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Pluronic (Plu) hydrogels mixed with variable fractions of Pluronic diacrylate (PluDA) have become popular matrices to encapsulate bacteria and control their growth in engineered living materials. Here we study the rheological response of 30 wt% Plu/PluDA hydrogels with PluDA fraction between 0 and 1. We quantify the range of viscoelastic properties that can be covered in this system by varying in the PluDA fraction. We present stress relaxation and creep-recovery experiments and describe the variation of the critical yield strain/stress, relaxation and recovery parameters of Plu/PluDA hydrogels as function of the covalent crosslinking degree using the Burgers and Weilbull models. The analyzed hydrogels present two stress relaxations with different timescales which can be tuned with the covalent crosslinking degree. We expect this study to help users of Plu/PluDA hydrogels to estimate the mechanical properties of their systems, and to correlate them with the behaviour of bacteria in future Plu/PluDA devices of similar composition.

Water-soluble bis( N -acylpiperidone)s with aldehyde-like reactivity are reported to react rapidly with polyvinylamine at room temperature, providing unprecedented clean reaction products. Unlike most amine/ketone reactions that result in arbitrary mixtures of imines, aminals, hemiaminals, or hydrates, in the present study hemiaminals, aminals, or hemiaminal/aminal mixtures are exclusively found. Detailed NMR spectroscopy of solutions, gels, and solids, aided by model reactions, reveals that the hemiaminal/aminal ratio depends on pH, water content, and cross-linking density. Network formation is fully reversible upon changes in pH, with the resulting moduli from rheology spanning almost 3 orders of magnitude. The self-healing ability of the system is probed by rheology as well, demonstrating maintained material properties of fractured and healed samples. The unusually clean, fast, and reversible chemistry highlights bispiperidones as a class of efficient building blocks with unprecedented possibilities in dynamic covalent chemistry.

Scaffolds based on bioconjugated hydrogels are attractive for tissue engineering because they can partly mimic human tissue characteristics. For example, they can further increase their bioactivity with cells. However, most of the hydrogels present problems related to their processability, consequently limiting their use in 3D printing to produce tailor-made scaffolds. The goal of this work is to develop bioconjugated hydrogel nanocomposite inks for 3D printed scaffold fabrication through a micro-extrusion process having improved both biocompatibility and processability. The hydrogel is based on a photocrosslinkable alginate bioconjugated with both gelatin and chondroitin sulfate in order to mimic the cartilage extracellular matrix, while the nanofiller is based on graphene oxide to enhance the printability and cell proliferation. Our results show that the incorporation of graphene oxide into the hydrogel inks considerably improved the shape fidelity and resolution of 3D printed scaffolds because of a faster viscosity recovery post extrusion of the ink. Moreover, the nanocomposite inks produce anisotropic threads after the 3D printing process because of the templating of the graphene oxide liquid crystal. The in vitro proliferation assay of human adipose tissue-derived mesenchymal stem cells (hADMSCs) shows that bioconjugated scaffolds present higher cell proliferation than pure alginate, with the nanocomposites presenting the highest values at long times. Live/Dead assay otherwise displays full viability of hADMSCs adhered on the different scaffolds at day 7. Notably, the scaffolds produced with nanocomposite hydrogel inks were able to guide the cell proliferation following the direction of the 3D printed threads. In addition, the bioconjugated alginate hydrogel matrix induced chondrogenic differentiation without exogenous pro-chondrogenesis factors as concluded from immunostaining after 28 days of culture. This high cytocompatibility and chondroinductive effect toward hADMSCs, together with the improved printability and anisotropic structures, makes these nanocomposite hydrogel inks a promising candidate for cartilage tissue engineering based on 3D printing.

Understanding polymer gelation over multiple length‐scales is crucial to develop advanced materials. An experimental setup is developed that combines rheological measurements with simultaneous time‐domain 1H NMR relaxometry (TD‐NMR) techniques, which are used to study molecular motion (<10 nm) in soft matter. This so‐called low‐field RheoNMR setup is used to study the impact of varying degrees of crosslinking (DC) on the gelation kinetics of acrylic acid (AAc) and N,N′‐methylene bisacrylamide (MBA) free radical crosslinking copolymerization. A stretched exponential function describes the T2 relaxation curves throughout the gelation process. The stretching exponent β decreases from 0.90 to 0.67 as a function of increasing DC, suggesting an increase in network heterogeneity with a broad T2 distribution at higher DC. The inverse correlation of the elastic modulus G′ with T2 relaxation times reveals a pronounced molecular rigidity for higher DC at early gelation times, indicating the formation of inelastic, rigid domains such as crosslinking clusters. The authors further correlate G′ with the polymer concentration during gelation using a T1 filter for solvent suppression. A characteristic scaling exponent of 2.3 is found, which is in agreement with theoretical predictions of G′ based on the confining tube model in semi‐dilute entangled polymer solutions. [ABSTRACT FROM AUTHOR]

A series of poly(sodium methacrylate) hydrogels, also called superabsorbents, having a theoretical degree of neutralization of 100 mol%, and degree of crosslinking varying from 0.6 to 20 mol%, are synthesized via conventional free radical polymerization. The networks are characterized in detail by inverse size‐exclusion chromatography and 1H‐NMR relaxometry in order to place particular emphasis on the investigation of the pore size distribution (PSD) and the chain mobility, respectively. The two previously mentioned parameters are compared to understand the correlation between the elastic chain mobilities and the average pore size of the hydrogel. From the resulting data, a new empirical equation is proposed, which is valid under the given experimental conditions and permits a rough estimation of the average PSD from the relaxation data. Thus, the equation permits to reduce the number of analytical techniques needed for the characterization of complex systems such as polymer networks. [ABSTRACT FROM AUTHOR]

In this study, the ability of cationic poly(acrylamide‐co‐(3‐acrylamidopropyl)trimethylammonium chloride) hydrogels to desalinate seawater is explored, where the salt separation is based on the partial rejection of mobile salt ions by the fixed charges along the polymer backbone. Water absorbency measurements reveal that artificial seawater‐containing divalent ions (Mg2+, Ca2+, and SO42−) drastically decrease the swelling capacity of previously employed anionic poly(acrylic acid‐co‐sodium acrylate) hydrogels, whereas no influence on the swelling behavior of the synthesized cationic hydrogels is found. The swelling behavior and mechanical properties are studied by varying the degree of crosslinking and degree of ionization systematically in the range of 1–5 and 25–75 mol%, respectively. Finally, artificial seawater (csea = 0.171 mol L−1) is desalinated in a custom‐built press setup with an estimated efficiency of Em³ = 17.6 kWh m−³ by applying an external pressure on the swollen hydrogels. [ABSTRACT FROM AUTHOR]

Salinity gradients between seawater and river water is a renewable source of energy having a worldwide potential capacity of about 3.1 TW. This energy can be extracted by e.g., an osmotic engine, using hydrogels with high water uptake capacity. Consecutive exposing hydrogels to fresh and saline water makes swelling–shrinking cycles, which can be utilized to move a piston in an osmotic engine. The production of power with this method is significantly suppressed by gelblocking, where voids between particles are blocked so that the water flow is limited and the absorbency significantly retarded. To improve the power production, the gelblocking is minimized within this article by using spherical mono‐dispersed hydrogels made by microfluidic technique. In this study mono‐disperse poly(acrylic acid‐co‐sodium acrylate) hydrogels with varying diameters (100‐600 µm) and varying degrees of neutralization (DN = 10–75 mol%) are synthesized. In addition, hydrogels with different DN are utilized for additional surface crosslinking to fabricate core–shell particles. The maximum power of 0.67 W kg−1 is obtained for hydrogels with a diameter of 105 µm, degree of crosslinking (DC) = 1.7 mol%, DN = 75 mol%, and a core‐shell architecture, which is three times higher compared to hydrogels having undefined size without a core–shell framework. [ABSTRACT FROM AUTHOR]

Objective: Resonant vibrations of implanted structures during a magnetic resonance imaging (MRI) procedure pose a risk to the patient in the form of soft tissue irritation and degradation of the implant. In this paper, the mechanical behavior of implant structures in air, water, and viscoelastic materials was explored. Methods: The static and dynamic transfer functions of various test samples in air and immersed in both water and hydrogels were analyzed. The laser-based acquisition method allowed for high-angular-resolution (10  $ \mu$ Deg) and high-dynamic-range (0–6 kHz) measurements. Additional MRI experiments were conducted to investigate the dependence of vibration strength on magnetic resonance (MR) sequence parameters in combination with the obtained transfer functions. Results: The largest forces were found to be in the micronewton to millinewton range, which is comparable to forces applied during implantation. Of additional concern was the damping introduced by viscoelastic tissue, which was less than expected, leading to an underdamped system. In contrast to current wisdom, the imaging experiments demonstrated drastically different vibration amplitudes for identical gradient slew rates, but different timing parameters TR, mainly due to resonant amplification. Conclusion: The results showed that a safe force-free MR procedure depends not only on the gradient slew rate, but also and more drastically on the choice of secure timing parameters. Significance: These findings delineate design improvements to achieve longevity of implants and will lead to increased patient safety during MRI. A prudent choice of mechanical characteristics of implanted structures is sufficient to avoid resonant excitation due to mismatched MR sequence parameters. [ABSTRACT FROM AUTHOR]

Abstract: Superabsorbent polymer (SAP) hydrogels have pronounced water‐absorbing and water‐storing capacities, which are essential for numerous potential applications. It remains a challenge to better understand the network topology because of their amorphous and anisotropic structures. Synthesis parameters such as monomer concentration, degree of neutralization and crosslinking, and surface crosslinking are varied to correlate structural changes in the network with low‐field proton double‐quantum (1H DQ) NMR results. 1H DQ‐NMR data are processed by a reliable, user‐independent analysis approach to determine the fractions of network defects, of mobile sol components, and of network chains as well as the residual dipolar coupling distribution in SAPs. In addition, results obtained by applying different distributions to describe the DQ buildup curves are quantified and compared. The correlation between topological and synthesis parameters as well as the impact of temperature, swelling, and solvent of SAP on DQ signals is investigated and discussed. [ABSTRACT FROM AUTHOR]

The energy consumption for a novel desalination approach using charged hydrogels under externally applied pressure is experimentally measured and calculated. The salt separation is based on a partial rejection of mobile salt ions caused by the fixed charges inside the polyelectrolyte network. Self-synthesized and commercial poly(acrylic acid) hydrogels are used to study the desalination performance in reference to sodium chloride solutions within the concentration range of 0.1-35 g L−1. The influence of various synthetic parameters, such as the degree of crosslinking (DC) and the size and shape of the particles, is investigated. Furthermore, the effect of process parameters including the amount of the feed solution, the applied pressure profile, and the swelling time of the hydrogel is discussed. The best energy estimation found so far, is 8.9 kWh m−3 fresh water if a poly(acrylic acid) with a DC of 5 mol% is used in an infinite large salt bath. [ABSTRACT FROM AUTHOR]

Poly(acrylic acid)‐based hydrogels can swell up to 100–1000 times their own weight in desalinated water due to osmotic forces. As the swelling is about a factor of 2–12 lower in seawater‐like saline solutions (4.3 wt% NaCl) than in deionized water, cyclic swelling, and shrinking can potentially be used to move a piston in an osmotic motor. Consequently, chemical energy is translated into mechanical energy. This conversion is driven by differences in chemical potential and by changes in entropy. This is special, as most thermodynamic engines rely instead on the conversion of heat into mechanical energy. To optimize the efficiency of this process, the degree of neutralization, the degree of crosslinking, and the particle size of the hydrogels are varied. Additionally, different osmotic engine prototypes are constructed. The maximum mean power of 0.23 W kg−1 dry hydrogel is found by using an external load of 6 kPa, a polymer with 1.7 mol% crosslinking, a degree of neutralization of 10 mol%, and a particle size of 370–670 µm. As this is achieved only in the first round of optimization, higher values of the maximum power average over one cycle seem realistic. [ABSTRACT FROM AUTHOR]

An in‐depth investigation of the network topology for a series of sodium acrylate hydrogels synthesized via conventional free radical polymerization (FRP) and reversible addition–fragmentation chain transfer (RAFT) polymerization is conducted. The role of the RAFT agent on the crosslinking process is demonstrated on a model system upon analysis of the reaction mixture via size‐exclusion chromatography before the gelation point. For a comprehensive study, both the impact of the amount of RAFT agent and of the degree of crosslinking on the microstructure of the final product are systematically investigated. In addition to swelling experiments and oscillatory shear rheology measurements, the resulting networks are analyzed via low‐field proton nuclear magnetic resonance (1H‐NMR) techniques such as transverse relaxation and double‐quantum coherence to evaluate the network mobility, which is then correlated to structural inhomogeneity. A broader mobility distribution is observed for the RAFT mediated networks compared to the FRP samples, which can be assigned to a higher content of dangling ends in the former case. The results are further elaborated to propose a mechanism for network formation in presence of a RAFT agent. [ABSTRACT FROM AUTHOR]

A series of well‐defined symmetric poly(methyl methacrylate)‐b‐poly(sodium methacrylate)‐b‐poly(methyl methacrylate) (PMMA‐b‐PSMA‐b‐PMMA) triblock copolymers with various block compositions is synthesized. The amphiphilic ABA triblock copolymers form polyelectrolyte hydrogels in water by self‐assembly. The hydrophobic PMMA endblocks act as physical cross‐links in the form of frozen micelles, while the hydrophilic PSMA midblocks span the 3D network. The influence of various synthetic parameters on the self‐assembly and the macroscopic properties of these hydrogels is systematically investigated by water absorbency, oscillatory shear rheology, and small‐angle X‐ray scattering. The polymer concentration during the hydrogel formation affects the ratio between looping and bridging chains. The number of MMA units per endblock (nA) determines the size and the relaxation rates of the physical cross‐links and thus, the mechanical stability of the hydrogels. More SMA units in the midblock (nB) increase the water absorbency, while the mechanical moduli decrease. Even lower G‐moduli are achieved by partly exchanging the symmetric ABA triblock with AB diblock copolymers, which can only form non‐elastic dangling ends. [ABSTRACT FROM AUTHOR]

Polyacrylate hydrogels PAAM (M = 7Li+, 23Na+, 39K+, 87Rb+, and 133Cs+) are synthesized by free‐radical copolymerization and are studied by Nuclear Magnetic Resonance (NMR) over wide time and length scales to get an insight into the polymer network structure and the dynamics of alkali ions and water. 1H double‐quantum NMR (1H DQ‐NMR) indicates that neutralization by different alkali ions influences the polymer structure in swollen PAAM hydrogels. Relaxation rates of alkali ions are found to be dominated by quadrupolar relaxation. 1H relaxation varies with the chain fraction obtained from 1H DQ‐NMR. Pulsed field gradient NMR reveals diffusion coefficients of alkali ions and water for different swelling degrees. Network structure and dynamics as well as the interactions between water, alkali ions, and polymer chains are discussed. [ABSTRACT FROM AUTHOR]

Dynamics of sodium ions and water in swollen superabsorbent polymer (SAP) hydrogels are studied by 23Na‐ and 1H‐NMR, respectively. The apparent diffusion coefficients of water in swollen SAPs, probed by 1H pulsed field gradient NMR, decreases with increasing diffusion time. The degree of hindrance depends on structural and synthesis parameters. It is quantified within a tortuosity model. Based on the results, the swelling degree has the highest impact on the ion mobility, apart from synthesis parameters leading to different levels of physical and chemical crosslinks. 23Na‐NMR relaxation and diffusion reveal the 23Na+ mobility in swollen SAPs. A higher degree of neutralization leads to faster relaxation and to a smaller apparent diffusion coefficient. Surface crosslinking restricts water mobility, but has a smaller impact on the dynamics of sodium ions. The experimental results indicate an influence of SAP structure on the dynamics of ions and water molecules. Dynamics of 23Na+ and water in swollen superabsorbent polymers (SAPs) are probed by 23Na‐ and 1H‐NMR, respectively. The steric hindrance of the SAP network on water mobility is quantified within a tortuosity model for the 1H diffusion coefficient. 23Na‐NMR relaxation and diffusion experiments reveal 23Na+ mobility in swollen SAPs. Their mutual interactions with hydrophilic networks are explored. [ABSTRACT FROM AUTHOR]

Thiswork presents an experimental study of the network structureof swollen, charged hydrogels. We provide an in-depth comparison oftwo proton low-field NMR methods, i.e., transverse relaxation anddouble-quantum spectroscopy, that are both sensitive to residual orientationcorrelations of the network chain segments. The results are in bothcases analyzed by help of integral inversion techniques, and bothmethods are demonstrated to provide comparable results despite themore qualitative nature of the transverse relaxation. We investigatemodel samples that are similar to commercial superabsorber materialsbased on partly neutralized and chemically cross-linked poly(acrylicacid) as prepared by free radical polymerization. The degree of cross-linkingand the monomer concentration are varied during synthesis. Both parametersare found to affect the network structure and the amount of elasticallyinactive defects in a systematic way, and a comparison with commercialsamples with homogeneous or purposely inhomogeneous core–shellstructures proves that the cross-linking bimodality of the latteris readily revealed despite the rather large intrinsic inhomogeneityof the swollen gels. [ABSTRACT FROM AUTHOR]