2.1.1 Intra-catheter transportation

Most hydrogel embolic agents are delivered using standard catheters or micro-catheters, making them compatible with existing interventional procedures,^11-13 An exception is the Embrace™ Hydrogel Embolic System, which features a specialized dual-lumen catheter for separately transporting the polymer precursor and initiator precursor.¹⁴ This system suggests that advancements in hydrogel embolic agent transportation could be highly promising. However, it is crucial to assess their compatibility with current interventional devices.

2.1.2 Intra-vessel transportation

Navigating vessels presents challenges due to their heterogeneity in branch, shape, and caliber.^{15, 16} Non-targeted embolization is a potential complication of transarterial embolization (TAE), where embolic agents inadvertently enter incorrect vessels and obstruct the blood supply to healthy tissue. This highlights the critical importance of precise intra-vessel transportation.^{17, 18} Dual-lumen balloon catheter-assisted embolization has been shown to mitigate non-targeted embolization in TAE when using fluid embolic agents and coils.^19-22 However, research on dual-lumen balloon-assisted hydrogel embolization is limited, as most hydrogel embolic agents remain in the preclinical stage. Jerry C Ku et al. reported using a photosensitive hydrogel-based embolic agent with balloon assistance to embolize wide-necked aneurysms in pigs, have shown promise in minimizing non-target embolization.²³

In summary, most hydrogel embolic agents prioritize accumulation over transportation. The next section delves into specific strategies to achieve precise accumulation.

2.2.1 Stable hydrogel

In recent years, HydroCoil™, a hydrogel-coated metallic coil, has garnered significant attention in animal and patient studies^{11, 29} (Figure 2A). The hydrogel coating reduces the thrombus occlusion ratio of the bare coil, potentially lowering the recanalization rate.^{30, 31} Additionally, the swelling properties of hydrogel improve its volumetric filling capacity.¹³ Norio Hongo et al. observed shorter embolization segments and coil lengths with HydroCoil™ compared to bare coil embolization in vessel disease patients,¹³ while the embolic efficacy was the same.

Beyond HydroCoil®, novel hydrogel coils have emerged. Jung Seung Chai et al. investigated bilayered polyvinyl alcohol hydrogel coils in canine models (Figure 2B), showing comparable packing density to metallic coils. However, three in four aneurysm model canines were observed recanalization 4 weeks after embolization.³² Marcelo Guimaraes et al. tested a nonmetallic injectable coil hydrogel on healthy pigs, demonstrating similar embolization effects to HydroCoil®.³³ Yinyu Zhang et al. introduced a shape memory hydrogel coil. The hydrogel softens at body temperature and stiffens at room temperature. To impart the coil shape memory property, the hydrogel was twisted on a steel coil, and then the coil was straightened at body temperature and cooled down to room temperature (Figure 2C). Initial trials on healthy pigs have verified the embolization efficiency of the coil.¹²

However, the hydrogel coils share some common limitations. First, the hydrogel may tear off from the coil and lead to non-targeted embolization. Second, the hydrogel coil is stiffer than the bare coil, which may cause the movement of the catheter tip. Third, the hydrogel coil may swell in the catheter and lead to occlusion of the catheter.^{12, 13, 31} New construction of the hydrogel coils and softer hydrogel was reported to solve the second limitation.³⁴ Additionally, techniques like employing a one-way stopcock between the catheter hub and touhy can mitigate risks associated with HydroCoil® use.³¹

2.2.2 Stimulus-responsive hydrogel

Smooth transportation and robust accumulation require contrary physical properties of the hydrogel. For proper accumulation, a viscosity range from 30 to 150 CP is considered suitable for small capillaries, high viscosity of more than 400 CP is preferred for large cavities like aneurysms.¹⁰ Except for using stable hydrogel to get it stuck at the target vessels, making use of the specific environment at the target vessels to stimulate the sol–gel transition of hydrogel offers another avenue. Additionally, to prevent venous washout, which may lead to downstream embolization, the ideal sol–gel transition in arterial vessels is less than 5 min.¹⁰

Temperature-responsive hydrogels offer a solution in embolization procedures, which convert from solution to gel phase at about body temperature. The transition temperature, termed the lower critical solution temperature (LCST), plays a pivotal role in their functionality³⁵ (Figure 3A).

Poly(N-isopropylacrylamide) (PNIPAM), a temperature-sensitive polymer, exhibits a sol–gel transition at 32°C, making it ideal for embolization applications.^{24, 36-39} Yanbing Zhao et al. synthesized a nanohydrogel dispersed with iohexol for X-ray visibility, showcasing a tuned LCST of 36.5°C. In vitro experiments revealed varying gelation times at different initial temperatures, while in vivo studies on VX2 liver tumor-bearing rabbits demonstrated efficient embolization.²⁴ Kun Qian et al. enhanced the nanohydrogel by loading doxorubicin, with an LCST around 33°C. Interestingly, the viscosity of hydrogel initially increased and then decreased with higher doxorubicin concentrations. The doxorubicin-loaded hydrogel embolization group exhibited significantly lower tumor growth rates compared to infusion alone.³⁹ Ling Li et al introduced an X-ray and ultrasound visible temperature-responsive hydrogel. The gelation temperature was about 37°C. Embolization in VX2 liver tumor-bearing rabbits demonstrated robust efficiency and sustained ultrasound visibility over 4 weeks.³⁶ PNIPAM was also added to other materials without temperature responding to generate temperature-responsive hydrogel. Zheng et al. grafted PNIPAM to lignin via atom transfer radical polymerization to synthesize a temperature temperature-responsive poly(N-isopropylamide)-grafted lignin hydrogel. The hydrogel showed low shear viscosity (44CP) at room temperature and high strength (G' is 3099 Pa) at body temperature. In vivo testing on kidneys of healthy rabbits revealed evident hydrogel filling in kidney vessels 12 weeks post-embolization.⁴⁰

Silk-Elastin-like Protein Polymer (SELP) is another promising temperature-responsive hydrogel embolic agent. Different from the PNIPAM, SELP is a product of genetic engineering.^41-43 The monomer of SELP consists of elastin-like and silk-like polypeptide sequences. Its temperature-sensitive polymerization involves two key steps: elastin-like domains trigger a rapid phase transition above a specific temperature, followed by silk-like domains inducing slow β-sheet formation to counter phase separation.³⁵ Azadeh Poursaid et al. crafted SELP hydrogel SELP-815 K, formulated at 12% w/w through shear processing. This hydrogel exhibited an optimal viscosity of around 100 CP at room temperature, with a gelation time of 3.5 minutes and remarkable stiffness (4.4E5 Pa). In a rabbit liver embolization experiment, SELP-815 K effectively occluded the terminal hepatic artery without downstream embolization evidenced histologically.⁴⁴ Poursaid, A. et al. further explored SELP by loading doxorubicin and sorafenib, observing minimal influence on hydrogel viscosity and stiffness, albeit with prolonged gelation time (over 5 min).⁴⁵

Chitosan, a biocompatible and biodegradable polymer derived from chitin deacetylation,⁴⁶ has gained attention for its sol–gel transition with β-glycerophosphate (C/GP) at body temperature.⁴⁷ Wang et al. conducted a preliminary trial of C/GP on healthy rabbit kidneys, resulting in complete kidney atrophy confirmed both macroscopically and histologically after 8 weeks.⁴⁸ Andrea Salis et al. further improved the C/GP by loading indocyanine green to it. The capacity to absorb and emit in the near-infrared spectral range of indocyanine green imparted the hydrogel a potential to realize intraoperative fluorescence imaging. The gelation time changed with the concentration of chitosan and β-glycerophosphate. Optimal rheological characterization was observed on C1.6/GP18d, gelling in 2 min and demonstrating shear-thinning behavior conducive to controlled accumulation. The embolization trial on ex vivo bovine liver with the visualization of Photo Dynamic Eye that can detect near-infrared fluorescence showed a uniform distributed fluorescence at the embolization site.⁴⁹

Attempts of other temperature-responsive materials as embolic agents presented promising results. Xu Yan et al. explored a temperature-responsive triblock polymer matrix integrated with reduced graphene oxide nanosheets and iron oxide nanoparticles (Fe₃O₄@rGO) for embolizing the feeding artery in a rabbit VX2 liver tumor model. This hydrogel demonstrated excellent injectability with an inject force of about 2 N in a 29G catheter. However, its gelation temperature of approximately 14°C limits its room temperature storage capability. Notably, the magnetic hyperthermia property of hydrogel will be discussed in the subsequent section on “interaction-specific hydrogels.”⁵⁰ Hantao Yanga et al. introduced a reversible temperature-responsive hydrogel using methoxy PEG-poly(D, L-lactide) copolymer, which gelled at around 30°C. In vivo embolization on the porcine pharyngeal artery and its branches demonstrated complete occlusion of the target vessels, although recanalization occurred one hour post-embolization.⁵¹ Poloxamer 407, also known as Pluronic F127, was employed to confer thermal response properties to hydrogels. Various formulations, including poloxamer 407/hydroxymethyl cellulose (HPMC)/sodium alginate (SA)-derived hydrogel, Pluronic F127 and HPMC with Iohexol powder, and poloxamer 407/SA/HPMC/iodixanol/Ca²⁺ hydrogel, were tested in vitro and in vivo (animal experiments).^{25, 52, 53}

Despite the rapid advancements in temperature-responsive hydrogel embolic agents, further investigation into precise control of hydrogel accumulation is warranted. Factors such as patient body temperature, catheter length in a vessel, viscosity change rate during gelation, and injection force can influence hydrogel accumulation. Moreover, improper gelation in the catheter may lead to catheter obstruction.

pH-responsive hydrogels make use of the pH value at embolization site, altering polymer chain charges from the catheter to the target vessel⁵⁴ (Figure 3B). Hwang et al. synthesized a biodegradable pH-responsive chitosan hydrogel Janus particles. The particles own zwitterionic nature, which provided the hydrogel with a pH-dependent ionic interaction. CT scans of healthy rabbit kidneys post-embolization showcased even particle accumulation at the target site.⁵⁵ Another pH-responsive hydrogel adapted to both physiological (pH 7.4) and tumor (pH 6.5–7.0) conditions, comprising poly(e-caprolactone), sulfamethazine, and poly(ethylene glycol). It transited from a liquid phase at high pH (8.0) to a gel phase at low pH (7.4 or less), with viscosity ranging from 1 Pa·s to nearly 10,000 Pa·s during gelation. The short-term embolization efficiency was confirmed on both CT scans and histologic slides (5 h after embolization) of the rabbit VX2 liver tumor model.⁵⁶ Interestingly, Dedai Lu et al. introduced a noninvasive pH-responsive hydrogel based on acidic microenvironment-responsive doxorubicin-loaded hyperbranched poly[l-threonine-b-(l-glutamic-ran-l-tyrosine)]s (HPTTG). The optimized ratio of l-Tyr:l-Glu was 58:18, exhibiting a sol–gel transition pH of 6.8. The embolization efficacy was demonstrated on subcutaneous tumor-bearing rats and VX2 liver tumor-bearing rabbits. The HPTTG-injected groups exhibited prolonged survival. The in vivo tracing of the labeled HPTTG showed a high concentration at the tumor site and low at other regions of rats and rabbits. Digital subtraction angiography (DSA) images of the rabbit hepatic tumor displayed significant blood flow blockage 8 and 12 h after injection of HPTTG.⁵⁷ PH-responsive hydrogels addressed the limitation of temperature-responsive hydrogels that its phase transition occurs in the catheter. However, further investigation into the precise accumulation of pH-responsive hydrogels is still required.

Shear-thinning hydrogels undergo reversible sol–gel transitions under mechanical shake or shear stress, quickly reforming networks post-external force removal^{58, 59} (Figure 3C). Jingjie Hu et al. deeply investigated a decellularized-cardiac-extracellular-matrix-based hydrogel with shear-thinning properties. The injection force of the hydrogel measured was less than 25 N, which indicated an easy injection. Histological slides of embolized rabbit kidneys showed the hydrogel in blood vessels with a diameter of 200 μm.⁶⁰

Various other stimuli-responsive hydrogels exist, including electro-responsive and photo-responsive types (Figure 3D). Verbrugghe et al. presented a Pluronic methacrylic acid hydrogel with strong electro-mechanical actuation, enabling significant volume contraction and expansion. This characteristic holds promise for treating large vessel diseases like aneurysms.⁶¹ Dobashi et al. introduced a photo-responsive poly(ethylene glycol diacrylate)-nanosilicate hydrogel, exhibiting increased viscosity and storage modulus post-ultraviolet irradiation. It was transported with a specific fiber-optical catheter. Preliminary trials on health, arteriovenous malformations, vascular injuries, and aneurysm model pigs demonstrated its therapeutic efficacy.⁶²

2.2.3 Intra-vessel chemical reaction

The Embrace™ Hydrogel Embolic System offers another solution by isolating the polymer precursor and initiator precursor until reaching the catheter tip. In this way, the little refluxed precursors will not reach the gelation concentration and will not accumulate at the inappropriate place^{63, 64} (Figure 4). Goh et al. reported the first-in-human of Embrace™ in 2022 involving eight patients with hepatocellular carcinoma, renal angiomyolipoma, and intrahepatic cholangiocarcinoma. All patients reached a technical success, and no recanalization of the target vessels was observed 30 days after the embolization. Besides, no serious adverse effect related to the embolic agent was observed during the 30- to 90-day follow-up.¹⁴

2.3.1 Physical effects

Hydrogel embolic agents harness physical effects such as thermal and radioactivity. Hwan-Jeong group has explored chitosan micro-hydrogels, which matched the multifunctional trend of recent hydrogel embolic agents. Jeong et al. explored chitosan micro-hydrogels, including an ¹²⁵I-labeled variant visible under SPECT/CT and an ¹³¹I-labeled version ideal for radioembolization. In vivo experiments on a rat hepatoma model demonstrated the tumor-suppressing effects of ¹³¹I-loaded hydrogel. Additional enhancement was achieved by incorporating doxorubicin (chemical effect)^68-70 (Figure 5A). As mentioned in the temperature-responsive hydrogel part, Yan et al. developed a temperature-responsive magnetic hyperthermia hydrogel, showing successful tumor artery occlusion in vivo. The temperature of this hydrogel reached 49°C under the alternating magnetic field in vitro. In vivo, embolization efficacy assessment on VX2 liver tumor-bearing rabbits indicated successful occlusion of the tumor artery. However, the tumor volume and survival after the embolization were not measured⁵⁰ (Figure 5B).

2.3.2 Chemical effects

Most chemical effects hydrogels involve chemotherapeutics-loaded formulations. As mentioned above, the PIB nanohydrogel, SELP, PEGDA-nSi, HPTTG, and ¹³¹I-loaded hydrogel was reported to carry doxorubicin, a widely used chemotherapeutic agent. Doxorubicin works by disrupting DNA metabolism in tumor cells and is associated with enhanced immune responses against tumors. However, systemic administration of doxorubicin can cause significant side effects, particularly cardiotoxicity. Fortunately, doxorubicin-loaded embolic agents can deliver the drug directly to the tumor, potentially reducing the dosage required and minimizing the incidence of side effects⁷¹ (Figure 5C). Except for doxorubicin-loaded SELP, which was only investigated in vitro, other doxorubicin-loaded hydrogels demonstrated good regression of tumors in animal models.^{39, 45, 57, 62, 70} Beyond chemotherapeutics, Chabrot et al. developed a C/GP hydrogel loaded with sodium tetradecyl sulfate (STS), leading to chemical tumor ablation. Rabbit auricular artery embolization experiments demonstrated improved embolic efficacy with STS-loaded hydrogel⁷² (Figure 5D).

2.3.3 Biological effects

Hydrogel-based embolic agents play a crucial role in biological systems, initiating intricate interactions upon administration. M.H.T. Reinges et al. reveal that hydrogel coils induce heightened neo-endothelium and fibrous responses compared to bare coils, particularly in isolating aneurysm cavities.⁷³ Joseph Touma et al. explored the application of bone marrow-derived mesenchymal stem cells in a hyaluronic acid hydrogel for treating pig models with arteriovenous malformations. Their findings indicated an enhanced occlusion rate compared to using the hydrogel alone⁷⁴ (Figure 5E). Peng Lv et al. loaded the calcium alginate hydrogel microsphere with oncolytic adenoviruses, leading to improved viral replication, biodistribution, reduced systemic toxicity, and heightened antitumor immune responses⁷⁵ (Figure 5F). PuraMatrix is a self-assembled 16 peptide consisting of repeated amino acid sequence, exhibiting low antigenicity and minimal inflammatory reactions during embolization in healthy pig models.⁷⁶ Jingjie Hu et al. synthesized a hydrogel with decellularized cardiac extracellular matrix (ECM)-based nanocomposite. The bio-deprived material showed antimicrobial and pro-regenerative effects in healthy pigs.⁷⁷