on the peptide-assembly hydrogel scaffolds in contrast to that on the vitronection-coated substrate. An ischemia-reperfusion injury mouse model was established to assess the therapeutic outcomes of the peptide hydrogel in combination with HPSCs and the results demonstrate that the cardiac functions are substantially improved by such hydrogel scaffolds encapsulated with HPSCs throughout a 30-day evaluation in terms of the ejection fraction and fractional shorting, without any evident toxicity against the tissues.