Synthesis and characterization of multi-sensitive microgel-based polyampholyte hydrogels with high mechanical strength

Microgel-Crosslinked Thermo-Responsive Hydrogel Actuators with High Mechanical Properties and Rapid Response

Smart hydrogels responsive to external stimuli are promising for various applications such as soft robotics and smart devices. High mechanical strength and fast response rate are particularly important for the construction of hydrogel actuators. Herein, tough hydrogels with rapid response rates are synthesized using vinyl-functionalized poly(N-isopropylacrylamide) (PNIPAM) microgels as macro-crosslinkers and N-isopropylacrylamide as monomers. The compression strength of the obtained PNIPAM hydrogels is up to 7.13 MPa. The response rate of the microgel-crosslinked hydrogels is significantly enhanced compared with conventional chemically crosslinked PNIPAM hydrogels. The mechanical strength and response rate of hydrogels can be adjusted by varying the proportion of monomers and crosslinkers. The lower critical solution temperature (LCST) of the PNIPAM hydrogels could be tuned by copolymerizing with ionic monomer sodium methacrylate. Thermo-responsive bilayer hydrogels are fabricated using PINPAM hydrogels with different LCSTs via a layer-by-layer method. The thermo-responsive fast swelling and shrinking properties of the two layers endow the bilayer hydrogel with anisotropic structures and asymmetric response characteristics, allowing the hydrogel to respond rapidly. The bilayer hydrogels are fabricated into clamps to grab small objects and flowers that mimicked the closure of petals, and it shows great application prospects in the field of actuators.

Recent Advances in Design Strategies of Tough Hydrogels

Hydrogels are a fascinating class of materials popular in numerous fields, including tissue engineering, drug delivery, soft robotics, and sensors, attributed to their 3D network porous structure containing a significant amount of water. However, traditional hydrogels exhibit poor mechanical strength, limiting their practical applications. Thus, many researchers have focused on the development of mechanically enhanced hydrogels. This review describes the design considerations for constructing tough hydrogels and some of the latest strategies in recent years. These tough hydrogels have an up-and-coming prospect and bring great hope to the fields of biomedicine and others. Nonetheless, it is still no small challenge to realize hydrogel materials that are tough, multifunctional, intelligent, and zero-defect. This article is protected by copyright. All rights reserved.

Tough hybrid microgel-reinforced hydrogels dependent on the size and modulus of the microgels

Microgel-reinforced (MR) hydrogels are tough hydrogels with dispersed rigid microgels embedded in a continuous soft matrix. MR gels have the great potential to provide not only mechanical toughness but also the desired functional matrix by incorporation of various functional microgels. Understanding the toughening mechanism of the MR hydrogels is critical for the rational design of the desired functionally tough MR gels. However, our current knowledge of the toughening mechanism of MR gels mainly comes from the MR hydrogels with both chemically crosslinked dispersed microgels and a continuous matrix. Little is known about the hybrid MR gels with physically crosslinked microgels embedded in a chemically crosslinked matrix. Herein, we synthesize such hybrid MR hydrogels with the ionic crosslinked calcium alginate microgels incorporated into the chemically crosslinked polyacrylamide (PAAm) matrix. The alginate microgels show strong size and modulus effects on the toughening enhancement: the larger microgels could toughen the MR gels more than the small ones, and the microgels with medium modulus could maximize the toughness of the MR gels. By comparison of the mechanical performances of the MR and the corresponding double network (DN) hydrogels, we have proposed that the hybrid MR gels may have the same toughening mechanism as the bulk DN gel. This work tries to better understand the structure-property relationships of both MR and DN gels and help in the design of more functionally tough MR gels with the desired properties.

Hybrid granular hydrogels: combining composites and microgels for extended ranges of material properties

Developing hydrogels with optimal properties for specific applications is challenging as most of these properties, such as toughness, stiffness, swelling or deformability, are interrelated. The improvement of one property usually comes at the cost of another. In order to decouple the interdependence between these properties and to extend the range of material properties for hydrogels, we propose a strategy that combines composite and microgel approaches. The study focuses first on tailoring the swelling performance of hydrogels while minimally affecting other properties. The underlying principle is to partially substitute some of the hydrogels with pre-swollen microgels composed of the same materials. Swelling reductions up to 45% were obtained. Those granular hydrogels were then reinforced with nano-fibrillated cellulose fibres obtaining hybrid granular materials to improve their toughness and to further reduce their initial swelling. Four different structures of neat, granular and composite hydrogels including 63 different hydrogel compositions based on 20 kDa poly(ethylene glycol)dimethacrylate showed that the swelling ratio could be tailored without significantly affecting elastic modulus and deformation performance. The results explain the role of the PEGDM precursors on the swelling of the microgels as well as the influence of the microgel and fibre contents on the final properties. Moreover, the precursors of hydrogels with similar mechanical or swelling performance were injectable with a wide range of complex viscosities from 0.1 Pa s to over 1000 Pa s offering new opportunities for applications in confined as well as in unconfined environments.

Enhanced Biocompatibility of Polyampholyte Hydrogels

Tissue-engineered scaffolds encounter many challenges including poor integration with native tissue. Nonspecific protein adsorption can trigger the foreign body response leading to encapsulation and isolation from the native injured tissue. This concern is mitigated with nonfouling polymer scaffolds. This study investigates the long-term biocompatibility of a nonfouling polyampholyte system composed of positively charged [2-(acryloyloxy)ethyl]trimethylammonium chloride monomers and negatively charged 2-carboxyethyl acrylate monomers, cross-linked with triethylene glycol dimethacrylate. This system has previously shown resistance to nonspecific protein adsorption and short-term cell attachment via conjugated proteins. However, longer-term cell survival has not been evaluated with this system. First, the environmental pH was monitored with varying amounts of counter ions present in the hydrogel synthesis buffer. The lowest level (3 M NaOH) and the level that resulted in pH values closest to physiological conditions (6.7 M NaOH) were chosen for further investigation. These two formulations were then compared in terms of their contact angle, qualitative protein adsorption and conjugation capacity, and quantitative cell adhesion, proliferation, and viability. The 3 M NaOH formulation showed higher initial protein conjugation and cell adhesion compared to the 6.7 M NaOH formulation. However, the 3 M NaOH hydrogels had low cell viability after 24 h due to the acidic component release into the culture environment. The 6.7 M NaOH formulation showed a lower initial conjugation and cell adhesion but overcame this limitation by providing a stable environment that maintained cell viability for over 5 days. The 6.7 M NaOH polyampholyte hydrogel formulation shows increased biocompatibility, while maintaining resistance to nonspecific protein adsorption, as demonstrated by the targeted cell adhesion and proliferation. Therefore, this polyampholyte formulation demonstrates strong potential as a tissue-engineered scaffold.

Synthesis of Polyampholyte Janus-like Microgels by Coacervation of Reactive Precursors in Precipitation Polymerization

Controlling the distribution of ionizable groups of opposite charge in microgels is an extremely challenging task, which could open new pathways to design a new generation of stimuli-responsive colloids. Herein, we report a straightforward approach for the synthesis of polyampholyte Janus-like microgels, where ionizable groups of opposite charge are located on different sides of the colloidal network. This synthesis approach is based on the controlled self-assembly of growing polyelectrolyte microgel precursors during the precipitation polymerization process. We confirmed the morphology of polyampholyte Janus-like microgels and demonstrate that they are capable of responding quickly to changes in both pH and temperature in aqueous solutions.

Modern Strategies To Achieve Tissue-Mimetic, Mechanically Robust Hydrogels

Hydrogels are frequently used biomaterials due to their similarity in hydration and structure to biological tissues. However, their utility is limited by poor mechanical properties, namely, a lack of strength and stiffness that mimic that of tissues, particularly load-bearing tissues. Thus, numerous recent strategies have sought to enhance and tune these properties in hydrogels, including interpenetrating networks (IPNs), macromolecular cross-linking, composites, thermal conditioning, polyampholytes, and dual cross-linking. Individually, these approaches have achieved hydrogels with either high strength (σ f > 10 MPa), high stiffness (E > 1 MPa), or, less commonly, both high strength and stiffness (σ f > 10 MPa and E > 1 MPa). However, only certain unique combinations of these approaches have been able to synergistically achieve retention of a high, tissuelike water content as well as high strength and stiffness. Applying such methods to stimuli-responsive hydrogels has also produced robust, smart biomaterials. Overall, methods to achieve hydrogels that simultaneously mimic the hydration, strength, and stiffness of soft and load-bearing tissues have the potential to be used in a much broader range of biomedical applications.

Stiff micelle-crosslinked hyaluronate hydrogels with low swelling for potential cartilage repair

Pluronic F127 diacrylate (F127DA) micelle-crosslinked methacrylated hyaluronic acid (MeHA) hydrogel with low-swelling and strong compressive properties was successfully synthesized for the regeneration of cartilages in vivo.

Polyampholyte Hydrogels in Biomedical Applications

Polyampholytes are a class of polymers made up of positively and negatively charged monomer subunits. Polyampholytes offer a unique tunable set of properties driven by the interactions between the charged monomer subunits. Some tunable properties of polyampholytes include mechanical properties, nonfouling characteristics, swelling due to changes in pH or salt concentration, and drug delivery capability. These characteristics lend themselves to multiple biomedical applications, and this review paper will summarize applications of polyampholyte polymers demonstrated over the last five years in tissue engineering, cryopreservation and drug delivery.

Tough, rapid-recovery composite hydrogels fabricated via synergistic core-shell microgel covalent bonding and Fe3+ coordination cross-linking

We developed tough, rapid-recovery composite hydrogels that are fabricated via core-shell microgel covalent bonding and Fe³⁺ dynamic metal coordination cross-linking. First, core-shell microgels are used as cross-linking agents and initiators to prepare homogeneous hydrogel networks with rapid recovery in the absence of an organic cross-linking agent. The toughness and recoverability of the composite hydrogels can be improved by adding the dynamic reversibility of ionic cross-linking. Owing to the synergistic effect of microgel covalent bonding, Fe³⁺ coordination cross-linking, and H-bond cross-linking, the multi-cross-linked composite hydrogels exhibit excellent toughness and a fast recovery rate. These characteristics demonstrate that the dynamic reversibility of the ionic cross-linking can significantly improve the toughness and recoverability of the hydrogels. In addition, the core-shell microgels play a key role in toughening the hydrogels and accelerating their recovery by transferring stress to grafted polymer chains and homogenizing the hydrogel network.