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Thesis (M. Sc. (Microbiology)) -- University of Limpopo, 2016 / Smart hydrogels could facilitate immobilisation of cellulases to allow recovery and decrease enzyme cost in the biofuel industry, as they have a soluble-gel transition. The aim of the study was to design and evaluate the use of smart hydrogels for immobilisation of cellulase system that can be recovered after hydrolysis of cellulosic biomass. Cellulases from Aspergillus niger FGSC A733 produced under solid state fermentation and commercial cellulases were used in immobilisation. Various support matrices prepared were poly-N-isopropylacrylamide (p-NIPAAm), poly-N isopropylacrylamide-co-Methacrylic acid (p-NIPAAm-co-MAA) and supermacroporous poly-crosslinked-Acrylamide-co-N,N’-Methylenebisacrylamide (p-crosslinked-AA-co MBA). Cellulases were coupled onto the support matrices by covalent attachment method through reactive groups of N-acryloxysuccinimide (NAS) or Methacrylic acid N-hydroxysuccinimide (NMS). The low critical solution temperature (LCST) of formed p-NIPAAm-co-MAA copolymer was determined by the inflection point method. The shrinking and swelling kinetics and pH sensitivity of p-NIPAAm-co-MAA copolymer and conjugates were characterised using a cloud point method. Hydrolysis of CMC using cellulase-microbeads-p-NIPAAm and cellulase-crosslinked-p-NIPAAm with different percentage gel showed activity trend of 0.05>1>10>5>0.1% and 5>2>10% respectively. HPLC analysis showed that supplementation of β-glucosidase in cellulase-crosslinked-p-NIPAAm conjugates increased glucose by 12 and 14-fold at 30 and 50 °C respectively in the avicel hydrolysate in comparison with no β glucosidase supplementation. In the hydrolysis of avicel using cellulase-crosslinked p-NIPAAm-co-MAA conjugate a total of 13.6 g/L of reducing sugar was liberated after three cycles. In comparison a total of 21.4 g/L of reducing sugars were released from avicel hydrolysis using cellulase-crosslinked-p-AA-co-MBA conjugate after 3 cycles. In contrast, reducing sugars released in thatch grass hydrolysis using free enzyme were 8 times greater than in cellulase-crosslinked-p-AA-co-MBA conjugate. Cellulase crosslinked-p-NIPAAm-co-MAA conjugates were more stable than free enzyme at 50 and 60 °C after 24 hour and 120 minutes of incubation respectively, but lost activities at 65 °C after 120 minute. Therefore the activity loss in the immobilised enzymes was more due to thermal inactivation during precipitation and recovery than incomplete recovery during precipitation cycles. The results show that cellulases immobilised on smart polymers with sol-gel transition could be used in hydrolysis of cellulose due to ease of recovery. Hydrolysis kinetics was efficient for both immobilised enzyme system (cellulase-crosslinked-p-AA-co-MBA and cellulase-crosslinked-p-NIPAAm-co MAA conjugate) since were re-used in hydrolysis of avicel. Therefore the use of these smart polymers for cellulase immobilisation can contribute in cost reduction of the enzymatic hydrolysis process in the biofuel industry. / National Research Foundation (NRF) , University of Limpopo financial aid office and Flemish Interuniversity Council (VLIR-UOS) fo

In this work, a hydrogel made from hyaluronic acid is synthesized and utilized to study neural stem cell behavior within a custom tailored three dimensional microenvironment. The physical properties of the hydrogel have been optimized to create an environment conducive for neural stem cell differentiation by mimicking the native brain extracellular matrix (ECM) environment. The physical properties characterized include degree of methacrylation, swelling ratios, enzymatic degradation rates, and viscoelastic moduli. One dimensional proton nuclear magnetic resonance (1HNMR) confirms modification of the hyaluronic acid polymers, and is used to quantify the degree of methacrylation. Rheological measurements are made to quantify the viscoelastic moduli. Further post-processing by lyophilization leads to generation of large voids to facilitate re-swelling and cell infiltration. ReNcell VM (RVM), and adult human neural stem cell line derived from the ventral mesencephalon, are cultured and differentiated inside the hydrogel for up to 2 weeks. Differentiation is characterized by immunocytochemistry (ICC) and real time quantitative polymerase chain reaction (qRT-PCR). / Bioengineering

One of the major technical challenges with creating 3D artificial tissue constructs is the lack of simple and effective methods to integrate vascular networks within them. Without these vascular-like networks, the cells embedded within the constructs quickly become necrotic. This thesis details the use of a commercially available, low-cost, 3D printer modified with a microfluidic printhead in order to generate instantly perfusable vascular-like networks integrated within gel scaffolds seeded with cells. The printhead featured a coaxial nozzle that allowed the fabrication of hollow, gel tubes (500µm–2mm) that can be easily patterned to create single or multi-layered constructs. Media perfusion of the channels caused a significant increase in cell viability. This microfluidic nozzle design was further modified to allow for multi-axial extrusion in order to 3D print and pattern bi- and tri-layered hollow channel structures. Most available methodologies lack the ability to create multi-layered concentric conduits inside natural extracellular matrices, which would more accurately replicate the hierarchal architecture of biological blood vessels. The nozzle used in this work allowed, for the first time, for these hierarchal structures to be embedded within layers of gels in a fast, simple and low cost manner. This scalable design allowed for versatility in material incorporation, thereby creating heterogeneous structures that contained distinct concentric layers of different cell types and biomaterials. This thesis also demonstrates the use of non-extrusion based 3D biofabrication involving planar processing by means of hydrogel adhesion. There remains a lack of effective adhesives capable of composite layer fusion without affecting the integrity of patterned features. Here, silicon carbide was found for the first time to be an effective and cytocompatible adhesive to achieve strong bonding (0.39±0.03kPa) between hybrid hydrogel films. Multi-layered, heterogeneous constructs with embedded high-resolution microchannels (150µm-1mm) were fabricated in this way. With the new 3D fabrication technology developed in this thesis, gel constructs with embedded arrays of hollow channels can be created and used as potential substitutes for blood vessel networks as well as in applications such as drug discovery models and biological studies. / Thesis / Doctor of Philosophy (PhD) / Additive manufacturing (AM) involves any three-dimensional (3D) fabrication technologies that is used to produce a solid model of a predetermined design. AM techniques have recently been used in tissue engineering applications for fabrication of 3D artificial tissues that resemble architectures and material properties similar to that of the native tissue. Utilizing AM for this purpose presents the advantage of increased control in feature patterning, which leads to the realization of more complex geometries. However, there still remains a lack of simple and effective methods to integrate vascular networks within these 3D artificially engineered scaffolds and tissue constructs. Without these vascular-like networks, the cells embedded within the constructs would quickly die due to a lack of nutrient delivery and waste transport. This remains one of the biggest challenges in true 3D tissue engineering. This thesis presents a number of fast, effective and low-cost AM biofabrication techniques to address this challenge.

This study presents a viable strategy using fmoc-protected peptides hydrogels, to encapsulate and stretch mesenchymal stem cells (MSC). To explore the peptide hydrogel potential, a custom mechanical stretching device with polydimethylsiloxane (PDMS) chambers were used to stretch MSCs encapsulated in Fmoc hydrogels. We investigated the impact of fmoc- FF prepared in dimethyl sulfoxide (DMSO), 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) and deionizied water in the self-assembly, and mechanical properties of the gels. The peptide hydrogel is formed through molecular self-assembly of peptide sequence into β-sheets that are connected with the π-π aromatic stacking of F-F groups. The hydrogels provided a stiff, hydrated gel with round nanofiber morphology representing an elastic modulus of 174-266 KPa. MSCs cultured on peptide hydrogels undergo viability, morphology, and alignment evaluations using MTT, live/dead, and phalloidin (F-actin) staining. The F-actins of 3D- cultured MSCs in Fmoc-FF/HFP, and Fmoc-FF/DMSO followed by mechanical stretching showed elongated morphology with defined microfilament fibers compared to the round and spherical F-actin shape of the control cells. Peptide gels with 5mM concentration preserved 100% viability of MSC. Results reveals the feasibility and conditions for successful cell encapsulation and alignment within peptide hydrogels. Encapsulation of MSC in peptide nanofiber followed by a stretching process present a promising tissue engineering platform. By enhancing our understanding of MSC-peptide hydrogel interactions, this research con- tributes to the development of biomaterials tailored for regenerative medicine.

Peptide-based biomaterials have gained much interest in various applications in drug delivery and tissue engineering in recent years, in large part due to their typically excellent biocompatibility and biodegradability. Composed of different amino acids, peptides can be designed with numerous sequences, providing flexibility and tunability in biomaterials. Peptides are easy to modify with small molecule drugs, inorganic components, and polymer chains to access multiple functions and tune properties relevant to biology and medicine. Stimuli-responsive peptide-based biomaterials can respond to environmental stimuli, such as light and ultrasound, in addition to local environmental factors, such as temperature, enzyme activity, and pH. Under environmental changes, these materials can be triggered to release therapeutic payloads, change conformations, or induce self-assembly in the target sites. In this work, I introduce the design, synthesis, and potential applications of several stimuli-responsive peptide-based biomaterials. The first half of this dissertation is based on enzyme-responsive, peptide-based biomaterials as extracellular matrix (ECM) mimics in tissue engineering. We synthesized linear and dendritic elastin-like peptides (ELPs) as crosslinkers and conjugated them with hyaluronic acid (HA) to form hydrogels. Trypsin was used as the enzyme trigger for cleaving the C-terminal lysine and to study how crosslinker topology affects enzymatic degradation. Hydrogels with dendritic ELPs degraded more slowly than linear ELPs, providing a novel strategy to tune the degradation rate of hydrogels as ECM mimics by the molecular design of crosslinker topology. Building on this peptide-polysaccharide platform for synthetic ECM design, we subsequently prepared hydrogels embedded with bioactive cryptic sites. These novel polymeric hydrogels mimicked native ECM cryptic sites by using depsipeptides that undergo an enzyme-triggered molecular rearrangement, "switching" from a non-functional epitope to a bioactive sequence. Mass spectrometry, 1H and 13C NMR spectroscopy, and fluorescence studies were applied to track structural changes in the peptide. SEM was used to image these polymer-peptide hybrid hydrogels. Finally, in vitro studies were conducted to evaluate cell interactions with the hydrogels. Switch peptide-modified alginate hydrogels showed increased cell adhesion upon induction of enzymatic activity, which provided a "gain of function" of the synthetic ECM. Critically, enzymes associated with the cells themselves could trigger the peptide switch and change in synthetic ECM behavior. With knowledge of stimuli-responsive peptide-based biomaterials applied in tissue engineering, I then studied how this system could be used in drug delivery by designing peptide-hydrogen sulfide (H2S) donor conjugates (PHDCs). H2S is a gasotransmitter that is produced endogenously, which has been explored in recent years with many potential therapeutical applications. We studied H2S release profiles in dual-enzyme-responsive PHDCs, with a further investigation into PHDC–Fe2+ complexes for potential tumor treatments via chemodynamic therapy. The PHDC–Fe2+ complexes were examined in a C6 glioma cell line, exhibiting an improved cell-killing effect compared with controls, by inducing toxic hydroxyl radical generation (•OH) via a Fenton reaction. To this end, we further discovered how side chains influence self-assembling nanostructures, H2S release profiles, and biological activities via three constitutionally isomeric PHDCs. Different morphologies and varied H2S release rates were observed, paving the way for tuning the properties of PHDCs by simple changes in molecular design. Finally, this dissertation discloses conclusions and future directions on stimuli-responsive peptide-based biomaterials using similar platforms with different designs in the drug delivery and tissue engineering fields. / Doctor of Philosophy / Peptides, short sequences of two or more amino acids linked by chemical bonds, are smaller versions of proteins. Forming naturally in nature, peptides are promising candidates in the design of biocompatible and biodegradable materials. To make these peptide-based materials "smart", certain sequences or functional groups are installed in the peptides, making them responsive to environmental changes, or stimuli. These external stimuli include light, ultrasound, temperature, enzyme activity, and pH changes. In this work, we have explored the design and synthesis of stimuli-responsive peptide-based biomaterials and their potential applications in tissue engineering and drug delivery. The first half of this dissertation focuses on the design and synthesis of two enzyme-responsive, peptide-based materials that function as extracellular matrix (ECM) mimics. The ECM is a three-dimensional microenvironment where cells reside, providing structural support and adhesive anchor points for cells. In the first system, we synthesized peptide-polysaccharide hydrogels with different peptide crosslinkers, comparing their enzymatic degradation performance to evaluate how peptide topology (architecture) influences degradation. A more branched topology led to a slower hydrogel degradation rate. To introduce biofunctionality into the ECM mimics, we embedded the second system with a "switchable" peptide sequence, which transformed from a non-functional peptide into a functional, bioactive epitope after being triggered by an enzyme. The functional peptide after the switch provided cell adhesion and increased cell spreading. The latter half of this dissertation explores the possibility of stimuli-responsive peptide-based biomaterials in drug delivery. We designed peptides that release hydrogen sulfide (H2S), a signaling gas is commonly known for its foul smell and toxicity, and studied the biological behaviors in cells. The peptide-H2S donor conjugates (PHDCs) were activated by the enzyme legumain, which cancer cells overproduce, leading to H2S release. With the combined treatment with Fe2+, the PHDC-Fe2+ system reduced cancer cell viability due to the high amount of hydroxyl radicals (•OH) generated by the Fenton reaction. This system may be a potential design platform for precise tumor treatments.

Hydrogen sulfide (H2S) is a gasotransmitter that is produced endogenously and freely permeates cell membranes. It plays important roles in many physiological pathways, and by regulating these pathways, it provides many therapeutic effects. For example, H2S dilates vascular vessels, promotes angiogenesis, and protects cells from oxidative stress. Due to its therapeutic effects, H2S has been used as a potential treatment for diseases like diabetes, ischemia-reperfusion injuries, lung diseases, ulcers and edemas, among others. To apply H2S for therapeutic applications, two challenges need to be addressed. The first challenge is the H2S donor, which not only provides H2S but must be stable enough to avoid side effects caused by overdose; and the second challenge is the delivery strategies, which transport the H2S to the target sites. A series of S-aroylthiooximes (SATOs), an H2S releasing compound, were synthesized and conjugated to peptide sequences to form H2S-releasing aromatic peptide amphiphile (APA) hydrogels. APAs formed nanofibers, which were stabilized by beta-sheets and aromatic stacking. The self-assembled structures were affected by the substituents on the aromatic rings of SATOs, leading to the formation of twisted nanofibers. After the addition of cysteine, H2S was released from the APAs with half-lives ranging from 13 min to 31 min. The electron-donating groups slowed down the H2S release rate, while the electron-withdrawing groups accelerated the release rate. Therefore, the release rates of H2S were controlled by electronic effects. When self-assembled structures were formed, the H2S release rate was slowed down even more, due to the difficulties in cysteine diffusion into the core of the structures. Antimicrobial effects were also discovered using the H2S releasing APA hydrogels. The H2S-releasing dipeptides S-FE and S-YE formed self-assembled twisted nanoribbons and nanotubes, respectively. The non H2S-releasing control oxime dipeptides C-FE and C-YE were also synthesized. The C-FE formed nanoribbons while the C-YE only showed non-specific aggregates. S-FE and S-YE released H2S with peaking times of about 41 and 39 min. Both the self-assembled structures and the release rates were affected by their packing differences. In vitro and ex vivo experiments with Staphylococcus aureus (Xen29), a commonly found bacterium on burn wounds, showed significant antimicrobial effects. APAs S-FE and C-FE eliminated Xen29 and inhibited the biofilm formation, while S-FE always showed better effects than C-FE. These antimicrobial H2S-releasing APA hydrogels provide a new approach to treat burn wound infections, and provide healing benefits due to the therapeutic effects of H2S. / Doctor of Philosophy / Hydrogen sulfide (H₂S) is a signaling gas that produced in our body. It regulates physiological pathways, and can be a potential treatment for diseases like diabetes, ischemia-reperfusion injuries, lung diseases, ulcers and edemas, among others. However, two issues need to be addressed before applying H₂S for disease treatments. The first issue is to choose an H₂S donor, which is stable enough to avoid side effects caused by overdose. The second issue is the delivery methods, which transport the H₂S to target sites. A series of S-aroylthiooximes (SATOs), an H₂S releasing compound, were synthesized and attached to peptide sequences to form H₂S-releasing self-assembled aromatic peptide amphiphile (APA) hydrogels. The APA hydrogels were found to be affected by the substituents on the SATO structures. For example, the H₂S released from APAs had halflives ranged from 13 min to 31 min, which were controlled by the substituents. When hydrogels were formed, the H₂S release was slowed down even more, due to the difficulties in cysteine diffusion into the SATO structures. The antimicrobial effects were also discovered using the H₂S releasing APA hydrogels. Two H₂S-releasing APA hydrogels, S-FE and S-YE, were formed. At the same time, two non H₂S-releasing oxime dipeptides, C-FE and C-YE, were also synthesized as controls. The H₂S-releasing peptides, S-FE and S-YE, released H₂S with peaking times of about 41 and 39 min, while no H₂S was released from C-FE and C-YE. The self-assembled structures and the release rates were affected by their structural differences. In vitro and ex vivo experiments with Staphylococcus aureus (Xen29), a commonly found bacterium on burn wound, showed significant antimicrobial effects. Both H₂S-releasing S-FE and non H₂S-releasing C-FE eliminated Xen29 and inhibited the biofilm formation, indicating the potential use of the designed peptides as antimicrobial treatment for wounds. The S-FE always showed better effects than C-FE, suggesting the benefit of H₂S during the elimination of bacteria. These antimicrobial H₂S-releasing APA hydrogels provide a new approach to treat burn wound infection and provide healing benefits due to the therapeutic effects of H₂S.

The work focused on addressing four main objectives. The first objective was to quantify protein and amino acid substitution reactions. Michael addition reactions were used to modify the amino acids and protein. Amino acids alanine, cysteine, and lysine, and protein ovalbumin (OA) were substituted with different concentrations of ethyl vinyl sulfone (EVS). The substituted products were analyzed using Raman spectroscopy and UV-spectroscopy based ninhydrin assay. In case of alanine, Raman and UV results correlated with each other. With cysteine at lower EVS substitutions amine on the main chain was the preferred site while the substitution shifted to thiols at higher substitutions. This could only be discerned using Raman spectroscopy. Lysine has amines on the main chain and side chain while main chain amine was the most reactive site at lower concentrations of EVS while at higher concentrations side chain amines were also substituted. This information could be discerned using Raman spectroscopy only and not UV spectroscopy. In case of protein as observed by Raman and UV spectroscopy the reaction continued at higher concentrations of EVS indicating the participation of glutamine and asparagines at higher substitutions. However, the reaction considerably slowed down at higher EVS substitutions. The second objective of the study was to decrease the glass transition temperature (Tg) of OA through internal plasticization and also study the effects of the substituents on the thermal stability of OA. The hypothesis was by covalently attaching substituents to OA, number of hydrogen bonds can be reduced while increasing the free volume and this would reduce Tg. EVS, acrylic acid (AA), butadiene sulfone (BS) and maleimide (MA) were the four groups used. EVS was the most efficient plasticizer of all the four substituents. The Tg decreased with the increasing concentration of EVS until all of the reactive of groups on OA were used up. Tg decreased slightly with AA and BS while no change was observed with MA. However, the substituents showed exact opposite trend in thermal stability as measured using thermogravimetric analysis (TGA). The thermal stability of MA substituted OA was the highest and that of EVS substituted OA was least. FT-IR spectroscopy results indicated that all four substituents caused structural changes in OA. This implied that there were intermolecular interactions between substituted protein chains in case of AA, BS, and MA. This caused an increase in the thermal stability. EVS on the other hand is a linear chain monomer with a hydrophobic end group and hence could not participate in the intermolecular interactions and hence caused a decrease in Tg. As mentioned above the limitation to this technique is the number of available reactive groups on the protein. However, we successfully demonstrated the feasibility of this method in decreasing Tg of protein. The third objective was to create hydrogels by crosslinking OA with divinyl sulfone (DVS). Protein hydrogels due to their biocompatible nature find applications in drug delivery and tissue engineering. For tissue engineering applications the hydrogels need to be mechanically stable. In this study the protein was substituted with EVS or AA and then crosslinked with DVS. The swelling ratio was measured as a function of pH. All the hydrogels showed the same trend and swelled the least at pH 4.5 which is the isoelectric point of the protein. At basic pH conditions EVS substituted hydrogels swelled the most while AA substituted hydrogels showed least swelling. The static and dynamic moduli of the hydrogels were determined using tensile tester and rheometer respectively. The static modulus values were three times the dynamic modulus. The modulus of the control which is crosslinked OA was least and that of AA substituted OA was highest. The stress relaxation test also showed similar results in which AA substituted OA relaxed the most and the control relaxed the least. FT-IR of the dry hydrogels showed that the amount of hydrogen bonding increased with AA substitution. The hydrophilic AA end groups interacted with each other forming hydrogen bonds. These hydrogen bonds served as additional crosslinks there by increasing the modulus of the hydrogels. EVS on the other hand was incapable of interactions due to the lack of hydrophilic end groups. We were successfully able to create protein hydrogels and control the swelling and mechanical properties by varying the amount of substituted group. The final objective of the study was to create and characterize microstructures from substituted alanine and lysine. Alanine and lysine were substituted with different concentrations of EVS. Bars and fibers were observed for alanine at moderate substitutions while at higher concentrations random structures were observed using scanning electron microscopy (SEM). Lysine formed tubes at moderate EVS substitutions and rosettes at high concentrations of EVS as evidenced by SEM. FT-IR results suggested that instead of carbonyl one of sulfonyl bonded to the available amine in modified amino acids. And only in this case fibers, tubes and rosettes were observed. X-ray diffraction (XRD) results supported this observation. Using these results we hypothesized that the self assembled structures very much depended on the amount of EVS present in the substituted product and sulfonyl forming β-sheet analogs with amine. / Ph. D.