Shivani Desai, graduate student in the Department of Chemical and Biomolecular Engineering (ChBE) at Lehigh University was chosen to present her rheology research findings as part of the Spring 2024 Society of Rheology (SOR): Future of Rheology Seminar Series.
The selection is through the self nomination to the Society of Rheology and decided by the organizing committee. Each year will have two rounds of abstract solicitations.
Bio
Desai is from Maharashtra, India and graduated with a B.Tech in Textile Engineering from the Institute of Chemical Technology, Mumbai in 2014. After earning her undergraduate degree, she worked as a research assistant at National chemical laboratory in Pune, India. In 2017, Desai enrolled at Rutgers University for a MS in Chemical engineering. During her time at Rutgers (2017-2019), she developed an interest in rheology of complex fluids as she worked on rheology of porous particle suspensions. After she joined Lehigh University in 2019, she continued to use rheology as a tool for her research in the lab group of Associate Professor Kelly Schultz.
Desai's work focuses on characterizing microstructure of covalent adaptable polymeric networks to design such networks for tissue engineering applications. Their group first measured microstructural changes during degradation of covalent adaptable networks with different compositions. This work was accepted in Soft Matter earlier this year. Currently, Desai is working towards characterizing cell mediated degradation in these networks when human mesenchymal stem cells (hMSCs) are encapsulated. The presentation for the Future of Rheology seminar will highlight work from the publication in Soft Matter.
Abstract
Covalent adaptable networks (CANs) are polymeric networks where cross-links can be rearranged in response to applied external chemical or physical stimuli. CANs are being designed for various applications including cell and drug delivery and dissolvable wound dressings. To design CANs for these applications it is important to characterize their degradation since degradation is necessary during use. In this work, we characterize degradation of thioester CANs. These networks are adaptable due to thioester exchange reactions that take place between excess thiol in the network and network cross-links. We form thioester networks between 8-arm poly(ethylene glycol) (PEG) thiol and PEG-thioester norbornene. Scaffolds are made with 0%, 50% and 100% excess thiol. We degrade these networks by incubating them in L-cysteine, which is an amino acid containing a thiol group. L-cysteine will exchange with network cross-links but has a single thiol, which means it will not
participate in network cross-linking. We characterize degradation using multiple particle tracking microrheology (MPT). MPT measures the Brownian motion of fluorescently labeled probe particles embedded in a network. MPT measures rearrangement of each network during degradation. Networks with 50% excess thiol can only form polymeric clusters during rearrangement while networks with 0% and 100% excess thiol form sample-spanning networks during rearrangement prior to degradation.
We then analyze our MPT data with time-cure superposition to calculate the critical relaxation exponent, n, for each network composition. The value of n is related to the network microstructure. The value of n changes when the amount of excess thiol in the thioester network is changed. Networks with 50% excess thiol are the most elastic networks with n = 0.23+0.04, followed by networks with 0% excess thiol with n = 0.34+0.07. Both these networks are elastic in nature. Networks with 100% excess thiol have n = 0.53+0.12, which indicates the network is an ideal, percolated network which is equally viscous and elastic in nature. We also measure equilibrium storage moduli of these networks with bulk rheology and a similar trend is measured. Networks with 0%, 50% and 100% excess thiol have moduli of 390+44 Pa, 504+107 Pa and 281+35 Pa, respectively. Together these results indicate that networks with 50% excess thiol have the highest cross-link density.
Previous work shows that increasing excess thiol can decrease cross-link density. In this work, we measure decreased cross-link density in thioester networks likely due to network non-idealities including loops, disulfide bond formation and unreacted functional groups. We hypothesize this reduced cross- link density results in the trend we measure viscoelastic properties of thioester networks. This work characterizes changes in microstructure along with macroscopic network properties which can help enable better engineering of these thioester networks for delivery applications and as materials that can enhance or restart wound healing.
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