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A special issue of Gels (ISSN 2310-2861). This special issue belongs to the section "Gel Chemistry and Physics".

Deadline for manuscript submissions: closed (20 December 2023) | Viewed by 21108

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Special Issue Editors

Dr. Melike Firlak

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Guest Editor

Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze 41400, Turkey
Interests: hydrogels; stimuli-responsive polymers; polypyrroles; conducting polymers; applications of hydrogels; polymeric fluorescence sensors; controlled release

Dr. John G. Hardy

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Guest Editor

1. Department of Chemistry, Faraday Building, Lancaster University, Lancaster LA1 4YB, UK
2. Materials Science Institute, Faraday Building, Lancaster University, Lancaster LA1 4YB, UK
Interests: polymer synthesis; supramolecular materials; biomaterials; stimuli-responsive materials; drug delivery; tissue engineering; sustainability
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Special Issue Information

Dear Colleagues,

The concept of gelation was first introduced by W.H. Carothers to describe systems in which a sample spanning three-dimensional (3D) networks of polymers/particles immobilizes the solvent in which they are dispersed/dissolved. The 3D network of polymers/particles is connected via covalent and/or non-covalent bonds, and it is important to note that the solvent content of such materials can be >90%. Gels have a variety of applications based on their properties, and those that respond to internal and/or external stimuli such as temperature, pH, solvent composition, ionic composition, magnetic field, electric field and light, etc. are described as “stimuli-responsive gels” or “smart gels”; this property of gels has attracted the attention of researchers working in different fields to generate gels for applications including but not limited to adsorption, recovery, drug delivery, tissue engineering, sensors, petroleum industry, etc.

This Special Issue serves as a focal point for high-quality research papers as well as review articles addressing crosslinked polymeric gels, their preparation, characterization, and various applications in the fields of chemistry, engineering, and biotechnology.

Dr. Melike Firlak
Dr. John G. Hardy
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Gels is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Polymeric gels
Crosslinked gels
Supramolecular gels
Synthesis of polymers
Characterization of gels
Smart gels
Stimuli-responsive gels
Applications
Drug delivery
Sensors
Tissue engineering
Adsorption

Published Papers (12 papers)

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Research

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11 pages, 2068 KiB

Open AccessArticle

Model Simulation and Rheological Research on Crosslinking Behavior of Polyethylene Resin

by Xuelian Chen and Qigu Huang

Gels 2024, 10(1), 35; https://doi.org/10.3390/gels10010035 - 31 Dec 2023

Viewed by 1095

Abstract

The crosslinking behavior of polyethylene (PE) determines its exceptional performance and application. In this study, we investigated the crosslinking behaviors of different PE resins through model simulation and rheological methods. Specifically, the mathematical equation of “S” model was established for PE resin. According to this equation, the optimal maximum gel content for high-density polyethylene (HDPE) was found to be around 85%. Moreover, the maximum crosslinking degrees for different PE resins depended largely on their density and molecular weight. The melt viscosities before crosslinking in PE resins were highly influenced by their melt index. The higher melt indexes resulted in the lower storage moduli, improving melt processability during processing. In addition, the crosslinking rates of PE resins were strongly influenced by peroxide concentration, independent of PE resin structures. For high molecular weight and low-density PE resins, they exhibited decreased ti values, increased A₀ values, and decreased k6 values. However, there were no noticeable variations in the values of k2 and phi among different PE resins. All simulated modeling outcomes showed remarkable consistency with the experimental rheological data. These findings are of strong significance in the industrial manufacture of PE resin. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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18 pages, 5490 KiB

Open AccessArticle

UV-Crosslinked Poly(N-isopropylacrylamide) Interpenetrated into Chitosan Structure with Enhancement of Mechanical Properties Implemented as Anti-Fouling Materials

by Isala Dueramae, Fumihiko Tanaka, Naoki Shinyashiki, Shin Yagihara and Rio Kita

Gels 2024, 10(1), 20; https://doi.org/10.3390/gels10010020 - 25 Dec 2023

Viewed by 1181

Abstract

High-performance properties of interpenetration polymer network (IPN) hydrogels, based on physically crosslinked chitosan (CS) and chemically crosslinked poly(N-isopropylacrylamide) (PNiPAM), were successfully developed. The IPN of CS/PNiPAM is proposed to overcome the limited mechanical properties of the single CS network. In this study, the viscoelastic behaviors of prepared materials in both solution and gel states were extensively examined, considering the UV exposure time and crosslinker concentration as key factors. The effect of these factors on gel formation, hydrogel structures, thermal stabilities of networks, and HeLa cell adhesion were studied sequentially. The sol–gel transition was effectively demonstrated through the scaling law, which agrees well with Winter and Chambon’s theory. By subjecting the CS hydrogel to the process operation in an ethanol solution, its properties can be significantly enhanced with increased crosslinker concentration, including the shear modulus, crosslinking degree, gel strength, and thermal stability in its swollen state. The IPN samples exhibit a smooth and dense surface with irregular pores, allowing for much water absorption. The HeLa cells were adhered to and killed using the CS surface cationic charges and then released through hydrolysis by utilizing the hydrophilic/hydrophobic switchable property or thermo-reversible gelation of the PNiPAM network. The results demonstrated that IPN is a highly attractive candidate for anti-fouling materials. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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17 pages, 5747 KiB

Open AccessArticle

The Enrichment of Whey Protein Isolate Hydrogels with Poly-γ-Glutamic Acid Promotes the Proliferation and Osteogenic Differentiation of Preosteoblasts

by Daniel K. Baines, Varvara Platania, Nikoleta N. Tavernaraki, Mattia Parati, Karen Wright, Iza Radecka, Maria Chatzinikolaidou and Timothy E. L. Douglas

Gels 2024, 10(1), 18; https://doi.org/10.3390/gels10010018 - 23 Dec 2023

Viewed by 989

Abstract

Osseous disease accounts for over half of chronic pathologies, but there is a limited supply of autografts, the gold standard; hence, there is a demand for new synthetic biomaterials. Herein, we present the use of a promising, new dairy-derived biomaterial: whey protein isolate (WPI) in the form of hydrogels, modified with the addition of different concentrations of the biotechnologically produced protein-like polymeric substance poly-γ-glutamic acid (γ-PGA) as a potential scaffold for tissue regeneration. Raman spectroscopic analysis demonstrated the successful creation of WPI-γ-PGA hydrogels. A cytotoxicity assessment using preosteoblastic cells demonstrated that the hydrogels were noncytotoxic and supported cell proliferation from day 3 to 14. All γ-PGA-containing scaffold compositions strongly promoted cell attachment and the formation of dense interconnected cell layers. Cell viability was significantly increased on γ-PGA-containing scaffolds on day 14 compared to WPI control scaffolds. Significantly, the cells showed markers of osteogenic differentiation; they synthesised increasing amounts of collagen over time, and cells showed significantly enhanced alkaline phosphatase activity at day 7 and higher levels of calcium for matrix mineralization at days 14 and 21 on the γ-PGA-containing scaffolds. These results demonstrated the potential of WPI-γ-PGA hydrogels as scaffolds for bone regeneration. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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26 pages, 1101 KiB

Open AccessArticle

Gelation Time of Network-Forming Polymer Solutions with Reversible Cross-Link Junctions of Variable Multiplicity

by Fumihiko Tanaka

Gels 2023, 9(5), 379; https://doi.org/10.3390/gels9050379 - 04 May 2023

Viewed by 1491

Abstract

The gelation time

t_{g}

The gelation time

t_{g}

necessary for a solution of functional (associating) molecules to reach its gel point after a temperature jump, or a sudden concentration change, is theoretically calculated on the basis of the kinetic equation for the stepwise cross-linking reaction as a function of the concentration, temperature, functionality f of the molecules, and multiplicity k of the cross-link junctions. It is shown that quite generally

t_{g}

can be decomposed into the product of the relaxation time

t_{R}

and a thermodynamic factor Q. They are functions of a single scaled concentration

x \equiv λ (T) ϕ

, where

λ (T)

is the association constant and

ϕ

is the concentration. Therefore, the superposition principle holds with

λ (T)

as a shift factor of the concentration. Additionally, they all depend on the rate constants of the cross-link reaction, and hence it is possible to estimate these microscopic parameters from macroscopic measurements of

t_{g}

. The thermodynamic factor Q is shown to depend on the quench depth. It generates a singularity of logarithmic divergence as the temperature (concentration) approaches the equilibrium gel point, while the relaxation time

t_{R}

changes continuously across it. Gelation time

t_{g}

obeys a power law

t_{g}^{- 1} \sim x^{n}

in the high concentration region, whose power index n is related to the multiplicity of the cross-links. The retardation effect on the gelation time due to the reversibility of the cross-linking is explicitly calculated for some specific models of cross-linking to find the rate-controlling steps in order for the minimization of the gelation time to be easier in the gel processing. For a micellar cross-linking covering a wide range of the multiplicity, as seen in hydrophobically-modified water-soluble polymers,

t_{R}

is shown to obey a formula similar to the Aniansson–Wall law. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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12 pages, 2616 KiB

Open AccessArticle

Formation of Alginate/Chitosan Interpenetrated Networks Revealed by EPR Spectroscopy

by Jean-Patrick Joly, Ludmila Aricov, George-Alin Balan, Elena Irina Popescu, Sorin Mocanu, Anca Ruxandra Leonties, Iulia Matei, Sylvain R. A. Marque and Gabriela Ionita

Gels 2023, 9(3), 231; https://doi.org/10.3390/gels9030231 - 16 Mar 2023

Cited by 2 | Viewed by 1467

Abstract

This study analyzes the physico-chemical properties of interpenetrated polymer networks (IPNs) and semi-IPN resulting from cross-linking chitosan with glutaraldehyde and alginate with Ca²⁺ cations, as a function of the order in which the cross-linking agents are added to the polymer mixture. Three physico-chemical methods were used to assess the differences between systems: rheology, IR spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. While rheology and IR spectroscopy are commonly used to characterize gel materials, EPR spectroscopy is rarely used, but has the advantage of providing local information about the dynamics of a system. The rheological parameters, which describe the global behavior of the samples, show that semi-IPN systems have a weaker gel behavior and the order of introducing the cross-linker in the polymer systems plays a role. The IR spectra of samples resulting by adding only Ca²⁺ or Ca²⁺ as the first cross-linker are similar to that of the alginate gel, while the spectra of samples in which glutaraldehyde is firstly added resemble the chitosan gel spectrum. Using spin-labeled alginate and spin-labeled chitosan, we monitored the changes occurring in the dynamic of the spin labels due to the formation of IPN and semi-IPN. The results show that the order of adding the cross-linking agents influences the dynamic of the IPN network, and that the formation of the alginate network determines the characteristics of the entire IPN system. The EPR data were correlated with the rheological parameters and IR spectra of the analyzed samples. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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15 pages, 3639 KiB

Open AccessArticle

Thermal Behavior of Water in Sephadex^® G25 Gels at Low Temperatures Studied by Adiabatic Calorimetry

by Noriko Onoda-Yamamuro, Hiroaki Minato, Eiji Nakayama and Norio Murase

Gels 2023, 9(2), 126; https://doi.org/10.3390/gels9020126 - 02 Feb 2023

Cited by 1 | Viewed by 1067

Abstract

Water in a crosslinked dextran gel, Sephadex^® G25, is known to remain partially unfrozen during cooling and undergoes ice crystallization during rewarming. However, the mechanism of ice crystallization during rewarming is still unclear. To elucidate the frozen state of water in the gel, thermal behavior at low temperatures was investigated by using adiabatic calorimetry. Heat capacities and enthalpy-relaxation rates of the gel-containing water of mass ratio h (=g H₂O/g dry G25) = 1.00 were measured between 80 and 300 K during rewarming, where the gel was intermittently heated at the rate of 50–100 mK min⁻¹. Although an exotherm indicating ice crystallization during rewarming was confirmed with the gel precooled rapidly, at 5 K min⁻¹, it disappeared when precooled slowly, at 20 mK min⁻¹. During rewarming after the rapid cooling, two glass transitions were observed at ca. 175 K and 240–242 K. A higher-temperature glass transition due to the water trapped by the polymer network was not so clear, as it was overlapped with an endotherm due to the melting of small ice crystals, which indicates that the ice crystals formed have a broad size-distribution and that water in the gel is vitrified when ice crystals of even the smallest size cannot be formed. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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20 pages, 3239 KiB

Open AccessArticle

Formulation Development and In Vitro/In Vivo Characterization of Methotrexate-Loaded Nanoemulsion Gel Formulations for Enhanced Topical Delivery

by Muhammad Shahid Latif, Asif Nawaz, Mufarreh Asmari, Jalal Uddin, Hidayat Ullah and Saeed Ahmad

Gels 2023, 9(1), 3; https://doi.org/10.3390/gels9010003 - 22 Dec 2022

Cited by 8 | Viewed by 2527

Abstract

Methotrexate-loaded oil-in-water nanoemulsion formulations were prepared using the high shear homogenization technique. A drug excipient study (ATR-FTIR) was carried out to investigate the compatibility between the drug, the polymers, and its admixtures. The thermal stability of the nanoemulsion formulations was evaluated by subjecting them to a heating and cooling cycle. The prepared nanoemulsion formulations (FNE1 to FNE6) were evaluated for particle size, PDI value, and entrapment efficiency (EE). They were analyzed for morphological information using transmission electron microscopy. The drug (methotrexate)-loaded nanoemulsion formulations (FNE2, FNE4, and FNE6) were then converted into nanoemulsion gel formulations by adding 1% chitosan (polymer) as a gelling agent. The nanoemulsion gel formulations (FNEG2, FNEG4, and FNEG6) were investigated for physicochemical parameters, viscosity, spreadability, extrudability, drug content, and skin irritation. Various penetration enhancers (olive oil, clove, and almond oil) were employed to examine the potency of the prepared nanoemulsion gel formulations. In vitro drug release, ex vivo permeation, skin drug retention, and stability tests were carried out for evaluation of the prepared nanoemulsion gel formulations (FNEG2, FNEG4, and FNEG6). The data obtained from the in vitro study were subjected to the kinetic model, and the Korsemeyer–Peppas model was best fitted to the data. The nanoemulsion gel formulation FNEG6 showed the maximum controlled drug release and followed an anomalous, non-Fickian release mechanism. The use of almond oil in the preparation of the nanoemulsion gel formulation FNEG6 helped the penetration of the drug across stratum corneum and the restructuring of the properties of skin and resulted in a higher penetration and retention of methotrexate in a deeper layer of the skin. The current study concluded that the methotrexate-loaded nanoemulsion gel formulation FNEG6 showed the best optimum release, permeation, and retention results as compared to the available oral tablets’ formulations, followed by a low serum concentration and the maximum drug retention, which is beneficial in treating skin infections and reducing systemic toxicity. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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16 pages, 3936 KiB

Open AccessArticle

Thermo-Sensitive mPEG-PA-PLL Hydrogel for Drug Release of Calcitonin

by Yu-En Cheng, I-En Wu, Yi-Chen Chen and I-Ming Chu

Gels 2022, 8(5), 282; https://doi.org/10.3390/gels8050282 - 02 May 2022

Cited by 6 | Viewed by 2579

Abstract

The oral route is the most popular way of drug administration because of good patient compliance and ease of use. However, the oral delivery of peptides and proteins is difficult, mainly due to poor oral bioavailability. In past decades, researchers have developed several strategies to improve oral bioavailability by avoiding losing activity in the gastrointestinal (GI) tract and enhancing the intestinal permeability of these drugs. Methoxy poly(ethylene glycol)-poly(l-alanine) (mPEG-PA) is a thermo-sensitive hydrogel exhibiting a sol-to-gel phase transition property. This characteristic is appropriate for encapsulating peptide or protein drugs. To enhance the adhesion ability to intestinal mucus, a thermo-sensitive polymer, mPEG-PA, modified with charged amino acid lysine was developed. This positively charged material would help to bind the negatively charged mucin in mucus. The synthesis was conducted by individually synthesizing mPEG-PA and poly(l-lysine) (PLL) of different lengths via ring-opening polymerization. Then, mPEG-PA and PLL were combined using an NHS ester reaction to synthesize the triblock copolymer (mPEG-PA-PLL). Biocompatibility and the release of calcitonin from the synthesized hydrogel particles under different pH were examined. The initial data showed that the newly design material had a promising potential for the oral delivery of peptide drugs. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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10 pages, 1581 KiB

Open AccessArticle

Do Polymeric Nanoparticles Really Enhance the Bioavailability of Oral Drugs? A Quantitative Answer Using Meta-Analysis

by Rania M. Hathout

Gels 2022, 8(2), 119; https://doi.org/10.3390/gels8020119 - 14 Feb 2022

Cited by 4 | Viewed by 1603

Abstract

The oral route remains one of the most popular and important routes of administration for drugs—one that warrants the development of advanced drug delivery systems, such as polymeric nanoparticles capable of enhancing the absorption and bioavailability of the used drugs. In this work, a systematic review of published works on several databases, followed by a meta-analysis, were utilized in order to navigate the published studies and access literature-based evidence about the capability of polymeric nanoparticulate systems to augment the absorption and bioavailability of orally administered drugs. The pharmacokinetic parameter of the area under the curve (AUC) was utilized as the “effect” of this meta-analytical study. The meta-analysis demonstrated a significant increase in AUC compared to conventional formulations. Furthermore, comparing the synthetic polymeric nanoparticles, versus their naturally-based administered counterparts, as subgroups of the meta-analysis, revealed no significant differences. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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Review

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15 pages, 6093 KiB

Open AccessReview

Preparation and Application of Polyrotaxane Cross-Linking Agent Based on Cyclodextrin in Gel Materials Field

by Siyuan Liu, Jingxi Zheng, Jiaqin Wang, Shanghao Liu, Xianli Zhang, Dan Bao and Peng Zhang

Gels 2023, 9(11), 854; https://doi.org/10.3390/gels9110854 - 28 Oct 2023

Viewed by 1085

Abstract

The cross-linking point of a conventional chemical cross-linking agent is fixed. Therefore, gels that are prepared with a conventional cross-linking agent have poor deformability, strength, shear resistance, and further properties. Some researchers have prepared a new cross-linking agent using cyclodextrin (CD). In a polyrotaxane cross-linking agent, the cross-linking points can slide freely along the molecule chain. The special “slide ring” structure can provide better elongation, strength, and other properties to gels, which can effectively expand the application of the gel’s materials. This paper summarizes the preparation methods and applications from different types of CD and compares the improvements of properties (swelling, viscoelastic properties, etc.). In addition, the current results of our group are presented, and some ideas are provided for the development of polyrotaxane cross-linking agents. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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20 pages, 1903 KiB

Open AccessEditor’s ChoiceReview

Engineered Vesicles and Hydrogel Technologies for Myocardial Regeneration

by Kaitlyn Ghassemi, Keiko Inouye, Tatevik Takhmazyan, Victor Bonavida, Jia-Wei Yang, Natan Roberto de Barros and Finosh G. Thankam

Gels 2023, 9(10), 824; https://doi.org/10.3390/gels9100824 - 18 Oct 2023

Cited by 1 | Viewed by 1308

Abstract

Increased prevalence of cardiovascular disease and potentially life-threatening complications of myocardial infarction (MI) has led to emerging therapeutic approaches focusing on myocardial regeneration and restoration of physiologic function following infarction. Extracellular vesicle (EV) technology has gained attention owing to the biological potential to modulate cellular immune responses and promote the repair of damaged tissue. Also, EVs are involved in local and distant cellular communication following damage and play an important role in initiating the repair process. Vesicles derived from stem cells and cardiomyocytes (CM) are of particular interest due to their ability to promote cell growth, proliferation, and angiogenesis following MI. Although a promising candidate for myocardial repair, EV technology is limited by the short retention time of vesicles and rapid elimination by the body. There have been several successful attempts to address this shortcoming, which includes hydrogel technology for the sustained bioavailability of EVs. This review discusses and summarizes current understanding regarding EV technology in the context of myocardial repair. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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46 pages, 2495 KiB

Open AccessReview

A Comprehensive Review of Cross-Linked Gels as Vehicles for Drug Delivery to Treat Central Nervous System Disorders

by Leshasha T. Mashabela, Mahlako M. Maboa, Ntombi F. Miya, Taiwo O. Ajayi, Rumbidzai S. Chasara, Marnus Milne, Shoeshoe Mokhele, Patrick H. Demana, Bwalya A. Witika, Xavier Siwe-Noundou and Madan S. Poka

Gels 2022, 8(9), 563; https://doi.org/10.3390/gels8090563 - 06 Sep 2022

Cited by 8 | Viewed by 3047

Abstract

Gels are attractive candidates for drug delivery because they are easily producible while offering sustained and/or controlled drug release through various mechanisms by releasing the therapeutic agent at the site of action or absorption. Gels can be classified based on various characteristics including the nature of solvents used during preparation and the method of cross-linking. The development of novel gel systems for local or systemic drug delivery in a sustained, controlled, and targetable manner has been at the epitome of recent advances in drug delivery systems. Cross-linked gels can be modified by altering their polymer composition and content for pharmaceutical and biomedical applications. These modifications have resulted in the development of stimuli-responsive and functionalized dosage forms that offer many advantages for effective dosing of drugs for Central Nervous System (CNS) conditions. In this review, the literature concerning recent advances in cross-linked gels for drug delivery to the CNS are explored. Injectable and non-injectable formulations intended for the treatment of diseases of the CNS together with the impact of recent advances in cross-linked gels on studies involving CNS drug delivery are discussed. Full article

(This article belongs to the Special Issue Recent Advances in Crosslinked Gels)

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