(2hs) Fabrication of Polymeric Systems for Biomaterials

Polymers are beneficial due to their vast characteristics and the ease with which their properties can be modulated. A moderate modification such as crosslinking predicts whether a material will melt, as in the case of thermosets, or if a material will dissolve in the presence of a solvent, as in the case of hydrogels. Hydrogels are particularly interesting because they can be designed to function permanently as a film or dissolve under specified conditions. Specifically, the 3-dimensional crosslinked network or mesh size, lends to its high-water absorption capacity and tunablity of mechanical and transport properties. This flexibility makes it favorable for use in a large range of applications. Further increasing their appeal, hydrogel fabrication has transitioned away from the use of petroleum-derived polymers to implementing biopolymers instead. Common biopolymers utilized include alginate, chitosan, agarose, and lignin. While the biopolymers listed have been well established, the heterogenous behavior of lignin poses a barrier to our understanding of how the introduction of lignin alters the network structure, obfuscating our ability to systematically tune mechanical and transport properties of these soft composites for a specific application.

In response, we have fabricated a collection of lignin–poly(vinyl alcohol) (PVA, Mw = 25,000 g/mol, Mw = 78,000 g/mol) hydrogel composites synthesized using Kraft lignin (raw, unfractionated lignin), as well as lignin that was cleaned and fractionated into lignin of prescribed molecular weights. Specifically, the lignin content varied between 0 mass % and 60 mass %, while the lignin MW varied between approximately 6,000 g/mol and 160,000 g/mol. Further, these soft composites were synthesized using two fabrication routes: (1) the “Freeze-Thaw” method, whereby physical crosslinks between PVA chains are created and (2) the “Freeze-Thaw” method, coupled with the co(non)solvency effect. Through these various fabrication methods, a variety of lignin-based hydrogel networks were achieved, including hierarchical network structures.

The first polymeric system to be explored was physically crosslinked PVA—lignin hydrogels. By way of the “Freeze-Thaw” method, free-standing hydrogel composites were fabricated. The network structure, equilibrium water uptake, and mechanical properties were investigated to aid in establishing structure-property-processing relationships. Scanning Electron Microscopy (SEM) revealed that the use of fractionated low molecular weight (LMW lignin) produced tighter pores while the high molecular weight (HMW) lignin produced wider pores. In support, the LMW hydrogel had the lowest equilibrium water uptake and the HMW hydrogel showed the most variance in the equilibrium water uptake due to the lignin aggregates identified via SEM imaging. Additionally, the equilibrium water uptake generally increased when the lignin concentration was increased to 20 wt % and decreased when the lignin concentration was increased to 60 wt %. This trend confirmed that lignin functions as a filler and does not crosslink with the PVA. Understanding the role of lignin in the polymer matrix aided with evaluating the influence on the mechanical properties. Mechanical characterization for this hydrogel system was performed using indentation (Young’s modulus measurements) and dynamic mechanical analysis (loss and storage modulus and tensile strength measurements). The mechanical characterization demonstrated that the Young’s modulus and tensile strength are readily altered with varying lignin MW and concentration. Further, both the storage and loss modulus generally decreased as lignin concentration increased. Overall, results of this study demonstrated that a “greener” alternative can be used for hydrogel fabrication while maintaining the properties of the hydrogel made from synthetic polymer.

The second hydrogel system explored coupled the “Freeze-Thaw” method and the co(non)solvency effect of dimethyl sulfoxide (DMSO) and water. Using varying ratios of DMSO and water and a fixed lignin MW of about 20,000 g/mol, free-standing hydrogel films with hierarchical structures were fabricated. SEM imaging demonstrated the presence of hierarchical pore structures that became more apparent with increased concentrations of DMSO. The pore sizes were reduced as the concentration of water in the PVA starting solution increased. Examination of the transport properties also supported this trend. The equilibrium water uptake decreased as water concentration increased from 10 wt % to 60 wt %, with the lignin samples having lower equilibrium water uptake than the neat samples. Permeability and diffusivity of methylene blue also decreased as the concentration of water increased; the lignin hydrogel was lower than the neat hydrogels. By leveraging crystallite formation and hierarchical structures, the mechanical properties of these hydrogels were expected to be reinforced and strengthened. Results from mechanical indentation demonstrated that stiffness generally increased as the DMSO concentration in the starting solution increased. However, the addition of lignin resulted in a reduction in Young’s modulus. Results from this portion of the study highlighted the refined transport properties achieved while allowing reinforced mechanical properties.

With success in fabricating robust lignin-based hydrogels without use of chemical crosslinkers and establishing parameters to fine-tune transport and mechanical properties, the viability of the hydrogels systems for bio-based applications was explored. In particular, the ease of modulating the stiffness and viscoelastic properties made the hydrogel attractive for use in wound healing, tissue engineering, and cartilage repair. On the other hand, the refined transport properties of the hierarchical systems made them attractive for protein separation or drug delivery. As such, biocompatibility testing was completed to explore the viability of the soft hydrogel composites to function as biomaterials. Specifically, the antimicrobial and antioxidant properties, as well as the cytotoxicity were examined. For the studies mentioned, both kraft lignin and hybrid poplar lignin were examined. For the antimicrobial studies, the inhibitory effects of lignin on the growth of E. coli was explored. The data suggested that the growth of bacterial colonies was hindered as the lignin concentration was increased. To examine the cytotoxicity of the soft composites, neuroblastoma cells were seeded on the surface of the lignin hydrogels and the growth was monitored over the course of a few days. The results suggested that the kraft lignin presents limited compatibility with the cells, while the hybrid poplar lignin demonstrated biocompatibility at low lignin concentrations. Lastly, the antioxidant assay confirmed that the lignin-based hydrogels can function as an effective antioxidant.

Results from this work emphasize the necessity of exploring a range of lignin molecular weights and utilizing cleaned lignin to elucidate the impact of lignin on the hydrogel’s network structure and the material properties. This work functions as a framework for the fabrication of next-generation, “green” polymeric systems.

Teaching Interests

As an educator, I aim to provide students with an environment that exudes enrichment and never judgement. I plan to be a professor whose students appreciate the course material because they are exposed to real-world applications and implications of the material. As such, my teaching interests center around teaching chemical engineering fundamentals and chemical product design. As part of my career plan, I would like to teach core chemical engineering courses such as thermodynamics and transport phenomena. The principles taught in these courses are often the driving force behind many mechanisms occurring in material fabrication. Additionally, I value the opportunity to teach polymer science. As chemical products such as pharmaceuticals, cosmetics, and adhesives are made from polymers, an understanding of polymeric behavior is beneficial. Finally, through combining those fundamental courses, I will develop a chemical product design course where students can explore the design space from the fundamental scientific requirements to the aesthetic properties added by the excipients used.

Research Interests

Throughout my academic career, I developed an interest in materials research. Specifically, I am interested in fabricating nontoxic materials for consumer use. This includes polymeric materials for use in drug delivery and cosmetics. With experience fabricating and characterizing sustainable hydrogels, I plan to explore options for hydrogel applications. This includes hydrogel wound-dressings, microneedle patches, and transdermal patches. Once I have established progress with hydrogel-based materials, I plan to explore other polymeric systems. This includes polymeric systems that can be used as “fake skin” to replace porcine skin during insertion tests and systems that can be used to improve performance and stability of cosmetics.