Microporous hydrogels for controlled gas storage and delivery

Abstract

This invention provides hydrogels including a plurality of microporous particles. The hydrogels have improved mass transport of gases for applications in medicine and catalysis.

Description

[0001] MICROPOROUS HYDROGELS FOR CONTROLLED GAS STORAGE AND DELIVERY

[0002] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0003] This invention was made with government support under N00014-22-1 -2739 and N00014-20-1- 2418 awarded by U.S. Office of Naval Research (NAVY / ONR). The government has certain rights in this invention.

[0004] Background

[0005] Hydrogels, a biphasic class of three-dimensional (3D) polymeric networks in water, engage in both inter- and intramolecular hydrogen bonding interactions while maintaining the ability to absorb and retain water. These hydrogels possess a combination of structural flexibility and processability, enabling biomedical and industrial applications such as drug delivery, tissue engineering, 3D printing, and cosmetics.

[0006] While a hydrogel’s high water content and modifiable polymer matrix makes them well-suited for sequestering and administering various molecular species in a target environment, it remains a challenge to store workable loads of gases within the hydrogel matrix. This is due to the limited solubility of gases in water and the intrinsic properties of the polymer precursors involved, which leaves achieving a high gas-carrying capacity inherently challenging. While previous approaches have focused on creating macro- to mesoscale pores or integrating reaction cascades within hydrogels, it has proven difficult to establish well-distributed and permanent dry microporosity that could enable efficient gas release and uptake.

[0007] Accordingly, there is a need for microporous hydrogels with improved gas-carrying capacity.

[0008] Summary of Invention

[0009] The invention provides microporous hydrogels made through incorporation of a porous aqueous liquid within a polymeric hydrogel matrix. Porous aqueous liquids are liquids that exhibit fluidity, permanent microporosity, and high gas-carrying capacities made of uniform, stable dispersions of colloidal nanocrystals with hydrophobic pore networks that thermodynamically preclude the intrusion of water, e.g., dry pores. The preservation of the dry microporous networks in water allows for the physisorption of gas molecules at higher densities compared to that of pure water. Incorporation of porous aqueous liquids into hydrogels combines structured, hydrophobic porosity with the physicochemical properties of a gel, and the resulting microporous hydrogels exhibit enhanced gas-carrying capacities and sustained gas release.

[0010] In one aspect, the invention features a hydrogel including a plurality of microporous particles having pores with an internal surface, wherein the pores are sized to allow entry of a gas, and having an external surface, wherein the hydrogel further comprises a gelling agent and a solvent. In some embodiments, the plurality of microporous particles includes a metal-organic framework (MOF), a zeolite, or a covalent-organic framework (COF). In some embodiments, the metalorganic framework includes a zeolitic imidazole framework (ZIF) or a zirconium-based MOF. In some embodiments, the zeolitic imidazole framework is ZIF-8 or ZIF-67. In some embodiments, the zirconium-based MOF includes an MIL-type MOF (Materials of Institute Lavoisier framework). In some embodiments, the MIL-type MOF includes an MIL-140, e.g., MIL-140A, MIL-140B, or a combination thereof. In some embodiments, the zeolite has a Mobil-five (MFI) structure type. In some embodiments, the MFI structure type comprises an atom percent ratio of silicon to aluminum between 1 to 0 and 20 to 1. In some embodiments, the zeolite is silicalite-1.

[0011] In some embodiments, the external surface is covalently bound to a polymer. In some embodiments, the external surface is non-covalently bound to the polymer. In some embodiments, the polymer includes polyethylene glycol (PEG) or a polysaccharide. In some embodiments, the polymer includes ethylene glycol, propylene glycol, or butylene glycol. In some embodiments, the polyethylene glycol has a molecular weight between 100 g / mol and 30000 g / mol.

[0012] In some embodiments, the external surface is hydrophobic. In some embodiments, the external surface is hydrophilic. In some embodiments, the internal surface is hydrophobic. In some embodiments, the external surface includes a hydrophilic, hydrophobic, or amphiphilic coating.

[0013] In some embodiments, the gelling agent includes an amphipathic hydrogen-bonding molecule that noncovalently links the plurality of microporous particles within the hydrogel, e.g., by hydrogen bonding with a polymer bound to the external surface. In some embodiments, the amphipathic hydrogen-bonding molecule includes a C1-10 alkane substituted with one or more hydrogen bonding groups such as amine, hydroxyl, carboxy, or amide, e.g., an alkylamine. In some embodiments, the alkylamine includes methylamine, ethylamine, propylamine, butylamine, pentylamine, or hexylamine.

[0014] In some embodiments, the gelling agent includes a polymer matrix. In some embodiments, the polymer matrix includes polymers of 2-hydroxyethylmethacrylate, 1-vinyl-2-pyrrolidinone, ethylene glycol dimethacrylate, a compound of the formula ° , wherein n is an integer between 2 and 14, a compound of the formula , wherein n is an integer between 2 and 14, or a combination thereof. In some embodiments, the polymer matrix forms upon exposure to light. In some embodiments, the polymer matrix noncovalently entraps the plurality of microporous particles.

[0015] In some embodiments, the microporous particles include nanoparticles, microparticles, or a combination thereof.

[0016] In some embodiments, the hydrogel further includes the gas located in the pores of the microporous particles. In some embodiments, the gas is argon, oxygen, nitrogen, carbon dioxide, carbon monoxide, xenon, methane, or hydrogen.

[0017] In some embodiments, the solvent is aqueous.

[0018] In another aspect, the invention provides a method of storing a gas in a hydrogel by providing a hydrogel described herein; and dissolving the gas in the hydrogel, thereby storing the gas in the pores. In another aspect, the invention provides a method of delivering a gas to a substrate by providing a hydrogel described herein and having the gas in the pores; and contacting the hydrogel with the substrate thereby delivering the gas to the substrate. The gas may include argon, oxygen, nitrogen, carbon dioxide, carbon monoxide, xenon, methane, helium, neon, or hydrogen.

[0019] Definitions

[0020] By “about” is meant ± 10% of the specific value.

[0021] By “hydrogel” is meant a biphasic class of three-dimensional (3D) polymeric networks in water that engage in both inter- and intramolecular hydrogen bonding interactions while maintaining the ability to absorb and retain water.

[0022] By “gas storage capacity” is meant the ability of a composition to store gas molecules within the composition. Gas storage capacity may be given in units of mmol / g / bar, referring to the amount of gas stored per mass of the storage media, per unit pressure. Gas storage capacity may vary with the gas to be stored, depending on the chemistry of said gas (e.g., size, polarity, etc.).

[0023] By “microparticle” is meant a particle having a diameter between 1 and 1000 micrometers.

[0024] By “microporous” is meant have an average pore diameter in the range of 3 A to 20 A.

[0025] By “nanoparticle” is meant a particle having a diameter between 1 and 1000 nanometers.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 shows an illustration comparing microporous water to microporous hydrogel. The release of trapped gas within the pores of particles in microporous water is rapid, whereas the release of trapped gas within the pores of particles in microporous hydrogel is slowed. FIGS. 2A-2F show hydrophobic features of the ZIF-8 surface and PEGylation of ZIF-8 nanocrystals for stable aqueous colloidal dispersions. (2A) The geometric structure of methyl imidazole-terminated ZIF-8 surfaces. (2B) Schematic illustration of the pristine ZIF-8 surface that is primarily hydrophobic. (20) Image of aggregated pristine ZIF-8 when dispersed in water. (2D) Reaction conditions for PEGylation on the surface imidazole ligands with methoxy polyethylene epoxide (mPEG; Mn = 750 g / mol). (2E) Schematic illustration of the mPEG functionalized ZIF-8 (mPEG-ZIF-8) surface with enhanced hydrophilicity at the nanocrystal surface. (2F) Image of mPEG-ZIF-8 dispersed in water.

[0028] FIGS. 3A-3D show the structural integrity and solid-state gas capacities of ZIF-8 nanocrystals after functionalization with mPEG. (3A) Low-magnification SEM image of mPEG-ZIF-8 nanocrystals. Inset: high-magnification SEM image of the corresponding SEM image. (3B) Powder X-Ray diffraction (PXRD) patterns of ZIF-8 nanocrystals before and after the functionalization. (30) N2 isotherms at 77 K for ZIF-8 nanocrystals before and after the functionalization. (3D) O2 isotherms at 293.15 K for ZIF-8 nanocrystals before and after the functionalization.

[0029] FIGS. 4A-4B show permanent dry microporosity and O2 carrying capacities of aqueous ZIF-8 liquid. (4A) Solution density (black circles) as a function of concentration for colloidal solutions of ZIF-8 nanocrystals in water. (4B) The amount of O2 released by an oxygenated aqueous solution of ZIF-8 nanocrystals relative to the theoretical amount calculated by assuming fully dry pores with O2 capacities equivalent to those measured in the solid state.

[0030] FIGS. 5A-5F show schematic illustration of interfacial chemical environments on the surface of mPEG-ZIF-8 in solution conditions. (5A) Hydrophilic nature of mPEG-ZIF-8 surface in aqueous conditions. (5B) Balancing hydrophilicity and hydrophobicity on the surface of mPEG-ZIF-8 in mixed water (W) and propylamine (PA) conditions. (50) PA-dominant mPEG-ZIF-8 surface in more PA-concentrated aqueous conditions. Computed intermolecular interaction energies and NCI surfaces (isodensity value = 0.04 a.u.) of water and propyl amine with (5D) the ZIF-8 cluster model with an open Zn-site ([open-ZIF-8]), (5E) the ZIF-8 cluster model ([ZIF-8]), and (5F) the mPEG-ZIF-8 cluster model ([mPEG-ZIF-8]). The solvation effect of the solvent medium (water, E = 78.4) was incorporated using the conductor-like polarizable continuum model (CPCM). Relative free energy differences (kcal mol-1) are in parenthesis.

[0031] FIGS. 6A-6D show molecular dynamics simulations and hydrogelation of aqueous ZIF-8 liquid. (6A) A snapshot of the initial state. (6B) A snapshot of the equilibrium state. (6C) Phase diagram for the sol-gel transition of aqueous ZIF-8 dispersion with respect to varying ZIF-8 concentration. Across all concentrations, 0.5 M propylamine was added to be 0.04 volumetric equivalent of the total dispersion volume. (6D) Phase diagram of the mPEG-ZIF-8 aqueous dispersion with respect to varying propylamine concentrations. A dispersion of 5 vol%, which was determined to be a lower bound for gelation, was selected for hydrogelation tests across all propylamine concentrations. All amines were added at a 0.04 volumetric equivalent of the total dispersion volume. Circles indicate successful hydrogelation, triangles partial gelation, and the red x’s minimal change in viscosity.

[0032] FIGS. 7A-7B show rheological characteristics of hydrogels created with aqueous mPEG-ZIF-8 fluids. (7A) Strain sweep and (7B) frequency sweep of mPEG-ZIF-8 hydrogels at 4.5 vol% and 6.7 vol% nanocrystal concentration. As expected of colloidal nanocrystals, the structural elasticity of the hydrogels depends largely on the volumetric concentration of the nanocrystals.

[0033] FIGS. 8A-8C show O2 release measurements of hydrogels. (9A) Image of the hydrogel O2 release set up. (9B) Schematic illustration of the gas release through the microporous hydrogel. (90) O2 release profile of the microporous gel and its liquid counterpart. Across an average of three measurements, the gel sample shows delayed release into its deoxygenated surroundings for up to ten hours.

[0034] FIG. 9 shows a chemical reaction with polymer precursors 2-hydroxyethylmethacrylate (HEMA), 1-vinyl-2-pyrrolidinone (NVP), and ethylene glycol dimethacrylate (EGDMA) initiated using light in the presence of a metal-organic framework (MOF) to form a MOF-contained hydrogel.

[0035] FIG. 10 shows an assembled O2 measurement wherein O2 trapped within a hydrogel submerged in water is allowed to escape, and O2 is measured with a Calvin probe, an O2 sensor.

[0036] FIG. 11 shows oxygen released over time from a microporous hydrogel including around 35 vol% ethylene glycol diacrylate polymer, around 60 vol% water, and around 5 vol% Zr-based MOF. The first measurement was performed on a hydrogel that had not been previously subjected to oxygenation. The second measurement was performed after soaking and shaking the hydrogel in deoxygenated water for 10 minutes. The third measurement was performed after soaking in water that had been bubbled with N2 gas for 1 day. The fourth measurement was performed after resoaking the hydrogel in air-equilibrated water for 1 day.

[0037] FIG. 12 shows an SEM image of pristine ZIF-8 nanoparticles. The scale bar represents 1 pm.

[0038] FIGS. 13A-13B show1H NMR and MS spectra of mPEG-ZIF-8.

[0039] FIG. 14 shows particle size distributions determined by dynamic light scattering (DLS) for mPEG-ZIF-8 dispersed in water. The graph shows that between the traces marked t = 0 and t = 1 week, there is minimal change in the distribution of diameters, indicating the diameter of the particles in the suspension remain unchanged over one week.

[0040] FIG. 15 shows the computed intermolecular interaction energies and NCI surfaces (isodensity value = 0.04 a.u.) of water and propyl amine with the isomer of mPEG-ZIF-8 cluster model ([iso- mPEG-ZIF-8]). The solvation effect of the solvent medium (water, E = 78.4) was incorporated using the conductor-like polarizable continuum model (CPCM). Relative free energy differences (kcal mol-1) are in parenthesis.

[0041] FIGS. 16A-16D show a molecular dynamics simulation of a covalently bound PEG chain attached to the external surface of a computationally generated ZIF-8 model, showing the chain and water without contact to the ZIF-8 surface (FIG. 16A) evolving to a final state after 300 picoseconds (FIG. 16B). After 300 picoseconds, the energy of the system appears to converge (FIG. 16C), and the cell volume remains constant and without intrusion of water (FIG. 16D). Hence, the ethylene glycol chain has attained a stable configuration, and water has been excluded from the pores.

[0042] FIG. 17 shows the thermogravimetric analysis (TGA) of (mPEG)ZIF-8 hydrogels at two representative concentrations. Both hydrogels show water content in agreement consistent with the expected values calculated from their respective volumetric (mPEG)ZIF-8 content (inset).

[0043] FIG. 18 shows the oxygen release profile of (mPEG)ZIF-8 liquid dispersion at 6.8 vol%. Unlike the hydrogel counterpart, the liquid sample shows approximately 86% of its capacity released over the same time period of 10 hours.

[0044] FIGS. 19A-19B show the stability of the (mPEG)ZIF-8 hydrogels formed with an amine gelator. (FIG. 19A) The stability of the microporous crystalline structure within the gel over a week is demonstrated by powder X-ray diffraction and scanning electron microscopy images (inset) and (FIG. 19B) surface area measurements after adding varying amounts of amines as a hydrogelator.

[0045] FIG. 20 shows the synthesis of MIL-140A and MIL-140B. MIL-140A composed of Zr ions and benzene-1 ,4-dicarboxylic acid (BDC). MIL-140B composed of Zr ions and naphthalene-2,6- dicarboxylic acid (NDC).

[0046] FIGS. 21A-21D show Zr-based MOF materials. (FIG. 21A) The crystal structure of MIL-140A. (FIG. 21B) The crystal structure of MIL-140B. (FIG. 21C) The simulated and experimentally measured PXRD patterns of MIL-140A. (FIG. 21 D) The simulated and experimentally measured PXRD patterns of MIL-140B.

[0047] DETAILED DESCRIPTION OF THE INVENTION

[0048] The invention provides microporous hydrogels made through incorporation of a porous aqueous liquid within a polymeric hydrogel matrix. Porous aqueous liquids are liquids that exhibit fluidity, permanent microporosity, and high gas-carrying capacities made of uniform, stable dispersions of colloidal nanocrystals with hydrophobic pore networks that thermodynamically preclude the intrusion of water, e.g., dry pores. The preservation of the dry microporous networks in water allows for the physisorption of gas molecules at higher densities compared to that of pure water. Incorporation of porous aqueous liquids into hydrogels combines structured, hydrophobic porosity with the physicochemical properties of a gel, and the resulting microporous hydrogels exhibit enhanced gas-carrying capacities and sustained gas release. The microporous hydrogel also features slower gas release in contrast to the porous aqueous liquid, which is beneficial for controllable dosing of gas to a target environment (FIG. 1).

[0049] While some hydrogels have been used in oxygen generation and transport by incorporating algae, peroxide, perfluorocarbons, and hemoglobin into the gel matrices, they suffer from poor stability and carrying capacities, often requiring high loadings of carrier substrates. Others mostly rely on the gas generation of the targeted species, which often limits their versatility to carry a wide range of gas molecules and is often capped in their carrying capacity due to the generation of peroxides and other unwanted byproducts. A rational design strategy for transforming porous aqueous liquids into microporous hydrogels by manipulating surface chemical environments of colloidal nanocrystals without cross-linking networks of covalent or ionic polymers is disclosed herein. The resulting microporous hydrogels show that preserving permanent dry porosity within hydrogels can lead to an order of magnitude increase in gas storage and release in hydrogels, which is not feasible with conventional hydrogels.

[0050] The invention provides a hydrogel including a plurality of microporous particles. The invention includes any microporous particles, such as zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs), that can be synthesized with hydrophobic pore surfaces, e.g., in nanocrystalline or microporous particle form. The great range and variety of such materials makes these materials an advantageous — and highly tunable — platform to provide liquid (e.g., aqueous) solutions with permanent porosity. Exemplary microporous particles are known in the art (see, e.g., WO 2022 / 076883, the materials of which are incorporated herein by reference). Certain embodiments of the invention include the use of various MFI-type zeolite nanoparticles, both in pure silica form (known commonly as “silicalite-1 ”) and in Al-containing form (known as “ZSM-5”). In some embodiments, the zeolite is silicalite-1 or ZSM-5. In some embodiments, the MFI-type zeolite includes an atom percent ratio of silicon to aluminum, e.g., 1 to 0 (pure silica), 20 to 1 (20 atomic equivalents of silica to one atomic equivalent of aluminum), 40 to 1 , 60 to 1, 80 to 1, 100 to 1, between 1 to 0 and 100 to 1, between 100 to 1 and 80 to 1, between 80 to 1 and 60 to 1 , between 60 to 1 and 40 to 1 , between 40 to 1 and 20 to 1 , and between 1 to 0 and 20 to 1. In some embodiments, the metal-organic framework is a zeolitic imidazole framework, e.g., ZIF-8, ZIF-67. In some embodiments, the metal organic framework is a zirconium-based framework, e.g., an MIL-type MOF. In some embodiments, the MIL-type MOF includes an MIL-140, e.g., MIL-140A, MIL-140B, or a combination thereof.

[0051] Microporous particles may also include activated carbon or amorphous porous silica particles. Microporous particles feature internal networks of angstrom-sized pores that lead to high internal surface areas, often exceeding 1 ,000 m2per g or mL of material. Porous particles of the invention may have average pore diameters of between about 3 A and about 20 A, e.g., between about 3-5 A, 4-6 A, 4-10 A, 5-10 A, 5-15 A, 6-8 A, 7-9 A, 9-11 A, 10-12 A, 10-15 A, IQ- 20 A, 12-15 A, 13-18 A, 14-18 A, 17-19 A, 18-20 A, or about 19-20 A, e.g., about 5 A, about 10 A, about 15 A, or about 20 A. Even when surface interactions with gas molecules are relatively weak, these high internal surface areas concentrate gas molecules to densities that surpass those which are possible in a conventional liquid and in the bulk gas phase — even after accounting for the space occupied by the atoms framing the pore. By preventing liquid, e.g., water molecules, from entering these pores, the invention provides porous liquids and brings the high gas capacities of porous materials to gels, e.g., hydrogels. In some embodiments, the microporous particles may include nanoparticles, microparticles, or a combination thereof.

[0052] Microporous particles of the invention may range in cross-sectional dimension (e.g., diameter) from about 5 nm to about 1000 nm, e.g., between about 5-100 nm (e.g., about 5-10 nm, 5-15 nm, 5-25 nm, 10-20 nm, 25-50 nm, 20-40 nm, 30-60 nm, 50-75 nm, 60-80 nm, 75-100 nm, 70- 90 nm, 80-95 nm, or 90-100 nm) or about 100-1000 nm (e.g., about 100-150 nm, 120-160 nm, 140-180 nm, 150-200 nm, 100-200 nm, 100-300 nm, 200-500 nm, 250-750 nm, 300-400 nm, 350-650 nm, 400-500 nm, 400-600 nm, 500-750 nm, 600-800 nm, 750-1000 nm, 600-950 nm, 700-900 nm, 800-1000 nm, or 900-1000 nm). The cross-sectional diameters may be measured by dynamic light scattering (DLS). The porous particles may account for less than 0.1 vol % or up to 90 vol % of the composition, for example between about 0.01 vol % to about 90 vol %, e.g., about 0.01 to about 1 vol % (e.g., about 0.01-0.05 vol %, 0.02-0.07 vol %, 0.04-0.09 vol %, 0.05-0.1 vol %, 0.06-0.12 vol %, 0.1-0.15 vol %, 0.1-0.2 vol %, 0.1-0.5 vol %, 0.2-0.6 vol %, 0.3- 0.7 vol %, 0.4-0.9 vol %, 0.5-0.9 vol %, 0.5-1 vol %, or 0.9-1 vol %) or, e.g., about 1 vol % to about 10 vol % (e.g., about 1-2 vol %, 1-3 vol %, 1-4 vol %, 2-5 vol %, 2-6 vol %, 3-7 vol %,4-8 vol %, 5-7 vol %, 5-10 vol %, 6-9 vol %, 7-10 vol %, 8-10, or 9-10 vol %), or about 10 vol % to about 90 vol (e.g., about 10-15 vol %, 10-20 vol %, 10-50 vol %, 15-45 vol %, 25-50 vol %, 30- 60 vol %, 40-80 vol %, 50-75 vol %, 50-90 vol %, 60-90 vol %, 65-85 vol %, 70-85 vol %, 75-90 vol %, or 80-90 vol %). In some embodiments, the microporous particles may be uniformly dispersed throughout the hydrogel. In some embodiments, the concentration of the microporous particles may be variable throughout the hydrogel. The microporous particles have pores with an internal surface, wherein the pores are sized to allow entry of a gas.

[0053] The invention also provides covalent surface functionalization approaches to producing porous particles which offer the potential for strongly bound and precisely located surface ligands that promote dispersibility and / or increased capability with a polymer membrane at lower loadings than more weakly associated surface ligands. Other surface chemistries are known in the art. The microporous particles have an internal and external surface, and the external surface may be functionalized with a polymer. The external surface may be covalently or non-covalently bound to the polymer.

[0054] In some embodiments, the polymer includes polyethylene glycol. The polyethylene glycol may have a molecular weight between 100 g / mol and 30000 g / mol, e.g., about 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1750, 2000, 2500, 3000, 4000, 5000, 7500, 10000, 12000, 15000, 20000, 25000, or 30000 g / mol, or between 100-150, 150- 200, 200-250, 250-300, 300-350, 350-400, 400-450, 450-500, 500-550, 550-600, 600-650, 700- 750, 750-800, 800-850. 850-900, 900-950, 950-1000, 1000-1050, 1050-1100, 1100-1150, 1150- 1200, 1200-1250, 1250-1300, 1300-1350, 1350-1400, 1400-1450, 1450-1500, 1500-1750, 1750-2000, 2000-2500, 2500-3000, 3000-4000, 4000-5000, 5000-7500, 7500-10000, 10000- 12000, 12000-15000, 15000-20000, 20000-25000, or 25000-30000 g / mol. In some embodiments, the polymer includes ethylene glycol, propylene glycol, or butylene glycol. Noncovalent surface functionalization with macromolecules such as polyethylene glycol (PEG) represents one approach for dispersing nanocrystals in solvents that would otherwise induce aggregation and precipitation. In some embodiments, the polymers occupy between 0% and 10% of the pore volume, e.g., less than 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10 of the pore volume, such that 90% or more of the pore volume is available for gas occupancy.

[0055] In some embodiments, the external surface is hydrophobic. Alternately, the external surface may be hydrophilic. In some embodiments, the internal surface is hydrophobic.

[0056] In some embodiments, the microporous particles include a hydrophilic, hydrophobic, or amphiphilic coating.

[0057] The hydrogel includes a gelling agent to form the gel. In one embodiment, the gelling agent is an amphipathic hydrogen-bonding molecule. An amphipathic hydrogen-bonding molecule may be added to the microporous particles to form a hydrogel. The amphipathic hydrogen-bonding molecule may noncovalently link the microporous particles within the hydrogel, e.g., by hydrogen bonding with a polymer bound to the external surface. In some embodiments, the amphipathic hydrogen-bonding molecule includes a C1-10 alkane substituted with one or more hydrogen bonding groups such as amine, hydroxyl, carboxy, or amide, e.g., an alkylamine. The alkylamine may include methylamine, ethylamine, propylamine, butylamine, pentylamine, or hexylamine. The amphipathic hydrogen-bonding molecule may have a concentration in solvent, e.g., water, between 0.1 M and 1.0 M before addition, e.g., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8. 0.9 or 1.0 M. A volume of the amphipathic hydrogen-bonding molecule may be added to a suspension of microporous particles, wherein the volume of the amphipathic hydrogen-bonding molecule that is added is between 1% and 10% of a total volume of the suspension, e.g., 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% of the suspension. In another embodiment, the gelling agent is a polymer. Common polymers that are used to form hydrogels include, but are not limited to, hyaluronic acid, chitosan, heparin, alginate, gelatin, fibrin, polyvinyl alcohol, polyethylene glycol, sodium polyacrylate, acrylate polymers and copolymers thereof. In some embodiments, the hydrogel includes a polymer matrix. The polymer matrix may include monomers including , e.g., 2-hydroxyethylmethacrylate, 1 -vinyl-2- pyrrolidinone, ethylene glycol dimethacrylate, a compound of the formula , wherein n is an integer between 2 and 14, a compound of the formula , wherein n is an integer between 2 and 14, or a combination thereof. In some embodiments, the polymer matrix forms upon exposure to light. In some embodiments, the polymer matrix noncovalently entraps the plurality of microporous particles.

[0058] In some embodiments, the hydrogel may include a gas located in the pores of the microporous particles, e.g., argon, oxygen, nitrogen, carbon dioxide, carbon monoxide, xenon, methane, or hydrogen.

[0059] In some embodiments, the solvent is aqueous. The hydrogel may include between 1 and 90 wt% water, e.g., 1, 2, 5, 10, 12, 15, 20, 25, 30, 40, 50, 60, 70, 80, or 90 wt% water, or 1-2, 2-5, 5-10, 10-12, 12-15, 15-20, 20-25, 25-30, 30-40, 40-50, 50-60, 60-70, 70-80, or 80-90 wt5 water.

[0060] The invention also provides methods of storing and releasing a gas. The high gas capacities of microporous hydrogels allow for in vitro or in vivo gas, e.g., O2, delivery.

[0061] In addition to oxygen, other gases may be employed with the invention including argon, nitrogen, carbon dioxide, carbon monoxide, xenon, methane, helium, neon, or hydrogen. The hydrogels may be used to deliver or store any such gas for any appropriate purpose. For example, the hydrogels may be employed to increase volumetric mass transfer of a gas in a mass transfer limited process, e.g., catalysis, such as electrocatalysis. A hydrogel may be equilibrated with a desired gas to load it into the pores. Placement of the hydrogel in an environment deficient in the gas will lead to net release and delivery of the gas from the pores.

[0062] Examples

[0063] Example 1 - Synthesis and fluidization of ZIF-8 nanocrystals.

[0064] ZIF-8, composed of zinc ions and 2-methyl imidazolate ligands, was chosen due to its chemical tunability and stability. ZIF-8 is known for its straightforward colloidal nanocrystal synthesis and a largely linker-terminated surface, which, in turn, facilitates chemical modification at the MOF surface and the subsequent integration into a hydrogel system without extensive structural modification (FIG. 2A). These properties of ZIF-8 allow for preserved crystallinity and, upon successful modification of its surface without polymer or solvent intrusion in its pores, dry porosity in water over extended periods of time. ZIF-8 nanocrystals with a size of about 90 nm were synthesized based on a colloidal synthesis (FIG. 12). When introduced into the water at a concentration of 40 mg / 1 ml_, ZIF-8 nanocrystals undergo rapid aggregation and sedimentation, confirming that the synthesized nanocrystal surface is primarily terminated by the hydrophobic 2-methylimidazole moieties rather than open zinc sites (FIGS. 2B and 2C). The surfaces of the ZIF-8 nanocrystals were covalently functionalized with polyethylene glycol (PEG) to achieve a stable aqueous colloid, as PEGylation of nanocrystals has been widely adopted for enhancing the dispersibility, biocompatibility, and stability of nanoparticles in aqueous media. Methoxy polyethylene epoxide (mPEG; Mn = 750 g / mol) was used to form a [3-hydroxyalkyl bond with exposed imidazolates via epoxide ring-opening reactions, with the expectation that the PEG could graft onto the surface of the ZIF-8 nanocrystals, as shown in FIG. 2D and 2E. The formation of the expected P-hydroxyalkyl covalent bond could be confirmed by digestion NMR and mass spectrometry (FIGS. 13A-13B). The functionalization also translated to enhanced aqueous colloidal stability in that upon the addition of water, the mPEG-functionalized ZIF-8 (mPEG-ZIF-8) exhibited excellent dispersion in aqueous media in contrast with the poor dispersibility observed with pristine ZIF-8 in water (FIG. 2F). Dynamic light scattering (DLS) measurements demonstrated the sustained dispersibility of mPEG-ZIF-8 in fluids over seven days, indicating the robustness of the material and its functionalization strategy (FIG. 14).

[0065] In order to validate the integrity of mPEG-ZIF-8 nanocrystals, both pristine ZIF-8 and mPEG- ZIF-8 samples were examined with scanning electron microscopy (SEM), PXRD, Brunauer- Emmett-Teller (BET), and O2 isotherm analyses, as depicted in FIGS. 3A-3D. The SEM images indicated that mPEG-ZIF-8 features rhombic dodecahedral-shaped nanocrystals, preserving its morphological integrity when compared to that of pristine ZIF-8 (FIG. 3A and FIG. 12). The PXRD patterns of mPEG-ZIF-8 were nearly identical to those of ZIF-8 (FIG. 3B). In addition, we found that pristine ZIF-8 and mPEG-ZIF-8 samples showed comparable BET surface areas and O2 capacities, as shown in FIGS. 3C and 3D. These results indicate that the covalent mPEG functionalization via epoxide ring opening is an effective way to enhance the dispersibility and colloidal stability of ZIF-8 nanocrystals in water without compromising their structural integrity and solid-state gas capacities.

[0066] All chemicals are commercially available and were used without further purification. Zn(NO3)2-6H2O, 2-methylimidazole, propylamine, isopropylamine, and triethylamine were purchased from Sigma-Aldrich and used as received. Methoxypolyethylene epoxide (PEG average Mn 750) was purchased from both Sigma-Aldrich and Biopharma PEG. Zn(NO3)2-6H2O (2.9 g, 9.75 mmol) and 2-methylimidazole(6.5 g, 79.2 mmol) were each dissolved in 90 mL of methanol. The two solutions were then mixed rapidly and stirred at 500 rpm for 1 h at room temperature. The solution was then centrifuged for 25 minutes at 7830 rpm (7197 ref) and the supernatant was decanted. The nanocrystal sediment was then redispersed in fresh methanol, vortexed, then stirred for at least an hour, and centrifuged. The process was repeated four times to ensure that no precursors remained in the nanocrystal pores.

[0067] Synthesized ZIF-8 nanocrystals were used as-dispersed in methanol. 80 mg of the ZIF-8 nanocrystals were mixed with methoxypolyethylene epoxide (mPEG; PEG average Mn= 750 g / mol; 300 mg) in 4 mL of methanol in a 20 mL scintillation vial. Upon full dissolution, 50 pL of 0.1 M NaOH was added to the mixture to facilitate the reaction between the surface 2- methylimidazole and the epoxide moiety of mPEG. After stirring for an hour at room temperature, the reaction was then heated at 45 °C for 72 h at 450 rpm. Upon completion of the reaction, the mixture was washed with methanol under centrifugation four times. The resulting functionalized nanocrystals were dried in vacuo and stored under ambient conditions.

[0068] Powder X-ray diffractions (XRD) were conducted using a Bruker D2 Phaser diffractometer equipped with CuKa radiation (A = 1 .54 A) at 40 kV and 40 mA with a scan rate of 3 7min. Samples were first ground and were loaded on a MTI zero diffraction plate for all measurements.

[0069] DLS was used to monitor the mPEG-ZIF-8 particle size distribution over time in water. Measurements were performed using a Malvern Zetasizer Ultra. We note that concentrated samples were diluted before measurement with MilIQ nanopure water to acquire accurate size distribution.

[0070] Density measurements on mPEG-ZIF-8 aqueous dispersions were performed using an Anton Paar DMA 4100 variable temperature density meter. Each measurement was taken by injecting approximately 1 .1 mL of sample into the cell. Between each measurement, the sample cell was cleaned by purging with MilliQ nanopure water and drying with an air pump.

[0071] All measurements of O2release into deoxygenated water were performed with a Unisense MicroRespiration O2microsensor. The sensor was calibrated through a 2-point calibration in which it was first immersed in fully deoxygenated water that has been sparged with N2for at least 1.5 h to obtain the mV that corresponds to 0 mg / L. The sensor was then placed in an airequilibrated water of known temperature and pressure to calibrate the mV reading to its corresponding O2concentration in mg / L.

[0072] Upon calibration, the measurement for dispersed liquid mPEG-ZIF-8 samples was taken by first filling an air-tight chamber of known volume equipped with two injection ports with fully deoxygenated water. Then, the O2probe was gently lowered through one of the ports to obtain the baseline reading of the deoxygenated system. Then, a known volume of a mPEG-ZIF-8 dispersion was injected into the chamber, and the corresponding rise in O2 was recorded to calculate the amount released into water. We note that the temperature was kept constant throughout the measurement by placing the sensor set-up inside a water bath.

[0073] Hydrogel samples were measured by first loading the hydrogel of desired concentration into a glass disc of 3.5 mm in height and 12 mm in diameter. Once loaded, the disc was topped with a round glass slide of 13 mm, making sure no sample gaps were present in the disc that could potentially affect the release properties. The loaded disc was then carefully transferred to an airtight chamber, on top of which deoxygenated water was gently layered. A degassed samarium-cobalt Teflon stir bar was then added to the chamber, and the probe was lowered through the top port. The setup was kept inside a water bath throughout the duration of the measurement for temperature control. The O2 release from the sample was measured up to approximately 16 hours, with an acquisition interval of 1 second. The final value was used to obtain the experimental O2 released in the given measurement period.

[0074] Scanning electron microscopy (SEM) (Zeiss Gemini 560 FE-SEM) was used to obtain high- resolution images of ZIF-8 nanocrystals. To prepare samples for SEM imaging, Si wafers were first cleaned by sonication in organic solvent (isopropanol and acetone) and dried with a stream of N2. A given aqueous colloidal solution was diluted with methanol and subsequently drop-cast onto the cleaned Si wafers. In the case of mPEG-ZIF-8 hydrogels, samples were not diluted but were directly spin coated onto the Si wafers. To facilitate imaging and reduce charging, samples were sputter coated with a 5 or 10 nm layer of 80:20 Pt:Pd.

[0075] Example 2 - Measuring porosity and gas carrying capacity of aqueous ZIF-8 fluids.

[0076] Density measurements were employed to estimate the hydration status of the ZIF-8 micropores when dispersed in water based on the crystallographic density (0.95 g mL1) and the experimentally measured pore volume (0.66 mL g1) of ZIF-8 (FIG. 3C).Given the low density of ZIF-8 nanocrystals, if the nanocrystals maintained their dry microporosity in aqueous media, the fluid densities of ZIF-8 dispersions should be lower than that of bulk water. Density measurements of mPEG-ZIF-8 dispersions ranging from 3.0 v / v% to 7.5 v / v% showed solution density values that corresponded to, on average, a 5.9 % pore filling by water across all concentrations measured, indicating the preservation of dry pores in aqueous conditions (FIG. 4A). Upon confirmation of dry pore networks, the gas storage and release capabilities of the porous dispersion were investigated through O2 release measurements using a deoxygenated water chamber and an O2 electrode, which enables the direct assessment of O2 carrying capacity. Aqueous dispersions of mPEG-ZIF-8 at varying concentrations were injected into a known volume of deoxygenated water, resulting in an immediate increase in O2 levels (FIG. 4B). The enhancement is significantly greater than what is expected for pure water at the same volume and is nearly identical to the anticipated solid-state O2 capacity of mPEG-ZIF-8, showcasing the ability of the preserved dry pore networks to store and release gas efficiently.

[0077] Solid pristine and functionalized samples were prepared by drying the as-synthesized, fully washed sample in vacuo at 50 °C for 1 hour and then at ambient temperatures overnight. For post-dispersion and hydrogel samples, the samples were either lyophilized or dried in a conventional oven at 130 °C overnight. All resulting solid samples were then loaded onto a glass sample tube and activated under vacuum at 120 °C for at least 20 h using a Micromeritics SmartVacPrep equipped with a turbomolecular pump. Solid state gas isotherms were measured using a Micromeritics MicroActive 3Flex 3500. Braunauer-Emmett-Teller (BET) surface areas and pore volumes for all samples were obtained via N2adsorption isotherms at 77K. Isotherm measurements at ambient temperatures were performed both using a recirculating dewar connected to an isothermal bath and Micromeritics ISO Controller for temperature control.

[0078] Example 3 - Synthesis of the microporous hydrogel via one-step gelation of aqueous ZIF-8 liquid.

[0079] Hydrogelation relies on striking a delicate balance between hydrophobic and hydrophilic interactions to enable effective self-assembly in aqueous environments. Moreover, the manipulation of gelation pathways provides the opportunity to create diverse hydrogel materials with distinct physicochemical properties and functions, leveraging intricate synthetic chemistries with various reactants. Nevertheless, such hydrogel matrices are often characterized by polydispersity, batch-to-batch variation, and inconsistent chemical compositions due to washing processes that are essential to remove residual crosslinkers, monomers, organic solvents, and byproducts, resulting in the loss of desired properties. To avoid these side effects in hydrogel synthesis, a one-step gelation may be employed. A molecule with sufficient amphiphilicity was selected as a hydrogen-bonding molecule. Specifically, we selected propylamine as it exists in a liquid phase at room temperature and is miscible in water, enabling the control of interfacial chemical environments in aqueous ZIF-8 liquids (FIGS. 5A and 5B). If the hydrogen-bonding molecule interacts strongly with the functionalized surface in aqueous conditions, adding a small amount of hydrogen-bonding molecule could effectively induce gelation while maintaining water content. However, when the hydrogen-bonding molecule is added in excess amounts into aqueous ZIF-8 dispersion, leading to an imbalance between hydrophobicity and hydrophilicity, the hydrogel would be transformed into an emulsion, as shown in FIG. 5C.

[0080] Exploratory studies using density functional theory (DFT) calculations with the conductor-like polarizable continuum model (CPCM) with the solvent medium (water, £ = 78.4) revealed that propylamine, which effectively interacts with the MOF surface, is suitable for our strategy. Initially, we constructed three types of Zn(Melm)3([open ZIF-8]), Zn(Melm)4([ZIF-8]), mPEG- ZIF-8 ([mPEG-ZIF-8]) cluster models based on the ZIF-8 crystal structure. The coordination bonding energies of water ([W]) and propyl amine ([PA]) on the open Zn site of [open ZIF-8] were calculated to be -23.3 kcal / mol and -40.0 kcal / mol, respectively. (FIG. 5D). In general, amines exhibit higher nucleophilicity than that of water as the lone pair on Namine in amines is more readily available, attributed to the lower electronegativity of nitrogen. For similar reasons, the intermolecular interaction energies of propyl amine on 2-methylimidazole of [ZIF-8] and the hydroxyl group of [mPEG-ZIF-8] are much stronger than that of water at a relative energy of - 2.8 kcal / mol and -7.33 kcal / mol, respectively, as shown in FIGS. 5E, 5F, and 15. Furthermore, the non-covalent interaction (NCI) surface indicated that not only is propylamine more advantageous for strong hydrogen bonding formation but also for large alkyl-imidazole interactions. These results highlight that propylamine exhibits stronger intermolecular interactions with all possible nanocrystal surface moieties, such as the open Zn-site, 2- methylimidazole, and the mPEG termination of mPEG-ZIF-8 surface when compared to water. Thus, when incorporated, propylamine can gradually alter the chemical environments on the surface of mPEG-ZIF-8 in aqueous media to afford a suitable balance between hydrophobic and hydrophilic interactions toward biphasic hydrogel formation.

[0081] To examine the chemical interaction behaviors in the periodic boundary conditions for the observed hydrophilic surface and the hydrophobic internal surface of mPEG-ZiF-8, as well as the strong affinity of propylamine with the surface of mPEG-ZIF-8, we carried out molecular dynamics (MD) simulations at 298.15 K and 1 bar, including water molecules (FIGS. 16A-16D). At reaching equilibrium after 300 ps, water molecules formed hydrogen bonding networks at the mPEG-ZIF-8 interface. However, both the mPEG polymer and water molecules are unable to intrude into the pores of ZIF-8. We also explored the behavior of propylamine in the equilibrated mPEG-ZIF-8 under aqueous conditions to determine whether the propyl amine interacts more favorably with the surface of mPEG-ZIF-8 compared to water, thereby creating hydrophobic environments on its surface (FIG. 6A). Notably, the snapshot for the equilibrium state after 0.5 ns indicated that all propylamine molecules effectively interact with the mPEG-ZIF-8 surface, resulting in the formation of hydrophobic environments (FIG. 6B).

[0082] Various concentrations of mPEG-ZIF-8 fluids, ranging from 3 vol% to 8 vol%, were combined with 0.5 M propylamine volumes equal to 0.04 volumetric equivalents of the total dispersion volume (FIG. 6C). Upon addition of dilute propylamine, immediate gelation of aqueous mPEG- ZIF-8 dispersions is observed, extending down to approximately 5 vol%, as confirmed visually by inversion of the sample (FIG. 6C). Below this concentration threshold, it is hypothesized that the nanoparticles are too dilute and distributed, preventing the collective hydrophobic interactions due to large interparticle distances. Near 5 vol %, the sample exhibits transient hydrogelation, in which partial transformation occurs. The surface chemistry of propylamine at the mPEG-ZIF-8 surface under fixed nanocrystal concentrations was investigated. When added at varying increments, 0.1 M and 0.25 M propylamine resulted in complete hydrogelation, while concentrations above 0.5 M resulted in transient hydrogelation (FIG. 6D). The nanocrystal and the amine need to interact in such a way that hydrophobicity and hydrophilicity are sufficiently maintained in order to form a colloidal hydrogel. Consequently, when the amine is present in excess of such balance, a hydrogel undergoes phase change into an amine-dominant emulsion, losing the optimal balance between hydrophobicity and hydrophilicity in the chemical systems. mPEG-ZIF-8 dispersion of a desired concentration was prepared by adding a known volume of deionized water (MilliQ dispensed) to a known mass of fully dried mPEG-ZIF-8. The sample then went under multiple cycles of stirring and sonication to ensure full dispersion in water. Upon dispersion, a known volume of amine (typically 20 pL of amine to 500 pL of dispersed microporous solution) with varying concentrations in water (ranging from 0.01 M to 10 M) was added. The efficacy of gelation was first-pass determined visually by inverting the mixture to see if the meniscus of the solution was constant over a period of time.

[0083] All calculations were conducted using DFT1implemented in the ORCA 4.2.0 program package with Becke’s three-parameter exchange functional B3LYP, including Grimme’s D3 dispersion correction levels of theory. All calculations were conducted using the def2 / J auxiliary basis set and RIJCOSX approximation. Geometry optimizations proceeded using the def2-SVP basis set for all atoms. The electronic energies of the optimized structures were reevaluated by additional single-point calculations on each optimized geometry using the same functional and def2-TZVP basis set. Analytical vibrational frequencies within the harmonic approximation were calculated using the def2-SVP to confirm the proper convergence to well-defined minima on the potential energy surface. Solvation energies were calculated using the conductor-like polarizable continuum model (CPCM) and were performed with the def2-SVP basis at the optimized gas phase geometry employing the dielectric constant of e = 78.4 for water. The Gibbs free energies in solution phase G(sol) were computed with the following protocol:

[0084] G(sol) = G(gas) + G(solv) (1)

[0085] G(gas) = H(gas) - TS(gas) (2)

[0086] H(gas) = E(SCF) + ZPE (3)

[0087] AE(SCF) = ZE(SCF) for products - ZE(SCF) for reactants (4)

[0088] AG(sol) = ZG(sol) for products - ZG(sol) for reactants (5)

[0089] G(gas) is the free energy in gas phase; G(solv) is the free energy of solvation; H(gas) is the enthalpy in gas phase; T is the temperature (298.15K); S(gas) is the entropy in gas phase; E(SCF) is “raw” electronic energy computed from the SCF (self-consistent field) procedure; ZPE is the zero-point energy. The entropy we refer to is specifically the vibrational / rotational / translational entropy of the solute(s), and the entropy of the solvent is implicitly included in the continuum solvation model. Non-covalent interaction (NCI) was calculated using NCIPLOT 3.0, and the surface of NCIs was visualized by Visual Molecular Dynamics (VMD) software. Molecular dynamics (MD) simulations were performed using Universal Force Field (UFF) for metal-organic frameworks (UFF4MOF-II) as implemented in the Amsterdam Modeling Suite (AMS) products of SCM (Software for Chemistry & Materials). During the simulations, the (110) surface of the ZIF-8 functionalized by mPEG, consisting of 36,514.7 A3supercells, was selected as the exposed facet in water and propylamine. The vacuum space was set to be at least 30 A to prevent the interactive effect between copies of the replica within the periodic boundaries and to fill water and propyl amine molecules. In the MD simulations, the time step was chosen to be 1 fs, and the system was performed using a combination of Nose-Hoover chains (NHC) thermostats with the temperature (298.15K) and Martyna-Tobias-Klein (MTK) extended Lagrangian barostat at a pressure of 1 bar. Annealing is executed to find a structure closer to a global minimum in the energy and optimize the structure of the samples. The results are shown in FIGS. 16A-16D.

[0090] Example 4 - Measuring the rheological properties and stability of the mPEG-ZIF-8 hydrogel.

[0091] The viscosity and viscoelastic properties of hydrogels created from aqueous mPEG-ZIF-8 fluids with different ZIF-8 concentrations was examined (FIGS. 7A-7B). The mPEG-ZIF-8 hydrogel shows a greater storage modulus (G’) than the loss modulus (G”) with strain-independence up to roughly 5% strain (FIG. 7A), indicative of its elasticity. Under frequency sweep, the sample also shows G’ > G”, with G’ largely independent, further demonstrating evidence of gelation and network formation amongst the colloidal nanocrystals (FIG. 7B).

[0092] The mPEG-ZIF-8 gels have a water content of at least 90% across all concentrations of gelation achieved (TGA - FIG. 17). The high water content results in a relatively low mechanical strength of the hydrogel itself, while the presence of the nanocrystals results in a rather high storage modulus compared to existing high-water content polymer-based hydrogels. This is in accordance with previous literature reports with nanoparticle-based gels (or nanoemulsion gels) in which the storage modulus of the system is dictated by the presence of solid particles, Indeed, the storage modulus, G’, of the mPEG-ZIF-8 gel increases with increasing nanocrystal concentration, reflecting the hydrogelation purely driven by subtle changes in the ZIF nanocrystal surface (FIGS. 7A-7B).

[0093] The presence of amines does not greatly alter the structural stability of mPEG-ZIF-8 hydrogel, as evidenced by PXRD (FIG. 19A). Moreover, even after the addition of the amines, the porosity of mPEG-ZIF-8 is fully intact, as evidenced by the preserved 77 K N2BET surface area on a lyophilized sample (FIG. 19B). This is a stark departure from many MOF / hydrogel composites that often sacrifice porosity to varying degrees upon integration of the two materials and demonstrates the efficacy of the small-molecule approach to inducing hydrogelation in mPEG-ZIF-8 aqueous dispersions.

[0094] All rheology measurements were performed on TA Instruments Discovery HR-2 Rheometer with a 40.0 mm parallel plate. Strain sweep was conducted at an angular frequency of 10.0 rad / s, from 0.1 % to 10.0 %. Frequency sweep was acquired at 0.1 % strain, well within the viscoelastic regime for all samples tested, from 100. rad / s to 0.1 rad / s.

[0095] All TGA measurements and analysis were performed on TA Instruments TGA 550 in open platinum pans. Samples were first equilibrated to 30 °C for 10 minutes, and then heated at a rate of 1 °C / min until 500 °C under a 15 mL / min N2flow. It was then held at the final temperature for 2 hours to determine the mass loss. All samples were loaded such that the initial equilibrated mass was less than 90 mg.

[0096] Example 5 - Measuring O2storage and carrying capacity of the microporous ZIF-8 hydrogel.

[0097] To examine the oxygen release kinetics of the mPEG-ZIF-8 hydrogels, a customized sample chamber was utilized, specifically designed to encapsulate the hydrogel samples securely (FIG. 8A). The encapsulation system is necessary to maintain the integrity of the mechanically soft hydrogel samples under stirring, with the stirring being required to achieve equilibration of O2into the surroundings in a reasonable timescale. The mPEG-ZIF-8 gels were packed in an open sapphire pan with a known volume. A gas permeable — but water impermeable — membrane was then secured on top to allow for gas exchange with the surroundings. The encapsulated gel was then placed inside a gas-tight chamber, upon which deoxygenated water was layered. A Clark electrode probe was then lowered carefully to monitor the O2levels in the deoxygenated layer under stirring until equilibrium was reached (FIG. 10).

[0098] The O2 levels of a typical mPEG-ZIF-8 hydrogel of 6.8 vol % nanocrystal loading were measured and compared with the release profiles of the aqueous dispersion injection measurements. The release profile showed an initial rapid release driven by the initial large partial pressure gradient of O2and a subsequent slow equilibration towards saturation. Within a measurement period of 10 hours, the gel reached approximately 55% of its capacity, in contrast to the 86% of its capacity reached by its aqueous counterpart under the same release encapsulation (FIG. 8C, FIG. 18). While the release of the encapsulated aqueous dispersion reaches equilibration values consistent with the rapid release injection measurements, albeit with a delay due to the encapsulation apparatus, the hydrogel reaches roughly half of that value. The slower release and lower final equilibration value is thought to be due to the higher viscosity and the viscoelasticity associated with hydrogelation that leads to the delayed mass transport of O2within the microporous hydrogel (FIG. 8B). Initially, the pressure gradient drives the release from the top layers of the hydrogel, allowing for a gradual rise in O2 over time. Upon depletion of O2 near the hydrogel / deoxygenated water interface, there are both greater barriers to mass transport within the gel and less pressure gradient across the entire system to drive the release at similar rates, leading to slowed O2 release. Ultimately, the release is slowed nearly to the detection limit of the instrument, leading to lower O2 released in the measurement time period.

[0099] This system is an example of a hydrogel that can store and release O2 in the base material without extensive O2 generation mechanisms and / or high volumetric loadings of the O2 carrier, and for which its O2 storage and release capacity can be directly probed. In particular, this system provides a way to control the gas release kinetics of the microporous hydrogel and is a first step towards ultimately achieving controllable, sustained O2 release into hypoxic environments. While demonstrated with O2as a carrier gas, we expect this system to be effective across various target gas substrates for utility across a wide range of applications.

[0100] A strategy for creating a microporous hydrogel composed of well-dispersed ZIF-8 nanocrystals that can retain porosity with high gas-carrying ability in aqueous media is disclosed. By manipulating the surface hydrophobicity and hydrogen bonding of ZIF-8 dispersions in water using a low molecular-weight hydrogen-bonding molecule, propylamine, straightforward hydrogelation was achieved without the need for complex polymer matrices and washing processes that would negatively influence the integrity of the hydrogel. Furthermore, the resulting hydrogels consist of a high water content of over 90% and biocompatible PEG polymers (Mn = 750 g / mol) that are considered biologically inert. As the porosity of the microporous material is not compromised in this novel hydrogel, the dry, microporous networks can store and release gases as effectively as both its solid-state and aqueous dispersion counterparts. The increased viscosity of the microporous hydrogel, in addition to its high gas capacities, lends itself towards delayed gas transport to its surroundings with the potential for sustained gas delivery. This simple strategy offers a synergistic approach to sustained gas delivery through hydrogel systems incorporating microporous materials for cosmetic and biomedical applications.

[0101] Example 6 - Encapsulation and oxygen release measurement of a hydrogel including a zirconium-based MOF within a polymer.

[0102] A hydrogel was formed through encapsulation of an aqueous dispersion of a zirconium-based MOF, MIL-140A or MIL-140B, within a polymer made of 2-hydroxyethylmethacrylate (HEMA), 1- vinyl-2-pyrrolidinone (NVP), and ethylene glycol dimethacrylate (EGDMA) (FIG. 9). MIL-140A or MIL-140B were synthesized by combining a zirconium metal source at 175 °C in DMF with either benzene dicarboxylate (BDC) or naphthalene dicarboxylate (NDC), respectively (FIG. 20). Successful synthesis of MIL-140A or MIL-140B was determined through PXRD by comparing the experimentally measured PXRD patterns with simulated patterns (FIGS. 21A-21 D).The HEMA, NVP, and EGDMA were cross-linked by application of blue light with a wavelength of 370 nm within a photoreactor. The resulting microporous hydrogel included around 35 vol% ethylene glycol diacrylate polymer, around 60 vol% water, and around 5 vol% Zr-based MOF. The oxygen release of the sample was then measured as described in Example 4 (FIG. 10). The first measurement was performed on a hydrogel that had not been previously subjected to oxygenation. The second measurement was performed after soaking and shaking the hydrogel in deoxygenated water for 10 minutes. The third measurement was performed after soaking in water that had been bubbled with N2gas for 1 day. The fourth measurement was performed after resoaking the hydrogel in air-equilibrated water for 1 day (FIG. 11).

[0103] Other embodiments are in the claims.

Claims

What is claimed is:CLAIMS1. A hydrogel comprising a plurality of microporous particles having pores with an internal surface, wherein the pores are sized to allow entry of a gas, and having an external surface, wherein the hydrogel further comprises a gelling agent and a solvent.

2. The hydrogel of claim 1 , wherein the plurality of microporous particles comprises a metalorganic framework, a zeolite, or a covalent-organic framework.

3. The hydrogel of claim 2, wherein the metal-organic framework comprises a zeolitic imidazole framework, a zirconium-based MOF, or a combination thereof.

4. The hydrogel of claim 3, wherein the zeolitic imidazole framework is ZIF-8 or ZIF-675. The hydrogel of claim 2, wherein the zirconium-based MOF is an MIL-type MOF.

6. The hydrogel of claim 5, wherein the MIL-type MOF is an MIL-140.

7. The hydrogel of claim 6, wherein the MIL-140 is MIL-140A, MIL-140B, or a combination thereof.

8. The hydrogel of claim 2, wherein the zeolite has an MFI structure type.

9. The hydrogel of claim 8, wherein the MFI structure type comprises an atom percent ratio of silicon to aluminum between 1 to 0 and 20 to 1.

10. The hydrogel of claim 2, wherein the zeolite is silicalite-1.

11. The hydrogel of claim 1, wherein the external surface is covalently bound to a polymer.

12. The hydrogel of claim 1, wherein the external surface is non-covalently bound to a polymer.

13. The hydrogel of claim 11 or 12, wherein the polymer comprises polyethylene glycol.

14. The hydrogel of claim 13, wherein the polyethylene glycol has a molecular weight between 100 g / mol and 30000 g / mol.

15. The hydrogel of claim 1, wherein the external surface is hydrophobic.

16. The hydrogel of claim 1, wherein the external surface is hydrophilic.

17. The hydrogel of claim 1, wherein the internal surface is hydrophobic.

18. The hydrogel of claim 1, wherein the gelling agent comprises an amphipathic hydrogenbonding molecule that noncovalently links the plurality of microporous particles within the hydrogel.

19. The hydrogel of claim 1, wherein the amphipathic hydrogen-bonding molecule comprises an alkylamine.

20. The hydrogel of claim 19, wherein the alkylamine comprises methylamine, ethylamine, propylamine, butylamine, pentylamine, or hexylamine.

21. The hydrogel of claim 1 , wherein the external surface further comprises a hydrophilic, hydrophobic, or amphiphilic coating.

22. The hydrogel of claim 1, wherein the gelling agent comprises a polymer matrix.

23. The hydrogel of claim 22, wherein the polymer matrix comprises 2- hydroxyethylmethacrylate, 1-vinyl-2-pyrrolidinone, ethylene glycol dimethacrylate, a compoundof the formula 0 , wherein n is an integer between 2 and 14, a compound of the formula, wherein n is an integer between 2 and 14, or a combination thereof.

24. The hydrogel of claim 22 or 23, wherein the polymer matrix noncovalently entraps the plurality of microporous particles.

25. The hydrogel of claim 1, wherein the microporous particles comprise nanoparticles, microparticles, or a combination thereof.

26. The hydrogel of claim 1, further comprising the gas located in the pores of the microporous particles.

27. The hydrogel of claim 1, wherein the gas comprises argon, oxygen, nitrogen, carbon dioxide, carbon monoxide, xenon, methane, or hydrogen.

28. The hydrogel of claim 1, wherein the solvent is aqueous.

29. A method of storing a gas in a hydrogel, comprising: providing a hydrogel of any one of claims 1-28; and dissolving the gas in the hydrogel, thereby storing the gas in the pores.

30. A method of delivering a gas to a substrate comprising: providing a hydrogel of any one of claims 1-28, wherein the hydrogel comprises the gas in the pores; and contacting the hydrogel with the substrate thereby delivering the gas to the substrate.

31. The method of claim 29 or 30, wherein the gas comprises argon, oxygen, nitrogen, carbon dioxide, carbon monoxide, xenon, methane, helium, neon, or hydrogen.

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