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Gels
Special Issues
Structure and Properties of Functional Hydrogels
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Structure and Properties of Functional Hydrogels

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

Deadline for manuscript submissions: closed (31 January 2024) | Viewed by 16291

Special Issue Editors

Dr. Chengtao Yu

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Guest Editor
State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou, China
Interests: hydrogels; polymer physics; soft materials; biodegradable materials
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Jian Hu

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Guest Editor
State Key Lab for Strength and Vibration of Mechanical Structures, Department of Engineering Mechanics, Xi’an Jiaotong University, Xi’an 710049, China
Interests: gels; soft materials; mechanical properties; soft electronics
Special Issues, Collections and Topics in MDPI journals
Dr. Yong Zheng

E-Mail Website
Guest Editor
Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Sapporo 001-0021, Japan
Interests: polymer gel; functional material; fracture mechanics
Special Issues, Collections and Topics in MDPI journals
Dr. Xiaohua Chang

E-Mail Website
Guest Editor
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, China
Interests: hydrogels; ionogels; polymer composites; stimuli-responsive polymers
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue is dedicated to developing new functional hydrogels with unique properties and original designs on hierarchical structure.

Hydrogels, composed of a three-dimensional, cross-linked polymer network and an abundance of water, are representative soft and wet materials. The soft and wet nature of hydrogels has already allowed them to find broad applications in various fields, such as tissue engineering, drug delivery, soft actuators, and flexible electronics. However, conventional hydrogels are mechanically brittle and weak, which severely limits their scope of applications. In the past two decades, a great amount of research has reported the successful preparation of mechanically strong and tough hydrogels by designing structures with sacrificial bonds; that is, the sacrificial bonds can break preferentially during deformation to dissipate energy, endowing the material with high toughness. Led by these pioneering studies, attempts were further made to construct novel structures in hydrogels, and subsequently, various hydrogels with special properties (e.g., anti-fracture, self-healing, adhesive, anti-fatigue, and anti-freezing) gradually emerged, greatly enriching the application fields of hydrogels.

The properties of hydrogels are highly dependent on their structures. Interestingly, functional hydrogels usually possess exquisite hierarchical structures in different scales, for example, at the nano-, micro-, meso-, and macro-scale. Understanding the structure–property relationship in hydrogels is vitally important for their functions and applications. Hence, within this topic, we aim to share state-of-the-art advances in developing functional hydrogels with excellent properties upon hierarchically structural design and expect to provide a powerful paradigm for developing new functional hydrogels. We sincerely welcome submissions in this exciting field and look forward to presenting these new works.

Dr. Chengtao Yu
Prof. Dr. Jian Hu
Dr. Yong Zheng
Dr. Xiaohua Chang
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 2100 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

  • functional hydrogels
  • hierarchical structure
  • multiscale structure
  • structure–property relationship
  • mechanical property

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Published Papers (6 papers)

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Research

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13 pages, 4338 KiB  
Article
Rapid Synthesis of Robust Antibacterial and Biodegradable Hydrogels via Frontal Polymerization
by Jinze Wang, Hao Li, Hai-Xia Shen, Wei Zhao, Qing Li, Cai-Feng Wang and Su Chen
Gels 2023, 9(12), 920; https://doi.org/10.3390/gels9120920 - 21 Nov 2023
Cited by 1 | Viewed by 1778
Abstract
Chitosan (CS) is widely used in biomedical hydrogels due to their similarity to extracellular matrix. However, the preparation method of CS-based hydrogel suffers the drawbacks of tedious operation, time-consuming and energy consumption. Thus, there is an urgent need to develop a rapid synthesis [...] Read more.
Chitosan (CS) is widely used in biomedical hydrogels due to their similarity to extracellular matrix. However, the preparation method of CS-based hydrogel suffers the drawbacks of tedious operation, time-consuming and energy consumption. Thus, there is an urgent need to develop a rapid synthesis pathway towards hydrogels. In this work, we used a modified CS as a cross-linking agent and acrylic acid (AA) as monomer to prepare a hydrogel through frontal polymerization (FP), which facilitates a facile and rapid method achieved in several minutes. The occurrence of pure FP was confirmed via the frontal velocity and temperature profile measurement. In addition, the as-prepared hydrogel shows excellent mechanical strength up to 1.76 MPa, and the Young’s modulus (ranging from 0.16 to 0.56 MPa) is comparable to human skin. The degradation mechanism is revealed by the micro-IR images through the distribution of the functional groups, which is attributed to the breakage of the ether bond. Moreover, the hydrogel exhibits excellent degradability, biocompatibility and antibacterial properties, offering great potentials in tissue engineering. We believe this work not only offers a facile and rapid FP method to fabricate a robust degradable hydrogel, but also provides an effective pathway for the investigation of the degradation mechanism at the chemical bond analysis level. Full article
(This article belongs to the Special Issue Structure and Properties of Functional Hydrogels)
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20 pages, 10901 KiB  
Article
An MXene-Grafted Terpolymer Hydrogel for Adsorptive Immobilization of Toxic Pb(II) and Post-Adsorption Application of Metal Ion Hydrogel
by Himarati Mondal, Mrinmoy Karmakar and Bhaskar Datta
Gels 2023, 9(10), 827; https://doi.org/10.3390/gels9100827 - 19 Oct 2023
Cited by 6 | Viewed by 2107
Abstract
Toxic metal ions present in industrial waste, such as Pb(II), introduce deleterious effects on the environment. Though the adsorptive removal of Pb(II) is widely reported, there is a dearth of research on the suitable utilization and disposal of the Pb(II)-adsorbed adsorbent. In this [...] Read more.
Toxic metal ions present in industrial waste, such as Pb(II), introduce deleterious effects on the environment. Though the adsorptive removal of Pb(II) is widely reported, there is a dearth of research on the suitable utilization and disposal of the Pb(II)-adsorbed adsorbent. In this work, an MXene-grafted terpolymer (MXTP) hydrogel has been designed for the adsorption of Pb(II) under ambient conditions of pH and temperature. The hydrogel MXTP was synthesized by facile one-pot polymerization in aqueous solvent, and the detailed structural characterization of terpolymer (TP), MXTP, and Pb(II)-loaded MXTP, i.e., Pb(II)-MXTP, was carried out by a combination of proton nuclear magnetic resonance (1H NMR), Fourier-transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffractometric (XRD), thermogravimetric/differential thermogravimetric (TG/ DTG), and field emission scanning electron microscopic (FESEM) analyses. The specific capacitance and conductivities of Pb(II)-MXTP were studied with cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS), which unambiguously indicate successful post-adsorption application. The specific capacitance of MXTP decreased after Pb(II) adsorption, whereas the conductivity increased significantly after Pb(II) adsorption, showing that MXTP can be successfully deployed as a solid electrolyte/anode after Pb(II) adsorption. This study covers the synthesis of a novel MXene-grafted terpolymer hydrogel for adsorptive exclusion of Pb(II) and assessment of the as-adsorbed Pb(II)-loaded hydrogel as a solid electrolyte/anode material and is the first demonstration of such post-adsorptive application. Full article
(This article belongs to the Special Issue Structure and Properties of Functional Hydrogels)
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15 pages, 3272 KiB  
Article
Rapid Preparation of Superabsorbent Self-Healing Hydrogels by Frontal Polymerization
by Ying Qin, Hao Li, Hai-Xia Shen, Cai-Feng Wang and Su Chen
Gels 2023, 9(5), 380; https://doi.org/10.3390/gels9050380 - 5 May 2023
Cited by 8 | Viewed by 2958
Abstract
Hydrogels have received increasing interest owing to their excellent physicochemical properties and wide applications. In this paper, we report the rapid fabrication of new hydrogels possessing a super water swelling capacity and self-healing ability using a fast, energy-efficient, and convenient method of frontal [...] Read more.
Hydrogels have received increasing interest owing to their excellent physicochemical properties and wide applications. In this paper, we report the rapid fabrication of new hydrogels possessing a super water swelling capacity and self-healing ability using a fast, energy-efficient, and convenient method of frontal polymerization (FP). Self-sustained copolymerization of acrylamide (AM), 3-[Dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]azaniumyl]propane-1-sulfonate (SBMA), and acrylic acid (AA) within 10 min via FP yielded highly transparent and stretchable poly(AM-co-SBMA-co-AA) hydrogels. Thermogravimetric analysis and Fourier transform infrared spectroscopy confirmed the successful fabrication of poly(AM-co-SBMA-co-AA) hydrogels with a single copolymer composition without branched polymers. The effect of monomer ratio on FP features as well as porous morphology, swelling behavior, and self-healing performance of the hydrogels were systematically investigated, showing that the properties of the hydrogels could be tuned by adjusting the chemical composition. The resulting hydrogels were superabsorbent and sensitive to pH, exhibiting a high swelling ratio of up to 11,802% in water and 13,588% in an alkaline environment. The rheological data revealed a stable gel network. These hydrogels also had a favorable self-healing ability with a healing efficiency of up to 95%. This work contributes a simple and efficient method for the rapid preparation of superabsorbent and self-healing hydrogels. Full article
(This article belongs to the Special Issue Structure and Properties of Functional Hydrogels)
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14 pages, 4181 KiB  
Article
pH-Responsive, Thermo-Resistant Poly(Acrylic Acid)-g-Poly(boc-L-Lysine) Hydrogel with Shear-Induced Injectability
by Maria-Eleni Karga, Maria-Eleni Kargaki, Hermis Iatrou and Constantinos Tsitsilianis
Gels 2022, 8(12), 817; https://doi.org/10.3390/gels8120817 - 12 Dec 2022
Cited by 4 | Viewed by 2493
Abstract
In this study we report the rheological behavior of aqueous solutions of an amphiphilic graft copolymer constituting a polyacrylic acid (PAA) grafted by poly(boc-L-lysine), P(b-LL). Due to the highly hydrophobic nature of the grafted chains, the copolymer self-assembles spontaneously in aqueous media forming [...] Read more.
In this study we report the rheological behavior of aqueous solutions of an amphiphilic graft copolymer constituting a polyacrylic acid (PAA) grafted by poly(boc-L-lysine), P(b-LL). Due to the highly hydrophobic nature of the grafted chains, the copolymer self-assembles spontaneously in aqueous media forming three-dimensional (3D) finite size networks (microgels). The rheological analysis demonstrated that the copolymer behaves as a strong elastic hydrogel, showing characteristics of a “frozen” network. Moreover, it is noteworthy that the formulation shows the above-described characteristics in very small concentrations (0.25–1.20 wt%) compared to other naturally cross-linked hydrogels that have been studied so far. Concentration significantly affects the rheological properties of the hydrogel, showing considerable increase in elastic modulus, following the scaling law G’~C1.93. At the same time, the hydrogels can be described as intelligent stimuli-responsive systems, showing pH and shear responsiveness as well as stability with temperature changes. Thanks to the pH dependance of the degree of ionization of the weak polyelectrolyte PAA backbone, stiffness and swelling of the hydrogels can be tuned effectively by adjusting the pH conditions. Simulating conditions such as those of injection through a 28-gauge syringe needle, the gel demonstrates excellent response to shear, due to its remarkable shear thinning behavior. The combination of pH-sensitivity and shear responsiveness leads to excellent injectability and self-healing properties, given that it flows easily upon applying a low stress and recovers instantly in the site of injection. Therefore, the physically cross-linked PAA-g-P(b-LL) hydrogel exhibits remarkable features, namely biocompatibility, biodegradability of cross-links, pH responsiveness, shear-induced injectability and instantaneous self-healing, making it a potential candidate for various biomedical applications. Full article
(This article belongs to the Special Issue Structure and Properties of Functional Hydrogels)
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23 pages, 5688 KiB  
Article
Bioresorbable Chitosan-Based Bone Regeneration Scaffold Using Various Bioceramics and the Alteration of Photoinitiator Concentration in an Extended UV Photocrosslinking Reaction
by Farah Alwani Azaman, Keran Zhou, María del Mar Blanes-Martínez, Margaret Brennan Fournet and Declan M. Devine
Gels 2022, 8(11), 696; https://doi.org/10.3390/gels8110696 - 28 Oct 2022
Cited by 13 | Viewed by 3574
Abstract
Bone tissue engineering (BTE) is an ongoing field of research based on clinical needs to treat delayed and non-union long bone fractures. An ideal tissue engineering scaffold should have a biodegradability property matching the rate of new bone turnover, be non-toxic, have good [...] Read more.
Bone tissue engineering (BTE) is an ongoing field of research based on clinical needs to treat delayed and non-union long bone fractures. An ideal tissue engineering scaffold should have a biodegradability property matching the rate of new bone turnover, be non-toxic, have good mechanical properties, and mimic the natural extracellular matrix to induce bone regeneration. In this study, biodegradable chitosan (CS) scaffolds were prepared with combinations of bioactive ceramics, namely hydroxyapatite (HAp), tricalcium phosphate-α (TCP- α), and fluorapatite (FAp), with a fixed concentration of benzophenone photoinitiator (50 µL of 0.1% (w/v)) and crosslinked using a UV curing system. The efficacy of the one-step crosslinking reaction was assessed using swelling and compression testing, SEM and FTIR analysis, and biodegradation studies in simulated body fluid. Results indicate that the scaffolds had comparable mechanical properties, which were: 13.69 ± 1.06 (CS/HAp), 12.82 ± 4.10 (CS/TCP-α), 13.87 ± 2.9 (CS/HAp/TCP-α), and 15.55 ± 0.56 (CS/FAp). Consequently, various benzophenone concentrations were added to CS/HAp formulations to determine their effect on the degradation rate. Based on the mechanical properties and degradation profile of CS/HAp, it was found that 5 µL of 0.1% (w/v) benzophenone resulted in the highest degradation rate at eight weeks (54.48% degraded), while maintaining compressive strength between (4.04 ± 1.49 to 10.17 ± 4.78 MPa) during degradation testing. These results indicate that incorporating bioceramics with a suitable photoinitiator concentration can tailor the biodegradability and load-bearing capacity of the scaffolds. Full article
(This article belongs to the Special Issue Structure and Properties of Functional Hydrogels)
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Review

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13 pages, 879 KiB  
Review
Reasoning on Pore Terminology in 3D Bioprinting
by Alexander Trifonov, Ahmer Shehzad, Fariza Mukasheva, Muhammad Moazzam and Dana Akilbekova
Gels 2024, 10(2), 153; https://doi.org/10.3390/gels10020153 - 19 Feb 2024
Cited by 6 | Viewed by 2341
Abstract
Terminology is pivotal for facilitating clear communication and minimizing ambiguity, especially in specialized fields such as chemistry. In materials science, a subset of chemistry, the term “pore” is traditionally linked to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, which categorizes [...] Read more.
Terminology is pivotal for facilitating clear communication and minimizing ambiguity, especially in specialized fields such as chemistry. In materials science, a subset of chemistry, the term “pore” is traditionally linked to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, which categorizes pores into “micro”, “meso”, and “macro” based on size. However, applying this terminology in closely-related areas, such as 3D bioprinting, often leads to confusion owing to the lack of consensus on specific definitions and classifications tailored to each field. This review article critically examines the current use of pore terminology in the context of 3D bioprinting, highlighting the need for reassessment to avoid potential misunderstandings. We propose an alternative classification that aligns more closely with the specific requirements of bioprinting, suggesting a tentative size-based division of interconnected pores into ‘parvo’-(d < 25 µm), ‘medio’-(25 < d < 100 µm), and ‘magno’-(d > 100 µm) pores, relying on the current understanding of the pore size role in tissue formation. The introduction of field-specific terminology for pore sizes in 3D bioprinting is essential to enhance the clarity and precision of research communication. This represents a step toward a more cohesive and specialized lexicon that aligns with the unique aspects of bioprinting and tissue engineering. Full article
(This article belongs to the Special Issue Structure and Properties of Functional Hydrogels)
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