Hydrogel compositions for treating vascular defects

Abstract

Compositions and methods for treating vascular defects are provided. In some embodiments, a composition for treating an aneurysm includes a freeze-thawed hydrogel comprising a polymer and a contrast agent, where the polymer is non-covalently crosslinked. The freeze-thawed hydrogel can be configured to be delivered into the aneurysm to occlude the aneurysm. The freeze-thawed hydrogel can have a storage modulus within a range from 1 kPa to 2 kPa, or from 3 kPa to 8 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel.

Description

[0001]MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO HYDROGEL COMPOSITIONS FOR TREATING VASCULAR DEFECTS CROSS-REFERENCED TO RELATED APPLICATION(S) The present application claims the benefit of priority to Greek Patent Application No.20240100299, filed April 24, 2024, which is incorporated by reference herein in its entirety. TECHNICAL FIELD The present technology generally relates to biocompatible materials, and in particular, to hydrogel compositions for treating vascular defects. BACKGROUND An intracranial aneurysm is a portion of an intracranial blood vessel that bulges outward from the blood vessel’s main channel. This condition often occurs at a portion of a blood vessel that is abnormally weak because of a congenital anomaly, trauma, high blood pressure, or for another reason. Once an intracranial aneurysm forms, there is a significant risk that the aneurysm will eventually rupture and cause a medical emergency with a high risk of mortality due to hemorrhaging. When an unruptured intracranial aneurysm is detected or when a patient survives an initial rupture of an intracranial aneurysm, vascular surgery is often indicated. One conventional type of vascular surgery for treating an intracranial aneurysm includes using a microcatheter to dispose a platinum coil within an interior volume of the aneurysm. Over time, the presence of the coil should induce formation of a thrombus. Ideally, the aneurysm’s neck closes at the site of the thrombus and is replaced with new endothelial tissue. Blood then bypasses the aneurysm, thereby reducing the risk of aneurysm rupture (or re-rupture) and associated hemorrhaging. Unfortunately, long-term recanalization (i.e., restoration of blood flow to the interior volume of the aneurysm) after this type of vascular surgery occurs in a number of cases, especially for intracranial aneurysms with relatively wide necks and / or relatively large interior volumes. Another conventional type of vascular surgery for treating an intracranial aneurysm includes deploying a flow diverter within the associated intracranial blood vessel. The flow diverter is often a mesh tube that causes blood to preferentially flow along a main channel of the blood vessel while blood within the aneurysm stagnates. The stagnant blood within the aneurysm MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO should eventually form a thrombus that leads to closure of the aneurysm’s neck and to growth of new endothelial tissue, as with the platinum coil treatment. One significant drawback of flow diverters is that it may take weeks or months to form aneurysmal thrombus and significantly longer for the aneurysm neck to be covered with endothelial cells for full effect. This delay may be unacceptable when risk of aneurysm rupture (or re-rupture) is high. Moreover, flow diverters typically require antiplatelet therapy to prevent a thrombus from forming within the main channel of the blood vessel at the site of the flow diverter. Antiplatelet therapy may be contraindicated shortly after an initial aneurysm rupture has occurred because risk of re-rupture at this time is high and antiplatelet therapy tends to exacerbate intracranial hemorrhaging if re-rupture occurs. For these and other reasons, there is a need for innovation in the treatment of intracranial aneurysms. Given the severity of this condition, innovation in this field has immediate life-saving potential. BRIEF DESCRIPTION OF THE DRAWINGS Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Instead, emphasis is placed on illustrating clearly the principles of the present disclosure. FIG. 1A is a partially schematic view of a treatment system configured in accordance with embodiments of the present technology. FIG. 1B is an enlarged cross-sectional view of a distal portion of the treatment system of FIG.1A. FIGS. 2A–2E illustrate an example method of treating an aneurysm using the treatment system of FIGS.1A and 1B, in accordance with embodiments of the present technology. FIG.3A illustrates a neck cover configured in accordance with embodiments of the present technology. FIGS.3B–3D illustrate an example method of treating an aneurysm using the neck cover of FIG.3A, in accordance with embodiments of the present technology. FIG. 4 is a schematic illustration of a process for preparing a freeze-thawed hydrogel, in accordance with embodiments of the present technology. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO FIG.5 illustrates a reaction scheme for synthesis of polyvinyl alcohol (PVA) from polyvinyl acetate through hydrolysis, in accordance with embodiments of the present technology. FIG.6A is a photograph of a PVA solution before freeze-thawing. FIG.6B is a photograph of a hydrogel formed from the PVA solution of FIG.6A after freeze-thawing. FIG.7A is a photograph of a freeze-thawed PVA hydrogel immersed in water. FIG.7B is a photograph of a freeze-thawed PVA hydrogel immersed in phosphate- buffered saline solution. FIGS. 8A–8D are fluoroscopic images of a simulated aneurysm during treatment with a freeze-thawed PVA hydrogel. FIGS. 9A–9D are fluoroscopic images of a simulated aneurysm during treatment with a freeze-thawed PVA hydrogel. FIGS. 10A–10C are fluoroscopic images of an aneurysm treated with a freeze- thawed PVA / polyvinyl pyrrolidone (PVP) hydrogel. DETAILED DESCRIPTION The present technology relates to compositions configured for delivery to a treatment site in a patient’s body, such as an aneurysm or other vascular defect, and associated methods. In some embodiments, for example, a composition for treating an aneurysm includes a freeze-thawed hydrogel including a polymer and a contrast agent. The polymer can be non- covalently crosslinked. For example, in embodiments where the polymer is polyvinyl alcohol, the hydrogel can be formed via hydrogen bonding between hydroxyl groups of the polyvinyl alcohol. The freeze-thawed hydrogel can be sufficiently soft and flowable to allow the hydrogel to be delivered into the aneurysm via injection through a catheter, and can be sufficiently stiff and cohesive to occlude the aneurysm without leaking into the parent vessel. In some embodiments, for example, the freeze-thawed hydrogel has a storage modulus within a range from 1 kPa to 2 kPa, or from 3 kPa to 8 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel; a loss modulus within a range from 100 Pa to 500 Pa, or from 200 Pa to 600 Pa at 37 °C over the MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO linear viscoelastic region of the freeze-thawed hydrogel; and / or a compression modulus within a range from 10 kPa to 15 kPa, or from 5 kPa to 20 kPa at 37 °C. The present technology can provide many advantages over conventional approaches for aneurysm treatment. For example, conventional treatment methods typically use either a low viscosity embolic agent that gels or solidifies in situ when exposed to physiological conditions at the treatment site, or separate precursor components that are mixed immediately before delivery to form the final embolic agent. However, these approaches may present challenges with long-term storage stability, require additional process steps, and / or introduce timing complications. For example, if the agent gels too quickly, it may clog the delivery device. If the agent gels too slowly, it may leak out of the treatment site, which can have catastrophic results in certain applications such as the treatment of cerebral aneurysms. In contrast, the compositions of the present technology can form an injectable hydrogel that can be supplied in a ready-to-use form and immediately introduced into a catheter at any desired time during an aneurysm treatment procedure, without the need to carry out any preliminary mixing steps prior to such introduction. This approach can improve the reliability, convenience, and efficacy of the treatment procedure. Moreover, many conventional embolic agents use covalent crosslinkers to form a gel, which are typically toxic and may result in an inflammatory response and / or other undesirable side effects at the treatment site if they leach out of the gel over time. Covalent crosslinking may also produce relatively stiff gels, which may require very high injection forces for delivery into the aneurysm and / or may require precise timing of the crosslinking reaction to avoid clogging or leaking, as discussed above. In contrast, the present technology provides non-covalently crosslinked hydrogels formed through a freeze-thaw process, thus avoiding the use of toxic covalent crosslinkers. The non-covalent crosslinks can also confer shear-thinning properties to the hydrogel, thus reducing the injection force needed to deliver the hydrogel and improving the safety margin of the treatment procedure. Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which example embodiments are shown. Embodiments of the claims may, however, be embodied in many different forms and should not be construed as limited to the MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO embodiments set forth herein. The examples set forth herein are non-limiting examples and are merely examples among other possible examples. The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed present technology. Embodiments under any one heading may be used in conjunction with embodiments under any other heading. I. Overview of Treatment Systems and Methods FIG. 1A shows a treatment system 100 (“system 100”) configured in accordance with embodiments of the present technology. Although the system 100 is described herein in the context of treating aneurysms such as cerebral aneurysms, this is not intended to be limiting, and the system 100 can also be used in the treatment of other types of vascular defects, and / or in any other application involving delivery of an embolic composition into a space within a patient’s body. As shown in FIG.1A, the system 100 includes a delivery system 102, a neck cover 104 (also referred to herein as an “occlusive member,” “occlusive device,” or a “neck protection device”), and an embolic kit 150. The neck cover 104 (shown schematically) is configured to be detachably coupled to the delivery system 102, and the delivery system 102 is configured to intravascularly position the neck cover 104 within an aneurysm. Representative examples of neck covers suitable for use with the system 100 are described in U.S. Patent No.8,142,456, U.S. Patent No. 9,855,051, U.S. Patent No. 10,327,781, U.S. Patent Application Publication No. 2020 / 0187953, U.S. Patent Application Publication No. 2021 / 0128169, and U.S. Patent Application Publication No.2021 / 0153872, the disclosures of which are incorporated by reference herein in their entirety. The embolic kit 150 includes an embolic composition 152 (e.g., a freeze-thawed hydrogel as described in Section II below) and an injector 154 configured to be fluidly coupled to a proximal portion of the delivery system 102 for injection of the embolic composition 152 into the aneurysm cavity. The embolic composition 152 can be delivered to a space between the neck cover 104 and the dome of the aneurysm to fill and occlude the aneurysm cavity. The neck cover 104 prevents migration of the embolic composition 152 into the parent vessel, and together the neck cover 104 and embolic composition 152 prevent blood from flowing into the aneurysm. As MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO described in greater detail below, bioabsorption of the embolic composition 152 (in embodiments where the embolic composition 152 is biodegradable) and / or endothelialization of the neck cover 104 may cause the aneurysm wall to fully degrade, leaving behind a successfully remodeled (aneurysm free) region of the blood vessel. As shown in FIG. 1A, the delivery system 102 has a proximal portion 106a configured to be extracorporeally positioned during treatment and a distal portion 106b configured to be intravascularly positioned at or within an aneurysm. The delivery system 102 may include a handle 108 at the proximal portion 106a and a plurality of elongate shafts extending between the handle 108 and the distal portion 106b. In some embodiments, for example as shown in FIG.1A, the delivery system 102 may include a first elongate shaft 110 (such as a guide catheter or balloon guide catheter), a second elongate shaft 112 (such as a microcatheter) configured to be slidably disposed within a lumen of the first elongate shaft 110, and a third elongate shaft 114 configured to be slidably disposed within a lumen of the second elongate shaft 112. The delivery system 102 and / or the third elongate shaft 114 is configured to be detachably coupled at its distal end portion to the neck cover 104 via a connector 122 (see FIG. 1B) of the neck cover 104. In some embodiments, the delivery system 102 does not include the first elongate shaft 110. The second elongate shaft 112 is generally constructed to track over a conventional guidewire in the cervical anatomy and into the cerebral vessels associated with the brain. The second elongate shaft 112 may also be chosen according to several standard designs that are generally available. For example, the second elongate shaft 112 can have a length that is at least 125 cm long, and more particularly may be between about 125 cm and about 175 cm long. The lumen of the second elongate shaft 112 is configured to slidably receive the neck cover 104 in a radially constrained state. The second elongate shaft 112 can have an inner diameter less than or equal to 0.006 inches (0.015 cm), 0.011 inches (0.028 cm), 0.015 inches (0.038 cm), 0.017 inches (0.043 cm), 0.021 inches (0.053 cm), or 0.027 inches (0.069 cm). The third elongate shaft 114 can be movable within the first and / or second elongate shafts 110, 112 to position the neck cover 104 at a desired location. The third elongate shaft 114 can be sufficiently flexible to enable manipulation (e.g., advancement and / or retraction) of the neck cover 104 through tortuous passages. Tortuous passages can include, for example, catheter lumens, microcatheter lumens, blood vessels, urinary tracts, biliary tracts, and airways. The third MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO elongate shaft 114 can be formed of any material and in any dimensions suitable for the task(s) for which the system 100 is to be employed. In some embodiments, at least the distal portion of the third elongate shaft 114 can comprise a flexible metal hypotube. The hypotube, for example, can be laser cut along all or a portion of its length to impart increased flexibility. In some embodiments, the third elongate shaft 114 can be surrounded over some or all of its length by a lubricious coating, such as polytetrafluoroethylene (PTFE). The third elongate shaft 114 can have an inner diameter less than or equal to 0.006 inches (0.015 cm), 0.011 inches (0.028 cm), 0.015 inches (0.038 cm), 0.017 inches (0.043 cm), 0.021 inches (0.053 cm), or 0.027 inches (0.069 cm) Referring still to FIGS.1A and 1B, the embolic composition 152 may be pre-loaded into the injector 154 (as shown) or may be provided separately. The embolic composition 152 can be any material suitable for forming a solid or semi-solid viscoelastic structure (e.g., a hydrogel) that partially or completely occludes the interior cavity of the aneurysm. In some embodiments, the embolic composition 152 is a preformed composition that is ready for use without any mixing of precursor materials. The embolic composition 152 can be sufficiently solid to fill and occlude the aneurysm in its preformed state, without requiring further steps (e.g., chemical reactions, physical interactions) and / or changes in material properties (e.g., viscosity, degree of crosslinking) to effectively occlude the aneurysm. Additional details of the embolic composition 152 are provided in Section II below. The injector 154 can be configured to pressurize the embolic composition 152 to a pressure that is sufficiently high to push the embolic composition 152 through the components of the delivery system 102 (e.g., through the lumen of the third elongate shaft 114). For example, the injector 154 can be configured to generate and withstand a pressure of at least 4,000 psi, 5,000 psi, 6,000 psi, 7,000 psi, 8000, psi, 9,000 psi, 10,000 psi, 11,000 psi, 12,000 psi, 13,000 psi, 14,000 psi, 15,000 psi, or higher. Representative examples of injectors suitable for use with the present technology are described in U.S. Patent Application Publication No.2023 / 0210540, the disclosure of which is incorporated by reference herein in its entirety. The system 100 can further include a conduit configured to guide the embolic composition 152 delivered from the injector 154 to a space between at least a portion of the neck cover 104 and the aneurysm dome. In some embodiments, the conduit is incorporated into the delivery system 102. For example, as depicted in the enlarged cross-sectional view of the distal MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO portion 106b shown in FIG.1B, the conduit can comprise a combination of the third elongate shaft 114 and an extension 116 fixed to a distal end portion of the third elongate shaft 114. The extension 116 can be a tubular member that extends distally from the third elongate shaft 114, through the connector 122, and through the neck cover 104, at least when the neck cover 104 is in an expanded state. When the neck cover 104 is collapsed within the lumen of the third elongate shaft 114 during delivery, a portion of the neck cover 104 may extend distally of the extension 116. The length of the extension 116 can be such that, when the distal portion 106b of the delivery system 102 is positioned at the aneurysm with the neck cover 104 in an expanded state (for example, as shown in FIG.2A), a distal terminus of the extension 116 is even with the distal end of the connector 122, distal of the connector 122 but proximal of a distal end of the neck cover 104, or even with or distal of the distal end of the neck cover 104. It may be beneficial for the extension 116 to be as short as possible to ensure the extension 116 remains sufficiently spaced apart from the fragile aneurysm wall. In some embodiments, the extension 116 comprises an atraumatic member, such as a soft, flexible coil. In other embodiments, the extension 116 comprises a flexible tube having a continuous sidewall (e.g., not formed of a coiled member). In any case, a distal end portion of the injector 154 can be fluidly coupled to a proximal end portion of the third elongate shaft 114 via a port 118. The port 118 can be located at the proximal portion 106a of the delivery system 102, such as on or proximal to the handle 108. The pressure generated at the injector 154 can cause the embolic composition 152 to flow through the lumen of the third elongate shaft 114, through the lumen of the extension 116, and into the aneurysm cavity. Once the embolic composition 152 has sufficiently filled the aneurysm cavity, the neck cover 104 and extension 116 can be detached via electrolytic detachment that severs a region of the extension 116 exposed between the third elongate shaft 114 and the neck cover 104. According to several embodiments, the conduit may comprise an additional elongate shaft (not shown). The additional elongate shaft can be delivered to the aneurysm through one or more of the first, second, and / or third elongate shafts 110, 112, 114, or may be delivered separately (e.g., outside of) the delivery system 102. In such embodiments, a proximal end portion of the elongate shaft is configured to be fluidly coupled to the injector 154 via the port 118. Methods for delivering the embolic composition 152 through a separate elongate shaft are discussed below. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO The neck cover 104 may comprise an expandable element having a low-profile or constrained state while positioned within a catheter (such as the second elongate shaft 112) for delivery to the aneurysm and an expanded, deployed state for positioning within the aneurysm. In some embodiments the neck cover 104 comprises a mesh 120 (shown schematically in FIG.1B) and a connector 122 coupled to the mesh 120. The connector 122 is configured to be coupled to one or more components of the delivery system 102, such as the third elongate shaft 114 and / or extension 116. The mesh 120 can be formed of a resilient material and shape set such that upon exiting the second elongate shaft 112, the mesh 120 self-expands to a predetermined shape. The mesh 120 can have any shape or size in the expanded state that enables the mesh 120 to cover the aneurysm neck. In some embodiments, for example as shown in FIG. 2A, the mesh 120 can be configured to assume a bowl shape. Other shapes are possible, e.g., as described in connection with FIGS.3A–3D below. The mesh 120 can have a porosity sufficient to prevent leakage of the embolic composition 152 into the parent vessel. In some embodiments, the mesh 120 is formed of a plurality of braided filaments that have been heat-set to assume a predetermined shape when released from the constraints of the delivery catheter. The mesh 120 may be formed of metal wires, polymer wires, or both, and the wires may have shape memory and / or superelastic properties. The mesh 120 may be formed of 24, 32, 36, 48, 64, 72, 96, 128, or 144 filaments. The mesh 120 may be formed of a range of filament or wire sizes, such as wires having a diameter of from about 0.0004 inches to about 0.0020 inches, or of from about 0.0009 inches to about 0.0012 inches. In some embodiments, each of the wires or filaments have a diameter of about 0.0004 inches, about 0.0005 inches, about 0.0006 inches, about 0.0007 inches, about 0.0008 inches, about 0.0009 inches, about 0.001 inches, about 0.0011 inches, about 0.0012 inches, about 0.0013 inches, about 0.0014 inches, about 0.0015 inches, about 0.0016 inches, about 0.0017 inches, about 0.0018 inches, about 0.0019 inches, or about 0.0020 inches. In some embodiments, all of the filaments of the braided mesh 120 may have the same diameter. For example, in some embodiments, all of the filaments have a diameter of no more than 0.001 inches. In some embodiments, some of the filaments may have different cross- sectional diameters. For example, some of the filaments may have a slightly thicker diameter to impart additional strength to the braid. In some embodiments, some of the filaments can have a diameter of no more than 0.001 inches, and some of the filaments can have a diameter of greater than 0.001 inches. The thicker filaments may impart greater strength to the braid without MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO significantly increasing the device delivery profile, with the thinner wires offering some strength while filling out the braid matrix density. In some embodiments, the mesh 120 can be a non-braided structure, such as a laser- cut stent. Moreover, while the mesh 120 shown in FIGS. 2A–2E is a dual-layer mesh, in some embodiments the mesh 120 may comprise more or fewer layers (e.g., a single layer, three layers, four layers, etc.). FIGS. 2A–2E illustrate an example method for treating an aneurysm using the system 100, in accordance with embodiments of the present technology. Referring first to FIG. 2A, a physician may begin by intravascularly advancing the second elongate shaft 112 towards an intracranial aneurysm A with the neck cover 104 in a low-profile, collapsed state and coupled to a distal end portion of the third elongate shaft 114. A distal portion of the second elongate shaft 112 may be advanced through a neck N of the aneurysm A to locate a distal opening of the second elongate shaft 112 within an interior cavity of the aneurysm A. The third elongate shaft 114 may be advanced distally relative to the second elongate shaft 112 to push the neck cover 104 through the opening at the distal end of the second elongate shaft 112, thereby releasing the neck cover 104 from the shaft 114 and enabling the neck cover 104 to self-expand into an expanded, deployed state. FIG.2A shows the neck cover 104 in an expanded, deployed state, positioned in an aneurysm cavity and still coupled to the third elongate shaft 114. In the expanded, deployed state, the neck cover 104 may generally conform to the curved inner surface of the aneurysm A. In some embodiments the neck cover 104 assumes a predetermined shape that is concave towards the aneurysm dome and encloses an interior region 124. As illustrated in FIG.2B, the embolic composition 152 can be injected through the third elongate shaft 114 and extension 116 to a space between the neck cover 104 and an inner surface of the aneurysm wall. In other embodiments, the embolic composition 152 can be delivered through another elongate shaft (not shown) separate from the third elongate shaft 114 and extension 116. As additional embolic composition 152 is delivered, it fills the interior region 124 and all or a portion of the volume of the aneurysm cavity. It may be beneficial to fill as much space in the aneurysm as possible, as leaving voids within the aneurysm sac may cause delayed healing and increased risk of aneurysm recanalization and / or rupture. While the scaffolding provided by MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO the neck cover 104 across the neck helps thrombosis of blood form in any gaps and healing at the neck N, the substantial filling of the cavity can prevent rupture acutely and does not rely on the neck cover 104. In some embodiments, the embolic composition 152 may fill greater than 40%, 50%, 60%, 65%, 70%, 75%, 80%, 85%, or 90% of the aneurysm sac volume. FIG.2C is a cross-sectional view of the neck cover 104 still attached to the delivery system just after completion of delivery of the embolic composition 152. During and after delivery, the embolic composition 152 exerts a substantially uniform downward pressure (e.g., towards the parent vessel) on the neck cover 104 that further seals and stabilizes the neck cover 104 around the neck N of the aneurysm A. Moreover, the embolic composition 152 along the distal wall provides additional occlusion. In some embodiments, the embolic composition 152 completely or substantially completely occludes the pores of the adjacent layer or wall of the neck cover 104 such that blood cannot flow past the embolic composition 152 into the aneurysm cavity. It may be desirable to occlude as much of the aneurysm as possible, as leaving voids of gaps can enable blood to flow in and / or pool, which may continue to stretch out the walls of aneurysm A. Dilation of the aneurysm A can lead to recanalization and / or herniation of the neck cover 104 and / or embolic composition 152 into the parent vessel and / or may cause the aneurysm A to rupture. Both conditions can be fatal to the patient. As shown in FIG.2D, once delivery of the embolic composition 152 is complete, the delivery system 102 and / or third elongate shaft 114 can be detached from the neck cover 104 (electrolytically or mechanically) and withdrawn from the patient’s body. In those embodiments comprising a separate elongate shaft for delivering the embolic composition 152, the elongate shaft can be withdrawn before, during, or after detachment of the third elongate shaft 114 from the neck cover 104. Over time natural vascular remodeling mechanisms and / or bioabsorption of the embolic composition 152 (in embodiments where the embolic composition 152 is biodegradable) may lead to formation of a thrombus and / or conversion of entrapped thrombus to fibrous tissue within the internal volume of the aneurysm A. These mechanisms also may lead to cell death at a wall of the aneurysm and growth of new endothelial cells between and over the filaments of the neck cover 104. Eventually, the thrombus and the cells at the wall of the aneurysm may fully degrade, leaving behind a successfully remodeled region of the blood vessel. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO In some embodiments, contrast agent can be delivered during advancement of the neck cover 104 and / or embolic composition 152 in the vasculature, deployment of the neck cover 104 and / or embolic composition 152 at the aneurysm A, and / or after deployment of the neck cover 104 and / or embolic composition 152 prior to initiation of withdrawal of the delivery system. The contrast agent can be delivered through the second elongate shaft 112, the conduit, or through another catheter or device commonly used to deliver contrast agent. The aneurysm (and devices therein) may be imaged before, during, and / or after injection of the contrast agent, and the images may be compared to confirm a degree of occlusion of the aneurysm. Alternatively, the contrast agent may be incorporated into the embolic composition 152, e.g., as described in Section II below. As shown in FIG. 2E, in some embodiments, the system 100 may comprise two separate elongate shafts (e.g., microcatheters), with one elongate shaft dedicated to delivery of the embolic composition 152 (e.g., a fourth elongate shaft 126), and the other elongate shaft dedicated to the delivery of the neck cover 104 (e.g., the third elongate shaft 114). In such embodiments, the fourth elongate shaft 126 can be fluidly coupled to the injector 154 to form at least part of the conduit for conveying the embolic composition 152 into the aneurysm A. The fourth elongate shaft 126 may be intravascularly advanced to the aneurysm A and through the neck N such that that a distal tip of the fourth elongate shaft 126 is positioned within the aneurysm cavity. In some embodiments, the fourth elongate shaft 126 may be positioned within the aneurysm cavity such that the distal tip of the fourth elongate shaft 126 is near the dome of the aneurysm A. The third elongate shaft 114 containing the neck cover 104 may be intravascularly advanced to the aneurysm A and positioned within the aneurysm cavity adjacent the fourth elongate shaft 126. The neck cover 104 may then be deployed within the aneurysm sac. As the neck cover 104 is deployed, it pushes the fourth elongate shaft 126 outwardly towards the side of the aneurysm A, and when fully deployed the neck cover 104 holds or “jails” the fourth elongate shaft 126 between an outer surface of the neck cover 104 and the inner surface of the aneurysm wall. The embolic composition 152 may then be delivered through the fourth elongate shaft 126 to a position between the inner surface of the aneurysm wall and the outer surface of the neck cover 104. For this reason, it may be beneficial to initially position the distal tip of the fourth elongate shaft 126 near the dome (or more distal surface) of the aneurysm wall. This way, the MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO “jailed” fourth elongate shaft 126 will be secured by the neck cover 104 such that the embolic composition 152 gradually fills the open space in the aneurysm sac between the dome and the neck cover 104. FIG.3A illustrates a neck cover 302 configured in accordance with embodiments of the present technology, and FIGS.3B–3D illustrate an example method for treating an aneurysm using the neck cover 302, in accordance with embodiments of the present technology. The neck cover 302 may be generally similar to the neck cover 104 of FIGS.1A and 1B, and may incorporate any of the features of the neck cover 104 described herein. For example, the neck cover 302 may comprise an expandable element (e.g., a mesh 304) having a low-profile or constrained state while positioned within a catheter (such as the second elongate shaft 112) for delivery to the aneurysm and an expanded, deployed state for positioning within the aneurysm. Referring first to FIG.3A, the neck cover 302 can be deployed within an aneurysm, e.g., using the system 100 of FIGS.1A and 1B. The proximal end of the neck cover 302 can be detachably coupled to a distal end of the third elongate shaft 114. For example, the third elongate shaft 114 can include a first connector 306, and the distal end of the neck cover 302 can include a second connector 308 configured to detachably couple with the first connector 306. The distal end of the neck cover 302 can be detachably coupled to a fourth elongate shaft 310 that is slidably disposed within the third elongate shaft 114. One or more connectors (not shown in FIG.3A) can be included at the proximal end of the neck cover 302 and the fourth elongate shaft 310 to permit deformation and inversion of a portion of the neck cover 302, as discussed in more detail below. The fourth elongate shaft 310 can be inserted into the system 100 before the neck cover 302 is expanded, while the neck cover 302 is expanded, or after the neck cover 302 has been expanded and then retracted to a partially inverted state, as discussed in more detail below. In some embodiments, the fourth elongate shaft 310 is configured to deliver an embolic composition 152 (e.g., received from the embolic kit 150 of FIG.1A) though exit port 312 to a position beyond the proximal end of the partially inverted neck cover 302. As such, the embolic composition 152 can become positioned between the neck cover 302 and an inner wall of the aneurysm cavity, as described in greater detail below. In FIG.3B, the neck cover 302 has expanded to substantially fill the interior volume of aneurysm A. In FIG. 3C, the interior volume of aneurysm A has been largely filled with an MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO embolic composition 152 injected through the exit port 312 at the end of the fourth elongate shaft 310. The embolic composition 152 can occupy the interior volume left by the retraction and partial inversion of the neck cover 302. As shown in FIG.3C, an annular ridge 314 can separate an outer first portion 316 of the neck cover 302 from an inverted inner second portion 318, thereby forming a concave (e.g., bowl) shape. In the embodiment shown in FIG. 3C, pressure exerted by the embolic composition 152 can help invert the proximal end of the neck cover 302. However, the neck cover 302 can alternatively or additionally be manually retracted using a suitable wire or other member (not shown in FIG.3C) prior to injection of the embolic composition 152. FIG.3D shows the aneurysm A after it has been completely filled with the embolic composition 152. The embolic composition 152 can form a cohesive, solid hydrogel mass that seals the aneurysm A and facilitates healing thereof. II. Embolic Compositions The present technology provides compositions that form an injectable hydrogel suitable for partially or fully occluding an aneurysm or other space within the body. In some embodiments, the compositions described herein include a hydrogel prepared via a freeze-thaw process, also referred to herein as a “freeze-thawed hydrogel.” A freeze-thawed hydrogel can be formed through one or more cycles of freezing and thawing that cause microphase separation and / or crystallization of a polymer, thereby facilitating non-covalent crosslinking between polymer chains. Advantageously, freeze-thawed hydrogels can be produced without the use of covalent crosslinking agents, which may be associated with toxicity and / or inflammatory responses. Moreover, the non-covalent crosslinking mechanism can confer shear-thinning properties to the hydrogel (e.g., the viscosity of the hydrogel decreases when shear stress is applied to the hydrogel and increases when the shear stress is removed), thus reducing the force needed to inject the freeze-thawed hydrogel while allowing the hydrogel to form a solid cohesive mass that occludes the aneurysm. FIG. 4 is a schematic illustration of a process for preparing a freeze-thawed hydrogel, in accordance with embodiments of the present technology. A polymer solution including a plurality of polymer chains 402 in an aqueous solvent 404 (e.g., water) is prepared (left panel). When frozen, the solution phase separates into polymer-rich regions 406 and ice crystals 408 (center panel). The polymer-rich regions 406 can include polymer chains 402 that are in close MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO contact with each other, thus forming crystalline structures and allowing for non-covalent crosslinks 410 (e.g., hydrogen bonds) to form between the polymer chains 402. Upon thawing (right panel), a cohesive hydrogel can be formed via the non-covalent crosslinks 410 within the crystalline polymer-rich regions 406. The hydrogel can include pores 412 at the spaces between the polymer chains 402 that were previously occupied by the ice crystals 408. The properties of the hydrogel can be modulated based on the number and duration of the freeze-thaw cycles. For example, more freeze-thaw cycles can increase the amount of polymer chains 402 that are incorporated into the polymer-rich regions 406 and non-covalently crosslinked to each other, thereby producing stronger and more stable hydrogels. In some embodiments, the freeze-thawed hydrogels described herein are prepared via one, two, three, four, five, six, or more freeze-thaw cycles. The temperature for the freezing phase of the freeze-thaw cycle can be less than or equal to 0 °C, -5 °C, -10 °C, -15 °C, -20 °C, -25 °C, -30 °C, -40 °C, or -50 °C; and / or within a range from 0 °C to -50 °C, -10 °C to -30 °C, or -15 °C to -25 °C. The duration of the freezing phase can be at least 1 hour, 4 hours, 8 hours, 12 hours, 16 hours, 18 hours, 20 hours, or 24 hours; and / or within a range from 1 hour to 24 hours, 4 hours to 16 hours, or 8 hours to 12 hours. In some embodiments, longer freezing phases can result in additional phase separation, formation of larger crystalline structures, and / or more hydrogen bonding, which can lead to stiffer and more cohesive gels. The temperature for the thawing phase of the freeze-thaw cycle can be greater than or equal to 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 40 °C, or 50 °C; and / or within a range from 0 °C to 50 °C, 10 °C to 30 °C, or 20 °C to 25 °C. The duration of the thawing phase can be at least 1 hour, 2 hours, 4 hours, 8 hours, 12 hours, or 24 hours; and / or within a range from 1 hour to 24 hours, 1 hour to 8 hours, or 2 hours to 4 hours. A. Polymers The freeze-thawed hydrogels described herein can be composed of at least one polymer that, when subjected to one or more freeze-thaw cycles, forms non-covalent crosslinks (e.g., hydrogen bonds) between polymer chains. For example, the polymer can include a repeating unit with a functional group that is capable of forming electrostatic interactions (e.g., hydrogen bonds), hydrophobic associations, coordination complexes, or π-π stacking. The functional group can be a hydroxyl group, a carboxyl group, an amine group (e.g., a primary amine group, a secondary amine group, a tertiary amine group), a thiol group, etc. The polymer can be a MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO biodegradable polymer or a nonbiodegradable polymer. The polymer can be a naturally occurring polymer or a synthetic polymer. The polymer can be a biocompatible polymer, e.g., the polymer produces little or no toxicity, inflammatory response, or other undesirable side effects in vivo. Examples of polymers that may be used to form a freeze-thawed hydrogel include polyvinyl alcohol (PVA), polyethyleneimine (e.g., linear polyethyleneimine, such as linear partially ethoxylated polyethyleneimine; branched polyethyleneimine), polyacrylic acid, ethylene-vinyl alcohol copolymer (e.g., with low polyethylene content), polyaniline, polyacrylamide, chitosan, alginate, gelatin, a gum (e.g., a carrageenan such as к-carrageenan, glucomannan, curdlan gum, xanthan gum, guar gum, locust bean gum), dextran, dextrin, cellulose, hyaluronate, starch, or a derivative thereof, or a combination (e.g., a copolymer or mixture) thereof. The hydrogel can include a single type of polymer or can include multiple different types of polymers (e.g., two, three, four, five, or more different polymers). The molecular weight of the polymer can be selected to provide desired hydrogel properties, e.g., higher molecular weight polymers may increase the degree of polymer chain entanglement and produce stiffer and more cohesive hydrogels, while lower molecular weight polymers may provide improved polymer chain mobility and thus produce softer and more injectable hydrogels. In some embodiments, the molecular weight of the polymer (weight average or number average molecular weight) is greater than or equal to 25 kDa, 50 kDa, 70 kDa, 75 kDa, 100 kDa, 125 kDa, 147 kDa, 150 kDa, 166 kDa, 175 kDa, 200 kDa, or 250 kDa; and / or is no more than 250 kDa, 200 kDa, 175 kDa, 166 kDa, 150 kDa, 147 kDa, 125 kDa, 100 kDa, 75 kDa, 70 kDa, 50 kDa, 25 kDa. The molecular weight of the polymer can be within a range from 10 kDa to 300 kDa, 10 kDa to 200 kDa, 10 kDa to 150 kDa, 10 kDa to 100 kDa, 10 kDa to 75 kDa, 10 kDa to 50 kDa, 50 kDa to 300 kDa, 50 kDa to 200 kDa, 50 kDa to 150 kDa, 50 kDa to 100 kDa, 50 kDa to 75 kDa, 75 kDa to 300 kDa, 75 kDa to 200 kDa, 75 kDa to 150 kDa, 75 kDa to 100 kDa, 100 kDa to 300 kDa, 100 kDa to 200 kDa, 100 kDa to 175 kDa, 100 kDa to 150 kDa, 125 kDa to 175 kDa, 125 kDa to 150 kDa, 150 kDa to 300 kDa, 150 kDa to 250 kDa, 150 kDa to 200 kDa, or 200 kDa to 300 kDa. The concentration of the polymer in the freeze-thawed hydrogel can be varied as desired. Higher concentrations of polymer may increase the degree of polymer chain entanglements and thus produce stiffer, stronger, and more viscous hydrogels, while lower concentrations of polymer may result in less polymer chain entanglement and thus produce softer MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO and more injectable hydrogels. In some embodiments, the concentration of the polymer(s) in the hydrogel as expressed in % w / w, individually or collectively, is greater than or equal to 1%, 2%, 3%, 3.5%, 4%, 4.5%, 5%, 5.25%, 5.5%, 5.75%, 6%, 6.25%, 6.5%, 6.75%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 12%, 15%, 17%, 20%, or 25%, and / or is no more than 25%, 20%, 17%, 15%, 12%, 10%, 9.5%, 9%, 8.5%, 8%, 7.5%, 7%, 6.75%, 6.5%, 6.25%, 6%, 5.75%, 5.5%, 5.25%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, or 1%. The concentration of the polymer(s) in the hydrogel, individually or collectively, can be within a range from 1% to 25%, 1% to 15%, 1% to 10%, 1% to 5%, 1% to 2%, 2% to 25%, 2% to 15%, 2% to 10%, 2% to 5%, 2% to 8%, 3% to 7%, 4% to 6%, 5% to 25%, 5% to 15%, 5% to 10%, 10% to 25%, 10% to 15%, or 15% to 25%. In some embodiments, the freeze-thawed hydrogel is a PVA hydrogel. PVA may be synthesized from polyvinyl acetate through hydrolysis, e.g., as shown in the reaction scheme of FIG.5. The degree of hydrolysis corresponds to the proportion of PVA repeating units that have a hydroxyl group, e.g., 100% hydrolyzation indicates that all PVA repeating units have a hydroxyl group. The degree of hydrolysis can be used to control the properties of the freeze-thawed hydrogel, e.g., greater hydrolyzation corresponds to more hydroxyl groups which can lead to stronger hydrogen bonding and better cohesive strength of the hydrogel. In some embodiments, the PVA hydrogels herein are at least 80%, 85%, 87%, 90%, 95%, or 99%+ hydrolyzed; and / or have a degree of a hydrolysis within a range from 80% to 99%, 80% to 90%, 85% to 95%, 87% to 99%, or 90% to 99%+. Optionally, the PVA hydrogels herein can be formed from a plurality of PVA polymers that differ from each other with respect one or more characteristics, such as molecular weight, degree of hydrolyzation, etc. This approach allows for more precise control over the extent of hydrogen bonding and chain entanglements within the hydrogel. For example, a PVA hydrogel can include a first PVA polymer having a first degree of hydrolyzation, and a second PVA polymer having a second degree of hydrolyzation. In some embodiments, the first PVA polymer has a higher degree of hydrolyzation (e.g., at least 90%, 95%, or 99%+ hydrolyzation), and the second PVA polymer has a lower degree of hydrolyzation (e.g., no more than 95%, 90%, 87%, or 85% hydrolyzation). The first PVA polymer can be present in the same or a greater concentration than the second PVA polymer, e.g., the ratio of the concentration of the first PVA polymer to the concentration of the second PVA polymer can be at least 1:1, 1.5:1, 2:1, 3:1, 4:1, 5:1, 10:1, or MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO 20:1. The first PVA polymer can have the same molecular weight as the second PVA polymer, or can have a different (e.g., greater or lower) molecular weight than the second PVA polymer. In some embodiments, the PVA hydrogels herein are formed from a first PVA polymer having a first, higher molecular weight, and a second PVA polymer having a second, lower molecular weight. The first PVA polymer can provide enhanced shear strength and / or cohesiveness, while the second PVA polymer can provide improved plasticity. The first molecular weight of the first PVA polymer can be at least 10 kDa, 20 kDa, 30 kDa, 40 kDa, or 50 kDa greater than the second molecular weight of the second PVA polymer. For example, the first molecular weight of the first PVA polymer can be at least 80 kDa, 90 kDa, 100 kDa, 110 kDa, or 120 kDa; and / or can be within a range from 80 kDa to 120 kDa, 90 kDa to 110 kDa, 95 kDa to 105 kDa. The second molecular weight of the second PVA polymer can be less than or equal to 75 kDa, 70 kDa, 65 kDa, 60 kDa, 55 kDa, or 50 kDa; and / or can be within a range from 50 kDa to 75 kDa, 60 kDa to 75 kDa, or 70 kDa to 75 kDa. The concentration of the first PVA polymer (% w / w) can be within a range from 5% to 9%, 6% to 8%, 6% to 6.5%, or 6.5% to 7.5%. The concentration of the second PVA polymer (% w / w) can be within a range from 0.5% to 8%, 1% to 5%, or 1% to 2%. In some embodiments, the PVA hydrogels herein include 6.1% of a 100 kDa PVA polymer and 1.7% of a 70 kDa PVA polymer. B. Additives In some embodiments, the freeze-thawed hydrogels described herein include at least one additive. One or more additives may be used to modulate hydrogel properties, such as by increasing or decreasing the modulus (e.g., storage modulus, loss modulus, compression modulus, tensile modulus), strength, cohesiveness, viscosity, shear-thinning behavior, etc., of the hydrogel. Modulation of hydrogel properties may be achieved through non-covalent interactions between the additive and the polymer. For instance, additives that form strong hydrogen bonds with the polymer (e.g., with the hydroxyl groups of PVA) can produce stiffer and more cohesive hydrogels. Examples of such additives include polyacrylic acid and other polymers with carboxylic acid groups. Alternatively, additives that reduce the strength of hydrogen bonding between polymer chains can produce softer and less cohesive hydrogels. Examples of such additives include polyvinyl pyrrolidone (PVP), polyethylene glycol, polyacrylamide, poloxamers, and laponite. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Softer and less cohesive hydrogels can also be produced by additives that act as plasticizers, such as glycerol, ethylene glycol, and propylene glycol. The hydrogel can include a single type of additive or can include multiple different types of additives (e.g., two, three, four, five, or more different additives). The additive(s) can be present in the hydrogel at any suitable concentration. In some embodiments, the concentration of the additive(s) in the hydrogel (% w / w), individually or collectively, is greater than or equal to 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 30%, 40% or 50%; and / or is no more than 50%, 40%, 30%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or 0.5%. The concentration of the additive(s) in the hydrogel, individually or collectively, can be within a range from 0.1% to 20%, 0.1% to 10%, 1% to 15%, 1% to 10%, 5% to 15%, 5% to 15%, or 10% to 15%. Moreover, the molecular weight of the additive may also be varied to further tune the mechanical and rheological properties of the hydrogel, e.g., in embodiments where the additive is a polymer such as polyacrylamide, polyethylene glycol, etc. In embodiments where the freeze-thawed hydrogel is a PVA hydrogel, the PVA hydrogel can include one or more PVA polymers (e.g., a higher molecular weight PVA polymer and a lower molecular weight PVA polymer), and one or more PVP polymers that reduce physical crosslinking between the PVA polymers to increase the plasticity of the PVA hydrogel. The one or more PVP polymers can include a first PVP polymer having a first, higher molecular weight, and a second PVP polymer having a second, lower molecular weight. The first molecular weight of the first PVP polymer can be at least 10 kDa, 20 kDa, 30 kDa, 40 kDa, or 50 kDa greater than the second molecular weight of the second PVP polymer. For example, the first molecular weight of the first PVP polymer can be at least 20 kDa, 30 kDa, 40 kDa, 50 kDa, or 60 kDa; and / or can be within a range from 20 kDa to 60 kDa, 30 kDa to 50 kDa, or 35 kDa to 45 kDa. The second molecular weight of the second PVP polymer can be less than or equal to 20 kDa, 15 kDa, 10 kDa, 8 kDa or 5 kDa; and / or can be within a range from 5 kDa to 15 kDa, 5 kDa to 10 kDa, or 10 kDa to 15 kDa. The concentration of the first PVP polymer (% w / w) can be within a range from 0.5% to 8%, 1% to 5 %, or 2% to 3%. The concentration of the second PVP polymer (% w / w) can be within a range from 0.5% to 8%, 1% to 5%, or 2% to 3%. In some embodiments, the PVA hydrogels herein include 2.2% of a 40 kDa PVP polymer and 2.2% of a 10 kDa PVP polymer. In some embodiments, the PVA hydrogels herein include 6.1% of a 100 kDa PVA polymer, 1.7% of a 70 kDa PVA polymer, 2.2% of a 40 kDa PVP polymer, and 2.2% of a 10 kDa PVP polymer. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO In other embodiments, however, the freeze-thawed hydrogels herein may not include any additives to modulate hydrogel properties, such that the characteristics of the hydrogel are determined primarily or entirely by the properties of the polymer(s) used to form the hydrogel. C. Contrast Agents In some embodiments, the freeze-thawed hydrogels described herein include at least one contrast agent that allows for visualization of the hydrogel during and / or after injection into the treatment site. For example, the contrast agent can be configured for radiographic imaging. Examples of contrast agents that may be used include tantalum, bismuth trioxide (bismuth (III) oxide), bismuth oxychloride, tungsten, tungsten carbide, barium sulfate, gadolinium, iodized oil (e.g., LIPIODOL®), iohexol (e.g., OMNIPAQUE™ from GE Healthcare), iopamidol (e.g., ISOVUE™ from Bracco Diagnostics, Inc.), ioxilan, iopromide, iodixanol (e.g., VISIPAQUE™ from GE Healthcare), iobitridol, ioversol, diatrizoate (e.g., HYPAQUE™ from GE Healthcare), metrizoate, iothalamate (e.g., CONRAY™ from Covidien), ioxaglate, iopromide, iothalamate / meglumine, ioxaglate / meglumine, diatrizoate / meglumine, iodamide sodium, metrizamide, or combinations thereof. The hydrogel can include a single type of contrast agent or can include a plurality of different types of contrast agents (e.g., two, three, four, five, or more different types of contrast agents). The concentration of the contrast agent(s) in the freeze-thawed hydrogel (% w / w), individually or collectively, can be greater than or equal to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%, and / or no more than 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, or 10%. In some embodiments, the concentration of the contrast agent(s) in the hydrogel, individually or collectively, is within a range from 10% to 90%, 10% to 80%, 10% to 60%, 10% to 50%, 10% to 40%, 10% to 30%, 20% to 90% 20% to 80%, 20% to 60%, 20% to 50%, 20% to 40%, 20% to 30%, 25% to 75%, 25% to 50%, 30% to 80%, 30% to 50%, 50% to 75% or 75% to 95%. In some embodiments, the contrast agent is present in the initial polymer solution that undergoes the freeze-thaw process to form the hydrogel. In other embodiments, however, the contrast agent may be added to the hydrogel after the freeze-thaw process. For instance, a freeze- thawed hydrogel may be immersed into a solution of the contrast agent after gelation to load the hydrogel with the contrast agent via exchange with water molecules present in the hydrogel. The MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO immersion period can be at least 1 hour, 2 hours, 4 hours, 8 hours, 10 hours, 12 hours, 16 hours, 20 hours, or 24 hours, for example. Optionally, after the freeze-thawed hydrogel is implanted into the treatment site, the contrast agent may gradually diffuse out of the hydrogel, thus resulting in diminishing radiopacity over time. This approach may be advantageous where the contrast agent produces imaging artifacts that may interfere with subsequent imaging of the treatment site. In some embodiments, the hydrogel is configured to release at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or 99% of the initial loading contrast agent within a target time period after implantation, where the target time period is at least 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days, 2 weeks, 3 weeks, 4 weeks, 1 month, 2 months, or 3 months after implantation. In other embodiments, however, the contrast agent may remain within the hydrogel without leaching out after implantation. In other embodiments, the contrast agent is optional and may be omitted from the freeze-thawed hydrogel. In such embodiments, the degree of occlusion of the treatment site by the hydrogel may be assessed using other techniques, such as based on extent of inversion of neck cover by the hydrogel (e.g., as discussed above with respect to FIGS.3A–3D). D. Solvents The freeze-thawed hydrogels disclosed herein can also include at least one solvent. The solvent can be a biocompatible aqueous solution, such as water (e.g., distilled water, deionized water), a saline solution (e.g., normal saline, Ringer’s lactate solution, phosphate-buffered saline), etc. The solvent can optionally be supplied as a part of other components of the hydrogel, such as the additive or contrast agent. The concentration of the solvent in the freeze-thawed hydrogel (% w / w) can be greater than or equal to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%, and / or no more than 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, or 10%. In some embodiments, the concentration of the solvent in the hydrogel is within a range from 10% to 90%, 10% to 80%, 10% to 60%, 10% to 50%, 10% to 40%, 10% to 30%, 20% to 90% 20% to 80%, 20% to 60%, 20% to 50%, 20% to 40%, 20% to 30%, 25% to 75%, 25% to 50%, 30% to 80%, 30% to 50%, 50% to 75%, or 75% to 95%. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO E. Hydrogel Properties The freeze-thawed hydrogels described herein can be configured for intravascular delivery into a treatment site (e.g., an aneurysm or other vascular defect) via injection through an elongate shaft (e.g., a microcatheter or other delivery catheter). In some embodiments, the hydrogels described herein can be used without carrying out any preliminary mixing and / or crosslinking of precursor materials. In some embodiments, the hydrogels herein can be delivered into the patient’s body without any prior mixing and / or crosslinking steps that occur within 1 minute, 2 minutes, 5 minutes, 10 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, 8 hours, 12 hours, 24 hours, 36 hours, 48 hours, 3 days, 4 days, 5 days, 6 days, 1 week, 2 weeks, 3 weeks, 4 weeks, 2 months, 3 months, 6 months, or 1 year before delivery. Instead, the hydrogels can be provided in an injectable, preformed state in a sterilized package (e.g., a vial or preloaded syringe) that is immediately ready for use. Accordingly, in contrast to conventional approaches, the hydrogel can be used without relying on an in situ polymerization reaction, crosslinking reaction, or other chemical reaction that may occur while the hydrogel is being injected, which may cause the hydrogel to become too viscous to be injected through a microcatheter. Excessive viscosity may cause plugging or rupture of the microcatheter, and may delay or even prevent a surgical aneurysm repair procedure. Additionally, the freeze-thawed hydrogels disclosed herein can be provided in a preformed, ex vivo state that does not need to undergo any significant changes in material properties before, during, and / or after delivery into the body to effectively occlude the treatment site. As previously discussed, conventional injectable compositions typically exhibit significantly different ex vivo and in vivo properties, and thus must undergo a phase transition within the patient’s body to reach a final, therapeutically effective state. For example, conventional compositions are typically in a liquid state ex vivo (e.g., a state having a low viscosity, a low (or zero) degree of crosslinking, and / or a loss modulus greater than the storage modulus), and transition to a solid or semi-solid state (e.g., a state having a high viscosity, a high degree of crosslinking, and / or a storage modulus greater than the loss modulus) when exposed to in vivo conditions (e.g., physiological temperature, pH, salt concentrations, etc.) and / or other conditions intended to induce a phase transition (e.g., physically induced, electromagnetically induced, etc.). MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO In contrast, the freeze-thawed hydrogels described herein can be a solid or semi- solid material (e.g., a viscoelastic, injectable hydrogel) that already exhibits sufficient mechanical strength and cohesiveness for occluding an aneurysm or other treatment site without requiring further crosslinking, phase transitions, and / or other significant changes in material properties. For example, in the ex vivo state, the hydrogel can be a unitary, cohesive, solid or semi-solid mass that sticks to itself and does not disperse or dissolve when placed in a physiological solution (e.g., phosphate-buffered saline). Similarly, when delivered into the body, the hydrogel can remain sufficiently solid to fill and seal the treatment site without dispersing or dissolving when exposed to in vivo conditions (at least until biodegradation and / or bioresorption of the polymer occurs, if applicable). As discussed above, this approach can reduce the likelihood of the hydrogel leaking out of the treatment site during and / or after delivery, thus lowering the risk of patient complications such as stroke. However, when subjected to pressure, the hydrogel can exhibit shear-thinning properties suitable for injection into the treatment site via a delivery catheter, such as a microcatheter having an inner diameter less than or equal to 0.02 inches, 0.015 inches, 0.014 inches, 0.013 inches, 0.012 inches, 0.011 inches, or 0.01 inches. The freeze-thawed hydrogels described herein can be sufficiently stiff to effectively occlude a vascular defect, but not so stiff that the hydrogel cannot be delivered via injection. The stiffness of the hydrogel may be related to the storage modulus of the hydrogel, in that a higher storage modulus corresponds to a stiffer hydrogel. In some embodiments, the storage modulus of the freeze-thawed hydrogel is greater than or equal to 1 kPa, 2 kPa, 3 kPa, 5 kPa, 6 kPa, 7 kPa, 8 kPa, 9 kPa, 10 kPa, or 15 kPa; and / or is no more than 15 kPa, 10 kPa, 9 kPa, 8 kPa, 7 kPa, 6 kPa, 5 kPa, 3 kPa, 2 kPa, or 1 kPa. The storage modulus of the freeze-thawed hydrogel can be within a range from 1 kPa to 20 kPa, 1 kPa to 10 kPa, 1 kPa to 5 kPa, 1 kPa to 3 kPa, 1 kPa to 2 kPa, 3 kPa to 8 kPa, 5 kPa to 15 kPa, 5 kPa to 10 kPa, 6 kPa to 10 kPa, or 7 kPa to 9 kPa. The storage modulus of the freeze-thawed hydrogel can be greater than the loss modulus of the hydrogel, such that the hydrogel behaves primarily like a solid or semi-solid substance rather than like a liquid. In some embodiments, the loss modulus of the freeze-thawed hydrogel is greater than or equal to 50 Pa, 100 Pa, 200 Pa, 300 Pa, 400 Pa, 500 Pa, 600 Pa, 700 Pa, or 800 Pa; and / or is no more than 800 Pa, 700 Pa, 600 Pa, 500 Pa, 400 Pa, 300 Pa, 200 Pa, or 100 Pa. The loss modulus of the freeze-thawed hydrogel can be within a range from 50 Pa to 1000 Pa, 100 Pa to 1000 Pa, 100 Pa to 500 Pa, 100 Pa to 200 Pa, 200 Pa to 800 Pa, 200 Pa to 600 Pa, or 300 MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Pa to 500 Pa. The ratio of the loss modulus to the storage modulus of the freeze-thawed hydrogel (tan delta) can be less than or equal to 0.5, 0.4, 0.3, 0.2, 0.15, 0.1, 0.075, 0.05, 0.04, 0.03, 0.02, or 0.01. The storage and loss moduli of the hydrogel can be measured via any suitable technique, such as using a parallel plate rheometer (e.g., 8 mm plate diameter) operating in shear mode at a suitable temperature (e.g., 37 °C). The storage and loss moduli can be measured within the linear viscoelastic region of the hydrogel, where the linear viscoelastic region of the freeze- thawed hydrogel corresponds to a critical strain value that is less than or equal to 10%, 8%, 5%, 4%, 3%, or 2%. The storage and loss moduli can be measured at a suitable angular frequency, such as an angular frequency of 0.1 rad / s, 1 rad / s, 10 rad / s, or 100 rad / s. The stiffness of the freeze-thawed hydrogel can also be correlated to the compression modulus of the hydrogel, in that a higher compression modulus corresponds to a stiffer hydrogel. In some embodiments, the compression modulus of the freeze-thawed hydrogel is greater than or equal to 1 kPa, 5 kPa, 10 kPa, 12 kPa, 15 kPa, 20 kPa, 25 kPa, or 30 kPa; and / or is no more than 30 kPa, 25 kPa, 20 kPa, 15 kPa, 12 kPa, 10 kPa, 5 kPa, or 1 kPa. The compression modulus of the freeze-thawed hydrogel can be within a range from 1 kPa to 50 kPa, 1 kPa to 25 kPa, 5 kPa to 25 kPa, 5 kPa to 20 kPa, 5 kPa to 15 kPa, 10 kPa to 20 kPa, 10 kPa to 15 kPa, or 15 kPa to 25 kPa. The compression modulus of the hydrogel can be measured via any suitable technique, such as using a parallel plate rheometer (e.g., 8 mm plate diameter) operating in compression mode at a suitable temperature (e.g., 37 °C). The compression modulus can be calculated from the average slope of the initial linear region (e.g., corresponding to 0.1% to 5% strain) of the stress / strain curve. In some embodiments, the freeze-thawed hydrogels described herein have sufficient cohesive strength such that when the hydrogel is used in combination with a mesh neck cover, the hydrogel does not leak through the pores of the mesh. For instance, the hydrogel can remain as a unitary cohesive mass when subjected to at least 50%, 75%, 80%, 90%, or 100% strain under shear; and / or at least 50%, 75%, 80%, or 90% strain under compression. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO F. Methods In some embodiments, the present technology provides a method for preparing a freeze-thawed hydrogel for occluding a treatment site, such as an aneurysm or other vascular defect. The method can include providing a composition including at least one polymer (e.g., PVA) in a solvent (e.g., water). The composition can optionally include other components, such as at least one additive (e.g., PVP) and / or at least one contrast agent. In some embodiments, the composition includes a polymer, an additive, a contrast agent, and a solvent. In some embodiments, the composition includes a polymer, an additive, and a solvent, without any contrast agent. The types and concentrations of polymer, additive, contrast, agent, and solvent can be varied as desired, e.g., as discussed in Sections II.A–II.D above. The method can further include forming a freeze-thawed hydrogel by subjecting the composition to one or more freeze-thaw cycles (e.g., one, two, three, four, five, or more freeze- thaw cycles). The freeze-thaw cycles can result in formation of non-covalent crosslinks between polymer chains of the polymer. For example, in embodiments where the polymer is PVA, the freeze-thaw cycles can cause phase separation and formation of hydrogen bonds between the hydroxyl groups of the PVA. Optionally, the freeze-thaw cycles can result in formation of non- covalent crosslinks (e.g., hydrogen bonds) between the polymer and an additive, if present. As discussed herein, the number, duration, and temperatures of the freeze-thaw cycles can be varied as appropriate to produce the desired hydrogel properties. In embodiments where a contrast agent was not initially present in the composition used to form the freeze-thawed hydrogel, the method can further include loading at least one contrast agent in the freeze-thawed hydrogel, after the freeze-thawed hydrogel has been formed via one or more freeze-thaw cycles as discussed above. The loading may be performed, for example, by immersing the freeze-thawed hydrogel in a composition including the contrast agent and a solvent. The solvent may be the same solvent used for the freeze-thawed hydrogel or may be a different solvent. In some embodiments, the present technology provides a method for treating a patient using a freeze-thawed hydrogel as described herein. The method can include delivering a freeze-thawed hydrogel to a treatment site. For example, the freeze-thawed hydrogel can be delivered into an aneurysm to partially or fully occlude the aneurysm. The freeze-thawed hydrogel MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO can be delivered via injection, e.g., via a catheter or other elongate shaft that is introduced to the treatment site via the vasculature. As discussed herein, the properties of the freeze-thawed hydrogel (e.g., storage modulus, loss modulus, compression modulus) can be configured so that the hydrogel is sufficiently soft and flowable to be delivered via injection, but also sufficiently stiff and cohesive so that the hydrogel forms a solid or semi-solid mass at the treatment site and does not leak out after delivery. In some embodiments, a freeze-thawed hydrogel is composed of 5% w / w to 10% w / w PVA (e.g., 6.5% w / w PVA), and 20% w / w to 40% w / w iohexol. The PVA can have a weight average molecular weight within a range from 100 kDa to 200 kDa (e.g., 146 kDa to 186 kDa). The freeze-thawed hydrogel can have a storage modulus within a range from 3 kPa to 8 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel. The freeze-thawed hydrogel can have a loss modulus within a range from 200 Pa to 600 Pa at 37 °C over the linear viscoelastic region of the freeze-thawed hydrogel. The freeze-thawed hydrogel can have a compression modulus within a range from 5 kPa to 20 kPa at 37 °C. In some embodiments, a freeze-thawed hydrogel is composed of 5% w / w to 8% w / w high molecular weight PVA (e.g., 6.1% w / w high molecular weight PVA), 1% w / w to 2% w / w low molecular weight PVA (e.g., 1.7% w / w low molecular weight PVA), 1.5% w / w to 2.5% w / w high molecular weight PVP (e.g., 2.2% w / w high molecular weight PVP), 1.5% w / w to 2.5% w / w low molecular weight PVP (e.g., 2.2% w / w low molecular weight PVP), and 10% w / w to 15% w / w bismuth trioxide (e.g., 13% w / w bismuth trioxide). The high molecular weight PVA can have a weight average molecular weight within a range from 80 kDa to 120 kDa, such as 100 kDa. The low molecular weight PVA can have a weight average molecular weight within a range from 60 kDa to 80 kDa, such as 70 kDa. The high molecular weight PVP can have a weight average molecular weight within a range from 30 kDa to 50 kDa, such as 40 kDa. The low molecular weight PVP can have a weight average molecular weight within a range from 5 kDa to 15 kDa, such as 10 kDa. The freeze-thawed hydrogel can have a storage modulus within a range from 1 kPa to 2 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel. The freeze- thawed hydrogel can have a loss modulus within a range from 100 Pa to 500 Pa at 37 °C over the linear viscoelastic region of the freeze-thawed hydrogel. The freeze-thawed hydrogel can have a compression modulus within a range from 10 kPa to 15 kPa at 37 °C. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Optionally, the freeze-thawed hydrogel can be used in combination with another device, such as a neck cover for an aneurysm (e.g., as described in Section I above). In such embodiments, the neck cover can be positioned within the aneurysm before the freeze-thawed hydrogel is delivered into the aneurysm. The freeze-thawed hydrogel can have sufficient stiffness and cohesive strength such that when the hydrogel is delivered into the aneurysm, the hydrogel pushes downward against the neck cover to ensure that the interior cavity of the aneurysm is substantially completely filled, without leaking through the neck cover. III. Examples The following examples are included to further describe some aspects of the present technology, and should not be used to limit the scope of the technology. Example 1: Preparation and Characterization of Freeze-Thawed PVA Hydrogels This example describes the preparation and characterization of PVA hydrogels prepared using a freeze-thaw process. Hydrogels were prepared using high molecular weight (HMW) PVA (weight average molecular weight 146 kDa–186 kDa, 99+% hydrolyzed or 87%–89% hydrolyzed) or low molecular weight (LMW) PVA (weight average molecular weight 55 kDa–85 kDa, 99+% hydrolyzed). The PVA was stirred at 95 °C in distilled water until fully dissolved. Once dissolved, the solution was brought to room temperature (22 °C). The solution was then subjected to one or more freeze-thaw cycles to form a hydrogel. For each freeze-thaw cycle, the freezing phase was performed by placing the solution in a freezer at -18 °C for 16 hours, and the thawing phase was performed by placing the solution at room temperature for 8 hours. For hydrogels containing tantalum or bismuth trioxide as a contrast agent, tantalum or bismuth trioxide was added to the polymer solution at room temperature, then the solution was vortex mixed for 1 minute before freeze-thawing to form the hydrogel. For hydrogels containing iohexol (Omnipaque) as a contrast agent, freeze-thawed hydrogels were immersed in an Omnipaque solution for 16 hours at room temperature, resulting in exchange of water molecules from the hydrogel with iohexol molecules from the Omnipaque solution. For hydrogels containing an additive (glycerol, polyacrylic acid (viscosity average molecular weight 1.25 MDa), or MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO polyvinyl pyrrolidone (weight average molecular weight 40 kDa)), the additive was added to the polymer solution before the freeze-thawing process. FIG.6A is a photograph of a PVA solution before freeze-thawing, and FIG.6B is a photograph of the hydrogel formed from the PVA solution after freeze-thawing. The PVA solution contained 10% HMW PVA without any contrast agent. After one freeze-thaw cycle, a cohesive gel was formed. FIG.7A is a photograph of a freeze-thawed PVA hydrogel immersed in water, and FIG.7B is a photograph of a freeze-thawed PVA hydrogel immersed in phosphate-buffered saline (PBS) solution. The PVA hydrogel was formulated using 10% HMW PVA and 20% tantalum, and with two freeze-thaw cycles. The PVA hydrogels were stable and did not dissolve or swell after immersion in PBS for 2 days at 40 °C, and in PBS and water after 4 weeks at room temperature, as determined by visual assessment. Table 1 below lists properties of freeze-thawed PVA hydrogels formulated with HMW PVA versus LMW PVA (no contrast agent). Both PVA polymers were 99+% hydrolyzed. In general, hydrogel stiffness and cohesive strength increased with increasing PVA concentration and with increasing number of freeze-thaw cycles. Hydrogels formulated with 5%–6.75% of the HMW PVA and two freeze-thaw cycles exhibited suitable properties for injection and embolization, whereas hydrogels formulated with the LMW PVA did not exhibit suitable properties. Table 1: PVA molecular weight versus hydrogel properties Molecular PVA Number of Formulation weight concentration freeze-thaw Hydrogel properties (kDa) (% w / w) cycles 155–85 17 1Very brittle, poorcohesive strength 2 55–85 17 2 Very brittle and soft 3 55–85 17 3 Brittle and soft 4 55–85 17 4 Brittle 5 55–85 17 5 Stiff, but brittle MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Molecular PVA Number of Formulation weight concentration freeze-thaw Hydrogel properties (kDa) (% w / w) cycles 6 55–85 17 6 Very stiff, but brittle 7 55–85 25 2 Too stiff 8146–186 5 2Good cohesivestrength9 146–186 5.25 2Good cohesivestrength10 146–186 5.5 2Good cohesivestrength11 146–186 5.75 2Good cohesivestrength12 146–186 6 2Good cohesivestrength13 146–186 6.25 2Good cohesivestrength14 146–186 6.5 2Good cohesivestrength15 146–186 6.75 2 Good cohesivestrength, a little stiff16 146–186 7 2 Too stiff Table 2 below lists properties of freeze-thawed PVA hydrogels formulated with varying concentrations of PVA (146 kDa–186 kDa, 99+% hydrolysis) (no contrast agent) and two freeze-thaw cycles. Hydrogel stiffness and cohesive strength increased with increasing PVA concentration. Hydrogels formulated with 5%–6.75% PVA exhibited suitable properties for injection and embolization. Hydrogels with lower PVA content were nonhomogeneous and too cohesively weak, while hydrogels with higher PVA content were too stiff for injection. Table 2: PVA content versus hydrogel properties MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO PVA Formulation concentration Hydrogel properties (% w / w) 1 2.5 Nonhomogeneous gel, cohesively weak 2 3.2 Nonhomogeneous gel 3 5 Acceptable cohesive strength 4 5.25 Good cohesive strength 5 5.5 Good cohesive strength 6 5.75 Good cohesive strength 7 6 Good cohesive strength 8 6.25 Good cohesive strength 9 6.5 Good cohesive strength 10 6.75 Good cohesive strength, a little stiff 11 7 Too stiff 12 7.5 Too stiff Table 3 below lists properties of freeze-thawed PVA hydrogels formulated with varying numbers of freeze-thaw cycles (no contrast agent). Both HMW PVA (146 kDa–186 kDa, 99+% hydrolysis) and LMW PVA (55 kDa–85 kDa, 99+% hydrolysis) were tested. For hydrogels formulated with the HMW PVA, a single freeze-thaw cycle was not sufficient to produce a cohesive gel unless high PVA concentrations (at least 7%) were used, whereas two freeze-thaw cycles produced gels with acceptable cohesiveness and stiffness for formulations with less than 7.5% PVA. For hydrogels formulated with the LMW PVA, the hydrogel properties were either too weak or too stiff for use. Table 3: Number of freeze-thaw cycles versus hydrogel properties MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO PVA Molecular Number of Formulation concentration weight freeze-thaw Hydrogel (% w / w) (kDa) cycles Properties1 5 146–186 1 Nonhomogeneousgel, cohesively weak2 5 146–186 2Acceptable cohesivestrength3 5 146–186 3Very cohesive, toostiff4 6 146–186 1 Too soft 5 6 146–186 2 Cohesive 6 6.25 146–186 1 Too soft 7 6.25 146–186 2 Cohesive 8 6.5 146–186 1 Too soft 9 6.5 146–186 2 Cohesive 10 6.75 146–186 1 Too soft 11 6.75 146–186 2 Cohesive 12 7 146–186 1 Cohesive, but soft 13 7 146–186 2 Cohesive, stiff14 7.5 146–186 1Cohesive, a littlestiff15 7.5 146–186 2 Cohesive, too stiff16 17 55–85 1 Poor cohesivestrength, soft17 17 55–85 2 Poor cohesivestrength, soft18 17 55–85 3Brittle and relativelysoft19 17 55–85 4 Stiff but brittle MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO PVA Molecular Number of Formulation concentration weight freeze-thaw Hydrogel (% w / w) (kDa) cycles Properties 20 17 55–85 5 Stiff but brittle 21 17 55–85 6 Very stiff but brittle Table 4 below lists properties of freeze-thawed PVA hydrogels formulated with varying degrees of hydrolyzation (146 kDa–186 kDa, 99+% hydrolysis or 87%–99% hydrolysis) (no contrast agent). The hybrid formulation (Formulation 6) was prepared with two PVA polymers: one with 99+% hydrolysis and one with 87%–99% hydrolysis. Formulations including 87%–99% hydrolyzed PVA only did not form gels even after undergoing a relatively high number of freeze-thaw cycles. Formulations including 99+% hydrolyzed PVA only and using the same number of freeze-thaw cycles as the partially hydrolyzed PVA formulations formed very stiff, cohesive gels. The hybrid formulation formed a hydrogel that was softer and tackier than the 99+% hydrolyzed PVA only hydrogels, suggesting that the presence of less hydrolyzed PVA reduced the degree of hydrogen bonding between PVA chains thereby producing weaker gels. Overall, these results demonstrate that increasing the PVA content and the number of freeze-thaw cycles produces relatively large changes in the stiffness and cohesion of the hydrogel, while addition of a small amount of partially hydrolyzed PVA reduces the amount of hydrogen bonding and allows for fine tuning of the mechanical properties of the hydrogel. Table 4: PVA hydrolysis versus hydrogel properties PVA Formulation %Number of concentration (% freeze-thaw Hydrogel w / w) c properties ycles 1 87%-89% 5 5 Did not gel 2 87%-89% 7.5 5 Did not gel 3 87%-89% 10 5 Did not gel 499+% 5 3Cohesive, verystiff5 99+% 7.5 2Cohesive, verystiff MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Formulation %PVA Number of Hydrolysisconcentration (% freeze-thaw Hydrogel w / w) cycles properties 5% PVA with 6 Hybrid 99+% hydrolysis,Soft and tacky5% PVA with 87%-2 gel99% hydrolysis Table 5 below lists properties of freeze-thawed PVA hydrogels formulated with and without additives (no contrast agent). Both HMW PVA (146 kDa–186 kDa, 99+% hydrolysis) and LMW PVA (55 kDa–85 kDa, 99+% hydrolysis) were tested. Glycerol is a low molecular weight additive that is expected to act as a plasticizer by allowing greater mobility of polymer chains; low molecular weight PVA hydrogels formulated with glycerol exhibited reduced brittleness compared to low molecular weight PVA hydrogels without any additive. Polyacrylic acid is expected to produce stronger hydrogen bonding via its carboxylic acid; low molecular weight PVA hydrogels formulated with polyacrylic acid exhibited improved cohesion compared to low molecular weight PVA hydrogels without any additive. Polyvinyl pyrrolidone is expected to reduce the strength of hydrogen bonding; the resulting gel was stiff and brittle. Overall, these results indicate that additives can be successfully used to modulate hydrogel properties. Table 5: Additives in PVA hydrogels versus hydrogel properties # of Additive FormulationPVA conc.Molecular freeze- Hydrogel (% w / w)Additive conc. (% weight (kDa) thaw prop. w / w) cycles 1 17 55–85 4 None Brittle Less 2 17 55–85 4 Glycerol 10% brittle, a little stiff Improved 317 55–85 4Polyacryliccohesion, acid 10%a little tacky 417 55–85 4 PolyvinylStiff, but pyrrolidone5%brittle 55 146–186 2 None N / ACohesivegel MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Rheological testing of hydrogels was performed using a parallel plate rheometer with an 8 mm plate diameter at 37 °C. For storage modulus and loss modulus measurements, the rheometer was operated in shear mode with a steady axial force of 30 g. Storage and loss moduli were measured at the linear viscoelastic region at 10 rad / s angular frequency. For compression modulus measurements, the rheometer was operated in compression mode with an initial axial force of 10 g. Compression moduli were calculated from the average slope of the initial linear portion of the stress / strain curve (0.1%–5% strain). Table 6 below lists rheological properties of freeze-thawed hydrogels formulated with varying concentrations of PVA and varying numbers of freeze-thaw cycles. Both HMW PVA (146 kDa–186 kDa, 99+% hydrolysis) and LMW PVA (55 kDa–85 kDa, 99+% hydrolysis) were tested. The hydrogels were formulated with Omnipaque, tantalum, or bismuth trioxide as a contrast agent. Of the tested formulations, only the hydrogel formulated with 6.5% HMW PVA and two freeze-thaw cycles exhibited properties suitable for injection and embolization. Table 6: Rheological properties of hydrogels PVA # of Contrast Compr nc. MW freeze- agentStoression Formu coagemodulusHydrogel modulus lation (% (kDa) thaw properties (kPa) (kPa) w / w) cycles 20–40% 15146–186 1w / w 2.3 ± 0.5 9 ± 1 Too soft Omnipaque 20–40% 5 17 55–85 6 w / w 18.4 ± 1 62 ± 4 Too stiff Omnipaque 617 55–85 3 20% w / wStiff and Tantalum5.7 ± 0.9 21 ± 2brittle MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO PVA # of Contrast . MW freeze- agentSCompression Formu conctoragemodulusHydrogel modulus lation (% (kDa) thaw properties (kPa) (kPa) w / w) cycles 1420% w / w Very 7 6.56–1862 Bismuth 12.1 ± 1 28 ± 4 cohesive trioxide and elastic these results demonstrate that molecular weight, concentration, and degree of hydrolyzation of the PVA polymer, as well as the number of freeze-thaw cycles, are significant parameters for the performance of the hydrogel. Careful selection of these parameters can be used to produce a hydrogel with good balance of properties in terms of cohesivity, injectability, and visibility under fluoroscopy. Example 2: Treatment of Simulated Aneurysms with Freeze-Thawed PVA Hydrogels This example describes assessment of freeze-thawed PVA hydrogels using an in vitro simulated aneurysm model. The hydrogel formulation included 6.5% w / w PVA (146–186 kDa, 99+% hydrolysis), 10%–50% w / w water, and estimated 20%–40% w / w Omnipaque. The hydrogel was prepared according to the protocols of Example 1 above, with two freeze-thaw cycles. Rheological testing was performed according to protocols in Example 1 above. The PVA hydrogel had a storage modulus of 5.5 kPa ± 0.2 kPa, a loss modulus of 400 Pa ± 100 Pa, and a compression modulus of 15 kPa ± 1 kPa. FIGS. 8A–9D provide fluoroscopic images obtained during treatment of a simulated aneurysm with the hydrogel. A silicone bifurcation aneurysm model with challenging tortuosity was used as the simulated aneurysm, with circulated PBS as simulated physiological fluid. Two experimental runs took place at the biplane under fluoroscopy. FIGS. 8A–8D are images obtained during the first run and FIGS.9A–9D are images obtained during the second run. FIGS.8A and 9A illustrate initial positioning of a braided neck cover in the aneurysm. FIGS.8B and 9B illustrate initial filling of the PVA hydrogel into the aneurysm. FIGS.8C and 9C illustrate continued filling of the PVA hydrogel into the aneurysm, with the hydrogel reaching the aneurysm dome. No leakage of the hydrogel through the braided neck cover was observed. FIGS. 8D and 9D illustrate that no flow of external fluid into the aneurysm was observed after the embolization MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO procedure. These results demonstrate that freeze-thawed PVA hydrogels can be successfully injected into an aneurysm to produce complete occlusion, while mitigating the risk of leaking through the neck cover due to the increased cohesive strength of the hydrogel. Example 3: Preparation and Characterization of Freeze-Thawed PVA / PVP Hydrogels This example describes the preparation and characterization of PVA / PVP hydrogels prepared using a freeze-thaw process. Hydrogels were prepared using HMW PVA (weight average molecular weight 100 kDa, 99+% hydrolyzed), LMW PVA (weight average molecular weight 70 kDa, 99+% hydrolyzed), HMW PVP (weight average molecular weight 40 kDa), and LMW PVP (weight average molecular weight 10 kDa). Individual solutions of HMW PVA, LMW PVA, HMW PVP, and LMW PVP were prepared and stirred at 95 °C in distilled water until fully dissolved. Once dissolved, the solutions were brought to room temperature (22 °C) and mixed. Bismuth trioxide was added to the solution and mixed. The solution was then subjected to one freeze-thaw cycle to form a hydrogel. For the freeze-thaw cycle, the freezing phase was performed by placing the solution in a freezer at -18 °C for 16 hours, and the thawing phase was performed by placing the solution at room temperature for 8 hours. Rheological testing of hydrogels was performed using a parallel plate rheometer with an 8 mm plate diameter at 37 °C. For storage modulus and loss modulus measurements, the rheometer was operated in shear mode with a steady axial force of 30 g. Storage and loss moduli were measured at the linear viscoelastic region at 10 rad / s angular frequency. For compression modulus measurements, the rheometer was operated in compression mode with an initial axial force of 10 g. Compression moduli were calculated from the average slope of the initial linear portion of the stress / strain curve (0.1%–5% strain). Table 8 below lists the PVA / PVP hydrogel formulations that were prepared. Table 9 below lists the properties of the PVA / PVP formulations. Table 8: PVA / PVP hydrogel formulations (all concentration are % w / w) Formulation HMWLMW HMW LMW Bismuth Water PVAPVA PVP PVP oxide 1 6.1 1.7 2.2 2.2 13 74.8 MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WOFormulation HMWLMW HMW LMW Bismuth Water PVAPVA PVP PVP oxide 2 7.8 1.7 2.2 2.2 13 73 3 4.3 1.7 2.2 2.2 13 76.5 4 6.1 7.0 2.2 2.2 13 69.6 5 6.1 0 2.2 2.2 13 76.5 6 6.1 1.7 7.0 2.2 13 70 7 6.1 1.7 0 2.2 13 77 8 6.1 1.7 2.2 7.0 13 70 9 6.1 1.7 2.2 0 13 77 Table 9: Properties of PVA / PVP hydrogel formulations Shear Shear Loss Compression Storage Formulation Modulus Tan(delta) Modulus Observations Modulus (Pa) (Pa) ±712489 ± Good balance of1 51 1740.143 ±0.004cohesiveness and 1682 injectability2 3371 ± 822 569 ± 2630.162 ±15211 ± Injection force 0.432792 too high 3 760 ± 113 101 ± 17 ± 9Not very 0.006 81cohesive4 1185 ± 244 186 ± 250.159 ±9296 ± Good balance of 0.13cohesiveness and 1169 injectability 126 Good balance of5 1267 ± 191 143 ± 120.114 ±54 ± 0.007cohesiveness and 703 injectability 221 183 ± 2111163 ± Good balance of6 1318 ±0.140 ±0.01cohesiveness and 2184 injectability 0.1410240 ± Good balance of7 1140 ± 139 162 ± 232 ±0.12cohesiveness and 1514 injectability MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Shear Shear Loss Compression Storage Formulation Modulus Tan(delta) Modulus Observations Modulus (Pa) (Pa) (Pa) 8899 ± 44 130 ± 230.145 ±12356 ± Too plasticized 0.18754 9 1304 ± 158 186 ± 14 12 Good balance of 728 ± cohesiveness and 4113 injectability Table 10 summarizes the that exhibited suitable injectability, plasticity, and cohesiveness. Table 10: Summary of Suitable PVA / PVP Hydrogel Formulations Hydrogel Example Minimum Maximum Component Concentration Concentration Concentration (%w / w) (%) (%) HMW PVA LMW PVA 1.7 0 <7 HMW PVP 2.2 0 7 LMW PVP 2.2 0 <7 Water 74.8 69.6 <77 Bismuth trioxide13 13 13Overall, these results demonstrate that by selecting the correct mixture of PVA and PVP polymers, a low toxicity hydrogel can be prepared, which balances cohesion with injectability. Careful selection of polymer molecular weights and polymer concentration within specific ranges ensures that the hydrogel demonstrates the desired properties. Suspension of bismuth trioxide opacifies the hydrogel and allows for controllable and safe delivery in the aneurysm to achieve immediate occlusion. Example 4: Treatment of Aneurysms with Freeze-Thawed PVA / PVP Hydrogels This example describes assessment of freeze-thawed PVA / PVP hydrogels using an in vivo aneurysm model. The hydrogel formulation included 6.1% w / w HMW PVA (weight MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO average molecular weight 100 kDa, 99+% hydrolyzed), 1.7% w / w LMW PVA (weight average molecular weight 70 kDa, 99+% hydrolyzed), 2.2% w / w HMW PVP (weight average molecular weight 40 kDa), 2.2 %w / w LMW PVP (weight average molecular weight 10 kDa), 13% w / w bismuth oxide, and 74.8% w / w water. The hydrogel was prepared according to the protocol of Example 3 above. The hydrogel was used to embolize aneurysms created in New Zealand white rabbits. The objective of the study was to assess the safety and efficacy of the PVA / PVP hydrogel with the embolization system described herein. The rabbit model was selected because it demonstrates similarities to human coagulation systems and equivalent hemodynamic forces. Moreover, reproducible techniques have been developed specifically for the rabbit model to produce aneurysms appropriate for evaluating endovascular embolization medical devices. These aneurysms remain stable for up to 6 months and therefore can be used for chronic preclinical studies. Finally, the anatomy of the vessels of rabbits in the head and neck area are similar to human anatomy. For aneurysm creation, the rabbits were placed in supine position under sterile positions. The desired vessels were accessed through a skin incision and aneurysms were surgically created. For embolization, the rabbits were fully anesthetized via a subcutaneous injection of ketamine and xylazine. Oxygen saturation, heart rate, and other physiological data was monitored at least every 15 minutes. Under fluoroscopic guidance, a guide catheter was inserted into a sheath and then positioned in the parent artery where the aneurysm was created. The delivery system was then placed in the aneurysm. A neck protection device was deployed, followed by the injection of the PVA / PVP hydrogel. Once complete occlusion of the aneurysm was confirmed with angiogram, the neck protection device was detached and the delivery system was removed. Two embolized rabbits were terminated after 3 days and the embolizations were evaluated through angiograms. Two additional rabbits were sacrificed after 30 days to evaluate for occlusion at a longer period of time. FIGS.10A–10C are fluoroscopic images of an aneurysm pre-treatment (FIG.10A), post-treatment (FIG.10B), and at termination (3 days after embolization) (FIG.10C). As shown, MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO the PVA / PVP hydrogel formulation resulted in complete and immediate occlusion of the aneurysm. Occlusion was maintained until termination. Additional Examples Various examples of aspects of the subject technology are described as numbered clauses (1, 2, 3, etc.) for convenience. These are provided as examples and do not limit the subject technology. Clause 1. A composition for treating an aneurysm, the composition comprising: a freeze-thawed hydrogel comprising a polymer and a contrast agent, wherein the polymer is non-covalently crosslinked, wherein the freeze-thawed hydrogel is configured to be delivered into the aneurysm to occlude the aneurysm, and wherein the freeze-thawed hydrogel has a storage modulus within a range from 1 kPa to 2 kPa, or from 3 kPa to 8 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel. Clause 2. The composition of Clause 1, wherein the polymer comprises one or more of the following: polyvinyl alcohol (PVA), polyethyleneimine, polyacrylic acid, ethylene- vinyl alcohol copolymer, polyaniline, polyacrylamide, chitosan, alginate, gelatin, a gum, dextran, dextrin, cellulose, hyaluronate, starch, or a derivative thereof. Clause 3. The composition of Clause 1 or 2, wherein the polymer comprises PVA. Clause 4. The composition of Clause 3, wherein the PVA is non-covalently crosslinked via hydrogen bonding between hydroxyl groups of the PVA. Clause 5. The composition of Clause 3 or 4, wherein the PVA is at least 87% hydrolyzed. Clause 6. The composition of any one of Clauses 3 to 5, wherein the freeze- thawed hydrogel comprises a first PVA having a first molecular weight and a second PVA having a second molecular weight, the first molecular weight being greater than the second molecular weight. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Clause 7. The composition of Clause 6, wherein the first molecular weight of the first PVA is within a range from 80 kDa to 120 kDa. Clause 8. The composition of Clause 6 or 7, wherein the second molecular weight of the second PVA is within a range from 50 kDa to 75 kDa. Clause 9. The composition of any one of Clauses 6 to 8, wherein the freeze- thawed hydrogel comprises from 5% w / w to 9% w / w of the first PVA. Clause 10. The composition of any one of Clauses 6 to 9, wherein the freeze- thawed hydrogel comprises from 1% w / w to 5% w / w of the second PVA. Clause 11. The composition of any one of Clauses 1 to 10, wherein the freeze- thawed hydrogel comprises polyvinyl pyrrolidone (PVP). Clause 12. The composition of Clause 11, wherein the freeze-thawed comprises a first PVP having a first molecular weight and a second PVP having a second molecular weight, the first molecular weight being greater than the second molecular weight. Clause 13. The composition of Clause 12, wherein the first molecular weight of the first PVP is within a range from 20 kDa to 60 kDa. Clause 14. The composition of Clause 12 or 13, wherein the second molecular weight of the second PVP is within a range from 5 kDa to 15 kDa. Clause 15. The composition of any one of Clauses 12 to 14, wherein the freeze- thawed hydrogel comprises from 1% w / w to 5% w / w of the first PVP. Clause 16. The composition of any one of Clauses 12 to 15, wherein the freeze- thawed hydrogel comprises from 1% w / w to 5% w / w of the second PVP. Clause 17. The composition of any one of Clauses 1 to 16, wherein the freeze- thawed hydrogel comprises a first PVA having a first degree of hydrolyzation and a second PVA having a second degree of hydrolyzation, the second degree of hydrolyzation being different than the first degree of hydrolyzation. Clause 18. The composition of Clause 17, wherein the first degree of hydrolyzation is at least 99% and the second degree of hydrolyzation is no more than 90%. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Clause 19. The composition of any one of Clauses 1 to 18, wherein the freeze- thawed hydrogel comprises from 1% w / w to 10% w / w of the polymer. Clause 20. The composition of any one of Clauses 1 to 19, wherein the polymer has a weight average molecular weight within a range from 100 kDa to 200 kDa. Clause 21. The composition of any one of Clauses 1 to 20, wherein the freeze- thawed hydrogel has a loss modulus within a range from 100 Pa to 500 Pa, or from 200 Pa to 600 Pa at 37 °C over the linear viscoelastic region of the freeze-thawed hydrogel. Clause 22. The composition of any one of Clauses 1 to 21, wherein the freeze- thawed hydrogel has a compression modulus within a range from 10 kPa to 15 kPa, or from 5 kPa to 20 kPa at 37 °C. Clause 23. The composition of any one of Clauses 1 to 22, wherein the freeze- thawed hydrogel is configured to be delivered into the aneurysm via injection through a catheter having a diameter no greater than 0.15 cm. Clause 24. The composition of any one of Clauses 1 to 23, wherein the contrast agent comprises one or more of the following: tantalum, bismuth trioxide, bismuth oxychloride, tungsten, tungsten carbide, barium sulfate, gadolinium, iodized oil, iohexol, iopamidol, ioxilan, iopromide, iodixanol, iobitridol, ioversol, diatrizoate, metrizoate, iothalamate, ioxaglate, iopromide, iothalamate / meglumine, ioxaglate / meglumine, diatrizoate / meglumine, iodomide sodium, or metrizamide. Clause 25. The composition of Clause 24, wherein the contrast agent is configured to diffuse out of the freeze-thawed hydrogel after the freeze-thawed hydrogel is delivered into the aneurysm. Clause 26. The composition of any one of Clauses 1 to 25, wherein the freeze- thawed hydrogel further comprises at least one additive configured to modulate a mechanical property of the freeze-thawed hydrogel. Clause 27. The composition of Clause 26, wherein the at least one additive comprises one or more of the following: polyacrylic acid, PVP, polyethylene glycol, polyacrylamide, poloxamers, laponite, glycerol, ethylene glycol, or propylene glycol. Clause 28. A method for treating an aneurysm, the method comprising: MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO delivering a freeze-thawed hydrogel into the aneurysm to occlude the aneurysm, wherein the freeze-thawed hydrogel comprises a polymer and a contrast agent, wherein the polymer is non-covalently crosslinked, and wherein the freeze-thawed hydrogel has a storage modulus within a range from 1 kPa to 2 kPa, or from 3 kPa to 8 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel. Clause 29. The method of Clause 28, wherein the polymer comprises PVA. Clause 30. The method of Clause 29, wherein the PVA is non-covalently crosslinked via hydrogen bonding between hydroxyl groups of the PVA. Clause 31. The method of Clause 29 or 30, wherein the PVA is at least 87% hydrolyzed. Clause 32. The method of any one of Clauses 29 to 31, wherein the freeze- thawed hydrogel comprises a first PVA having a first molecular weight and a second PVA having a second molecular weight, the first molecular weight being greater than the second molecular weight. Clause 33. The method of Clause 32, wherein the first molecular weight of the first PVA is within a range from 80 kDa to 120 kDa. Clause 34. The method of Clause 32 or 33, wherein the second molecular weight of the second PVA is within a range from 50 kDa to 75 kDa. Clause 35. The method of any one of Clauses 32 to 34, wherein the freeze- thawed hydrogel comprises from 5% w / w to 9% w / w of the first PVA. Clause 36. The method of any one of Clauses 32 to 35, wherein the freeze- thawed hydrogel comprises from 1% w / w to 5% w / w of the second PVA. Clause 37. The method of any one of Clauses 28 to 36, wherein the freeze- thawed hydrogel comprises polyvinyl pyrrolidone (PVP). Clause 38. The method of Clause 37, wherein the freeze-thawed comprises a first PVP having a first molecular weight and a second PVP having a second molecular weight, the first molecular weight being greater than the second molecular weight. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Clause 39. The method of Clause 38, wherein the first molecular weight of the first PVP is within a range from 20 kDa to 60 kDa. Clause 40. The method of Clause 38 or 39, wherein the second molecular weight of the second PVP is within a range from 5 kDa to 15 kDa. Clause 41. The method of any one of Clauses 38 to 40, wherein the freeze- thawed hydrogel comprises from 1% w / w to 5% w / w of the first PVP. Clause 42. The method of any one of Clauses 38 to 41, wherein the freeze- thawed hydrogel comprises from 1% w / w to 5% w / w of the second PVP. Clause 43. The method of any one of Clauses 29 to 42, wherein the freeze- thawed hydrogel comprises a first PVA having a first degree of hydrolyzation and a second PVA having a second degree of hydrolyzation, the second degree of hydrolyzation being different than the first degree of hydrolyzation. Clause 44. The method of Clause 43, wherein the first degree of hydrolyzation is at least 99% and the second degree of hydrolyzation is no more than 90%. Clause 45. The method of any one of Clauses 28 to 44, wherein the freeze- thawed hydrogel comprises from 1% w / w to 10% w / w of the polymer. Clause 46. The method of any one of Clauses 28 to 45, wherein the polymer has a weight average molecular weight within a range from 100 kDa to 200 kDa. Clause 47. The method of any one of Clauses 28 to 46, wherein the freeze- thawed hydrogel has a loss modulus within a range from 100 Pa to 500 Pa, or from 200 Pa to 600 Pa at 37 °C over the linear viscoelastic region of the freeze-thawed hydrogel. Clause 48. The method of any one of Clauses 28 to 47, wherein the freeze- thawed hydrogel has a compression modulus within a range from 10 kPa to 15 kPa, or from 5 kPa to 20 kPa at 37 °C. Clause 49. The method of any one of Clauses 28 to 48, wherein the freeze- thawed hydrogel is delivered into the aneurysm via injection through a catheter having a diameter no greater than 0.15 cm. MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO Clause 50. The method of any one of Clauses 28 to 49, wherein the contrast agent comprises one or more of the following: tantalum, bismuth trioxide, bismuth oxychloride, tungsten, tungsten carbide, barium sulfate, gadolinium, iodized oil, iohexol, iopamidol, ioxilan, iopramide, iodixanol, iobitridol, ioversol, diatrizoate, metrizoate, iothalamate, ioxaglate, iopromide, iothalamate / meglumine, ioxaglate / meglumine, diatrizoate / meglumine, iodomide sodium, or metrizamide. Clause 51. The method of Clause 50, wherein the contrast agent is configured to diffuse out of the freeze-thawed hydrogel after the freeze-thawed hydrogel is delivered into the aneurysm. Clause 52. The method of any one of Clauses 28 to 51, wherein the freeze- thawed hydrogel further comprises at least one additive configured to modulate a mechanical property of the freeze-thawed hydrogel. Clause 53. The method of Clause 52, wherein the at least one additive comprises one or more of the following: polyacrylic acid, PVP, polyethylene glycol, polyacrylamide, poloxamers, laponite, glycerol, ethylene glycol, or propylene glycol. Clause 54. The method of any one of Clauses 28 to 53, further comprising positioning a neck cover within the aneurysm before delivering the freeze-thawed hydrogel into the aneurysm. Clause 55. The method of Clause 54, wherein the freeze-thawed hydrogel does not leak through the neck cover during delivery into the aneurysm. Conclusion Although many of the embodiments are described above with respect to systems, devices, and methods for treating saccular intracranial aneurysms, the technology is applicable to other applications and / or other approaches. For example, suitable features of described systems, devices, compositions, and methods for treating saccular intracranial aneurysms can be implemented in the context of treating non-saccular intracranial aneurysms, abdominal aortic aneurysms, thoracic aortic aneurysms, renal artery aneurysms, arteriovenous malformations, tumors (e.g., via occlusion of vessel(s) feeding a tumor), perivascular leaks, varicose veins (e.g., via occlusion of one or more truncal veins such as the great saphenous vein), hemorrhoids, and MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO sealing endoleaks adjacent to artificial heart valves, covered stents, and abdominal aortic aneurysm devices, among other examples. Additionally, several other embodiments of the technology can have different configurations, components, or procedures than those described herein. A person of ordinary skill in the art, therefore, will accordingly understand that the technology can have other embodiments with additional elements, or the technology can have other embodiments without several of the features shown and described above with reference to FIGS.1A–10C. The descriptions of embodiments of the technology are not intended to be exhaustive or to limit the technology to the precise form disclosed above. Where the context permits, singular or plural terms may also include the plural or singular term, respectively. Although specific embodiments of, and examples for, the technology are described above for illustrative purposes, various equivalent modifications are possible within the scope of the technology, as those skilled in the relevant art will recognize. For example, while steps are presented in a given order, alternative embodiments may perform steps in a different order. The various embodiments described herein may also be combined to provide further embodiments. As used herein, the terms “generally,” “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent variations in measured or calculated values that would be recognized by those of ordinary skill in the art. Moreover, unless the word “or” is expressly limited to mean only a single item exclusive from the other items in reference to a list of two or more items, then the use of “or” in such a list is to be interpreted as including (a) any single item in the list, (b) all of the items in the list, or (c) any combination of the items in the list. As used herein, the phrase “and / or” as in “A and / or B” refers to A alone, B alone, and A and B. Additionally, the term “comprising” is used throughout to mean including at least the recited feature(s) such that any greater number of the same feature and / or additional types of other features are not precluded. To the extent any materials incorporated herein by reference conflict with the present disclosure, the present disclosure controls. It will also be appreciated that specific embodiments have been described herein for purposes of illustration, but that various modifications may be made without deviating from the technology. Further, while advantages associated with certain embodiments of the technology MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described herein.

Claims

MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO CLAIMS What is claimed is:

1. A composition for treating an aneurysm, the composition comprising: a freeze-thawed hydrogel comprising a polymer and a contrast agent, wherein the polymer is non-covalently crosslinked, wherein the freeze-thawed hydrogel is configured to be delivered into the aneurysm to occlude the aneurysm, and wherein the freeze-thawed hydrogel has a storage modulus within a range from 1 kPa to 2 kPa, or from 3 kPa to 8 kPa at 37 °C over a linear viscoelastic region of the freeze-thawed hydrogel.

2. The composition of claim 1, wherein the polymer comprises polyvinyl alcohol (PVA), optionally wherein the PVA is non-covalently crosslinked via hydrogen bonding between hydroxyl groups of the PVA.

3. The composition of claim 2, wherein the freeze-thawed hydrogel comprises a first PVA having a first molecular weight and a second PVA having a second molecular weight, the first molecular weight being greater than the second molecular weight.

4. The composition of claim 3, wherein the first molecular weight of the first PVA is within a range from 80 kDa to 120 kDa, and / or wherein the second molecular weight of the second PVA is within a range from 50 kDa to 75 kDa.

5. The composition of claim 3 or 4, wherein the freeze-thawed hydrogel comprises from 5% w / w to 9% w / w of the first PVA, and / or wherein the freeze-thawed hydrogel comprises from 1% w / w to 5% w / w of the second PVA.

6. The composition of any one of claims 1 to 5, wherein the freeze-thawed hydrogel comprises polyvinyl pyrrolidone (PVP).MDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO 7. The composition of claim 6, wherein the freeze-thawed comprises a first PVP having a first molecular weight and a second PVP having a second molecular weight, the first molecular weight being greater than the second molecular weight.

8. The composition of claim 7, wherein the first molecular weight of the first PVP is within a range from 20 kDa to 60 kDa, and / or wherein the second molecular weight of the second PVP is within a range from 5 kDa to 15 kDa.

9. The composition of claim 7 or 8, wherein the freeze-thawed hydrogel comprises from 1% w / w to 5% w / w of the first PVP, and / or wherein the freeze-thawed hydrogel comprises from 1% w / w to 5% w / w of the second PVP.

10. The composition of any one of claims 1 to 9, wherein the freeze-thawed hydrogel comprises from 1% w / w to 10% w / w of the polymer.

11. The composition of any one of claims 1 to 10, wherein the polymer has a weight average molecular weight within a range from 100 kDa to 200 kDa.

12. The composition of any one of claims 1 to 11, wherein the freeze-thawed hydrogel has a loss modulus within a range from 100 Pa to 500 Pa, or from 200 Pa to 600 Pa at 37 °C over the linear viscoelastic region of the freeze-thawed hydrogel.

13. The composition of any one of claims 1 to 12, wherein the freeze-thawed hydrogel has a compression modulus within a range from 10 kPa to 15 kPa, or from 5 kPa to 20 kPa at 37 °C.

14. The composition of any one of claims 1 to 13, wherein the freeze-thawed hydrogel is configured to be delivered into the aneurysm via injection through a catheter having a diameter no greater than 0.15 cm.

15. The composition of any one of claims 1 to 14, wherein the contrast agentMDT Ref. No. A00011927WO01 Fortem Ref. No. MDTNV.296WO comprises one or more of the following: tantalum, bismuth trioxide, bismuth oxychloride, tungsten, tungsten carbide, barium sulfate, gadolinium, iodized oil, iohexol, iopamidol, ioxilan, iopromide, iodixanol, iobitridol, ioversol, diatrizoate, metrizoate, iothalamate, ioxaglate, iopromide, iothalamate / meglumine, ioxaglate / meglumine, diatrizoate / meglumine, iodomide sodium, or metrizamide.

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