Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item The Hydrogel Reimagined: Gel-Derived Sponges and Sheets as Absorbents for Water, Blood, and Oil(2022) Choudhary, Hema; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer hydrogels, i.e., crosslinked networks of polymer chains swollen in water, are well-studied materials. Superabsorbent polymer (SAP) gels that can absorb more than 100x their dry weight in water are widely used in personal hygiene products – but only in the form of microscale beads. If dry SAP gels were larger, they would either take too long to swell or would be brittle solids. This dissertation seeks to reimagine polymer gels in very different physical forms: as soft sponges or foldable, fabric-like sheets. We want these macroscale dry materials to retain the ability to absorb large amounts of liquid, either aqueous or organic. In short, we would like to make polymer gels in convenient, usable forms similar to everyday absorbents like towels and sponges. The key to making gels as macroscale absorbents is to make the gels porous. In our first study, we devised a way to create porous gels by foam-templating. The approach involves in situ foaming of a monomer solution followed by fast polymerization. We generate the foams using a double-barrelled syringe that has acid and base in its two barrels. Gas (CO2) is formed at the mixing tip of the syringe by the acid-base reaction, and gas bubbles are stabilized by an amphiphilic polymer in one of the barrels. The monomers are then polymerized by ultraviolet (UV) light to form the gel around the bubbles, and the material is dried under ambient conditions to give a porous solid. We show that this dry, porous gel absorbs water at a rate of 20g/s until equilibrium is reached at ~ 300x of its weight. This is the fastest swelling and expansion ever achieved by a hydrogel. We convert the chemical potential energy from gel expansion into mechanical work: the gel is able to lift weights against gravity, with a power-density of 260 mW/kg. Next, we synthesize porous gels in the form of large sheets that resemble cloth or paper towels. For this, we polymerize thin films of the foams and ambient-dry the films after plasticization. Our gel sheets are flexible, foldable, and can be cut with scissors like fabrics. At the same time, the sheets absorb more than 30x of their dry weight in various aqueous fluids (water, blood, polymer solutions). Remarkably, these gel sheets expand as they absorb water, unlike any commercial towels. The expanded sheets retain absorbed fluid when lifted upright whereas fluid drips out of commercial absorbent sheets. Because of these superior properties, our gel sheets could be used to absorb aqueous liquids in various settings such as homes, labs and hospitals. Lastly, we design oleo-sheets, which are counterparts to the above that can absorb oils, i.e., non-polar liquids. We synthesize oleo-sheets by templating foams in which the continuous phase is non-aqueous and contains hydrophobic monomers. The oleo-sheets are hydrophobic and can selectively absorb oil from water. They show a high absorption capacity (> 50 g/g) for a range of organic solvents. The sheets can also be made magnetically responsive and an oil-soaked oleo-sheet can be lifted up by a magnet. We also fabricate a ‘Janus omni-absorbent sheet’ that has two sides: one side selectively absorbs water while the other side absorbs oil/solvents. Our oleo-sheets and omni-absorbent sheets could both be used in homes, hospitals, and various industries for cleaning up different spilled liquids.Item Soft Aqueous Structures with Smart Skins or Membranes: Regulating the Release of Solutes(2022) Subraveti, Sai Nikhil; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In nature, various soft materials have a water-rich core covered by a hydrophobic layer, i.e., a membrane or skin. These ‘smart’ or ‘adaptive’ membranes regulate the molecules that can enter or leave the core. Membranes enclose cells (microscale structures) as well as vesicles in the cells (nanoscale structures). At the macroscale, fruits and vegetables, as well as the human body are covered with skins. Currently, researchers are developing many soft, aqueous materials across length scales, including hydrogels, capsules, and vesicles, which are being used in areas such as drug delivery, pharmaceutics, and cosmetics. In this study, we will explore the synthesis of ‘smart’ skins or membranes around these structures with a goal of regulating solute release. In our first study, we put forward a simple technique for synthesizing a hydrophobic skin to cover any hydrogel. An analogy is made to the peelable skin around fruits and vegetables. To make the skin, we employ an inside-out polymerization, where one component of the polymerization (the initiator) is present only in the gel core while other components (the monomers) are present only in the external medium. The thin polymeric skin (~ 10 to 100 µm in thickness) grows outward from the core in a few minutes. We show that the skin prevents the gel from swelling in water and also from drying in air. Hydrophilic solutes are completely prevented from leaking out into the external solution, while harmful microbes are prevented by the skin from attacking the gels. The properties of the skin are tunable, including its thickness and its mechanical properties. A polyurethane skin is elastomeric, transparent, and peelable from the core hydrogel. Conversely, other skins can be hard and brittle (glass-like). Next, we alter the recipe for the skin around hydrogels by incorporating redox-responsive monomers. In the presence of an oxidizing agent, the initially hydrophobic skin becomes hydrophilic, thereby ‘turning on’ the release of solutes out of the gel. The release rate of various solutes can be easily controlled by changing the parameters such as solute loading, skin thickness as well as the concentration of oxidative species in the external medium. Conversely, solute release can also be ‘turned off’ at a later time by adding a reducing agent that reverts the skin to its hydrophobic state. Thus, our smart skin enables the on-off release of solutes out of a gel, and this concept is likely to be useful in many applications. Lastly, we turn our attention to smaller nanocontainers, i.e., vesicles. We have come up with a way to make the membranes of vesicles responsive to multiple stimuli such as reactive oxygen species (ROS), temperature and ultraviolet (UV) light. The vesicle membrane is formed by a combination of cationic and anionic surfactant molecules, and the stimuli alter the geometry of these molecules. In turn, the vesicles are converted into micelles, resulting in the burst release of solutes out of the vesicle core. High-energy radiation used in cancer treatment is known to generate ROS – so, one application of these ‘smart’ vesicles could be in the radiation-induced burst-release of chemotherapeutic drugs, which could increase the effectiveness of cancer treatment.Item Electrically Induced Gelation, Rupture, and Adhesion of Polymeric Materials(2017) Gargava, Ankit; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There has been considerable interest in developing stimuli-responsive soft materials for applications in drug delivery, biosensing, and tissue engineering. A variety of stimuli have been studied so far, including temperature, pH, light, and magnetic fields. In this dissertation, we explore the use of electric fields as a stimulus for either creating new soft materials or for rupturing existing ones. Our materials are based typically on biocompatible polymers such as the polysaccharides alginate, chitosan, and agarose. We also discuss the advantages and disadvantages of electric fields over other stimuli. First, we describe the use of electric fields to form transparent and robust alginate gels around an initial mold made of agarose. Moreover, we can melt away the agarose by heat, leaving us with hollow alginate tubes. In our technique, a tubular agarose mold with dissolved calcium chloride (CaCl2) is placed in a solution of sodium alginate. A voltage of ~ 10 V is then applied, with the mold as the anode and the container as the cathode. As the Ca2+ ions migrate from the mold towards the cathode, they contact the alginate chains at the mold surface. In turn, the Ca2+ crosslinks the alginate chains into a gel, and the gel grows outward with time. The technique can be used to grow multiple layers of alginate, each with a different content, and it is also safe for encapsulation of biological species. Complex tubular structures with multiple branches and specific patterns can be created. Next, we report that electric fields can be used to rupture particles formed by ionic complexation. The particles under study are typically in the microscale (~ 200 µm radius) and are either uniformly crosslinked microbeads (e.g., alginate/Cu2+) or microcapsules formed by complexation of oppositely charged polymers (alginate and chitosan). When these particles are placed in aqueous solution and subjected to an electric field of about 10 V/cm (applied remotely, i.e., electrodes not in contact), the particles rupture within about 5 min. A possible mechanism for the electric-field-induced disruption is discussed. We also use the above particles to create electrically actuatable valves, where the flow of a liquid occurs only when the particle blocking the flow is disrupted by the field. In our final study, we show that polyelectrolyte gels and beads can be rapidly induced to adhere by an electric field. We typically work with crosslinked acrylate hydrogels made with cationic co-monomers, and anionic beads made by contacting alginate with Ca2+. When the cationic gel (connected to an anode) is contacted for just a few seconds with the anionic bead (connected to a cathode) under a voltage of ~ 10 V, the two form a strong adhesive bond. When the polarity of the electrodes is reversed, the phenomenon is reversed, i.e., the gel and bead can be easily detached. We suggest that the adhesion is due to electrophoretic migration of polyelectrolyte chains, resulting in the formation of polyion complexes. Applications of this reversible adhesion are discussed for the pick-up and drop-off of soft cargo, and for the sorting of beads.Item SOFT HYDROGEL BATTERIES: THE DANIELL CELL CONCEPTUALIZED IN HYBRID HYDROGELS(2015) Goyal, Ankit; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energy storage devices such as batteries are important elements in many electronic devices. Currently, researchers are seeking to create new electronic devices that are "soft", i.e., bendable and stretchable. However, the batteries that power such devices are still mostly hard structures. In the current thesis, we have attempted to develop a "soft" battery out of hydrogels. Specifically, we have made a soft version of the Daniell Cell, which is a classic electrochemical cell. Our design involves a hybrid gel composed of three distinct layers. The top and bottom layers are gels swollen with a zinc salt and a copper salt, respectively, while the middle layer is akin to a "salt bridge" between the two. The hybrid gel is made by a polymerization technique developed in our laboratory and it retains good mechanical integrity (i.e., the individual layers do not delaminate). Zinc and copper foils are then attached to the hydrogel, thus creating an overall battery, and its discharge performance is reported. One unique aspect of these gel batteries is that they can be dehydrated and stored in a dry form, whereupon they are no longer batteries. In this inactive state, the materials are safe and light to transport. Upon rehydration, the gels revert to being functional batteries. This concept could be useful for military or other applications where an emergency energy storage is needed.