Tuning Hydrogel Mechanics Using the Hofmeister Effect

Abstract

6503wileyonlinelibrary.com mechanical properties. The large morphological and volume change at the LCST of PNIPAM-based gels [ 10 ] make mechanical studies meaningless. A series of studies on the Hofmeister effect on supramolecular hydrogels showed a dependence of the mechanical properties, but this was merely induced by the hugely different morphologies found for the different assembly conditions of the gels. [11][12][13] Here, we describe for the fi rst time that we can use the Hofmeister effect to controllably manipulate the mechanical properties of polymer hydrogels. As expected, the addition of salts directly affects the gelation temperature and, for our thermoresponsive hydrogel, this changes the stiffness of the gel over two orders of magnitude at a constant temperature, for instance 37 °C. We construct hydrogels that range from very soft to stiff, but all at identical concentration. To achieve such change in mechanical response, one commonly needs to vary parameters such as concentration, morphology and bundle (or fi ber) diameter and stiffness. [ 14 ] Some of these are diffi cult to control, while others simultaneously change many important network characteristics; for instance, the concentration changes the biomedically relevant pore size and stiffness of the gel (as well as nonlinear gel properties). The salts, however, are able to shift the mechanics, without changing porosity and network morphology.The polymers we use are ethylene glycol-functionalized polyisocyanides (PICs, Figure 1 b), which form a thermoreversible gel upon heating when dissolved in water. [ 15 ] At the LCST, the polymers become hydrophobic and form a network of entangled semi-fl exible bundles of polymer chains. The mechanical properties of the PIC hydrogels mimic those of biological gels, [ 15 ] including the nonlinear mechanics at large stress (or strain), which is markedly different than that of other synthetic hydrogels. The mechanics of PIC hydrogels is readily tuned by changing concentration, temperature, and polymer length. [ 16 ] Here, we add two additional parameters to this list: the nature of the salt (more precisely the anion) and its concentration.For a long time, the molecular basis of the Hofmeister series was related to the effect of ions on the bulk structure of water. [ 17 ] More recently, however, it was shown that salts generally do not affect the bulk water structure. [ 18,19 ] Instead, more recent theories hypothesize that direct interactions between ions and macromolecules and their fi rst hydration shell can explain the effects fi rst described by Hofmeister. [ 7,[20][21][22][23] The Hofmeister effect is usually more pronounced for anions than for cations and some ions have a stronger effect than others. The general order of the anions, termed the Hofmeister series is shown in Figure 1 a. [ 7,24 ] The ions on the The mechanical properties of hydrogels are commonly modifi ed by changing the concentration of the molecular components. This approach, however, does not only change hydrogel mechanics, but also th...

A Polyanionic Tartrate‐containing Temperature‐responsive Hydrogel

“…The ability of cations to interact with and influence the rheological properties of EPCT thermogels is further observed in the presence of increasing CaCl 2 concentrations. As expected, a higher salt content generally increases EPCT thermogel stiffness and lowers T gel (Table S1, Figure S6), which is partly driven by the ability of kosmotropic Cl − to augment the polymer‐water interfacial tension and thus decreases the solubility of EPCT's hydrophobic segments [21d] . This results in higher energetic penalties for dissociation, slower dissociating kinetics, and consequently stiffer gels at a fixed frequency above the G’‐G’’ crossover frequency, ω c [25c] .…”

A Polyanionic Tartrate‐containing Temperature‐responsive Hydrogel

“…The ability of cations to interact with and influence the rheological properties of EPCT thermogels is further observed in the presence of increasing CaCl 2 concentrations. As expected, a higher salt content generally increases EPCT thermogel stiffness and lowers T gel (Table S1, Figure S6), which is partly driven by the ability of kosmotropic Cl − to augment the polymer‐water interfacial tension and thus decreases the solubility of EPCT's hydrophobic segments [21d] . This results in higher energetic penalties for dissociation, slower dissociating kinetics, and consequently stiffer gels at a fixed frequency above the G’‐G’’ crossover frequency, ω c [25c] .…”

Strain stiffening and compression-induced softening of composite fibrous hydrogels derived from rod-shaped nanoparticles and a synthetic copolymer

“…S15, SI). We hypothesize that in our work, the values of m were below 1.5 (theoretically predicted for ideal semiflexible polymer networks), 13,37,59,60 because of the composite nature of the constituent fibers containing nanoparticles and the copolymer. Notably, for biological networks, a deviation of m from its ideal value of 1.5 is expected.…”

Controlling ligand density and viscoelasticity in synthetic biomimetic polyisocyanide hydrogels for studying cell behaviours: the key to truly biomimetic hydrogels

“…Interestingly, we observed a significant decrease in the gelation temperature ( T gel ) with increasing azide content, likely due to an increase in the polymer hydrophobicity and the number of charged units along the polymer, an analogous observation to increasing the salt concentration of PIC solutions. 40,42 Simultaneously, G 0 consistently increases in conjunction with the increasing azide content from 235 Pa in PIC-0 ( L = 145 nm), to 563 Pa in PIC-6.0 ( L = 146 nm), despite the consistent polymer length. To further explore this observation, we conducted small-angle neutron scattering (SANS) experiments that indicated an increase in the polymer-bundle diameter correlating with the increase in azide content (Table 3).…”

A Polyanionic Tartrate‐containing Temperature‐responsive Hydrogel

“…The ability of cations to interact with and influence the rheological properties of EPCT thermogels is further observed in the presence of increasing CaCl 2 concentrations. As expected, a higher salt content generally increases EPCT thermogel stiffness and lowers T gel (Table S1, Figure S6), which is partly driven by the ability of kosmotropic Cl − to augment the polymer‐water interfacial tension and thus decreases the solubility of EPCT's hydrophobic segments [21d] . This results in higher energetic penalties for dissociation, slower dissociating kinetics, and consequently stiffer gels at a fixed frequency above the G’‐G’’ crossover frequency, ω c [25c] .…”

Strain stiffening and compression-induced softening of composite fibrous hydrogels derived from rod-shaped nanoparticles and a synthetic copolymer

“…S15, SI). We hypothesize that in our work, the values of m were below 1.5 (theoretically predicted for ideal semiflexible polymer networks), 13,37,59,60 because of the composite nature of the constituent fibers containing nanoparticles and the copolymer. Notably, for biological networks, a deviation of m from its ideal value of 1.5 is expected.…”

Controlling ligand density and viscoelasticity in synthetic biomimetic polyisocyanide hydrogels for studying cell behaviours: the key to truly biomimetic hydrogels

“…Interestingly, we observed a significant decrease in the gelation temperature ( T gel ) with increasing azide content, likely due to an increase in the polymer hydrophobicity and the number of charged units along the polymer, an analogous observation to increasing the salt concentration of PIC solutions. 40,42 Simultaneously, G 0 consistently increases in conjunction with the increasing azide content from 235 Pa in PIC-0 ( L = 145 nm), to 563 Pa in PIC-6.0 ( L = 146 nm), despite the consistent polymer length. To further explore this observation, we conducted small-angle neutron scattering (SANS) experiments that indicated an increase in the polymer-bundle diameter correlating with the increase in azide content (Table 3).…”

A Polyanionic Tartrate‐containing Temperature‐responsive Hydrogel

“…The ability of cations to interact with and influence the rheological properties of EPCT thermogels is further observed in the presence of increasing CaCl 2 concentrations. As expected, a higher salt content generally increases EPCT thermogel stiffness and lowers T gel (Table S1, Figure S6), which is partly driven by the ability of kosmotropic Cl − to augment the polymer‐water interfacial tension and thus decreases the solubility of EPCT's hydrophobic segments [21d] . This results in higher energetic penalties for dissociation, slower dissociating kinetics, and consequently stiffer gels at a fixed frequency above the G’‐G’’ crossover frequency, ω c [25c] .…”

Strain stiffening and compression-induced softening of composite fibrous hydrogels derived from rod-shaped nanoparticles and a synthetic copolymer

“…S15, SI). We hypothesize that in our work, the values of m were below 1.5 (theoretically predicted for ideal semiflexible polymer networks), 13,37,59,60 because of the composite nature of the constituent fibers containing nanoparticles and the copolymer. Notably, for biological networks, a deviation of m from its ideal value of 1.5 is expected.…”

Controlling ligand density and viscoelasticity in synthetic biomimetic polyisocyanide hydrogels for studying cell behaviours: the key to truly biomimetic hydrogels

“…Interestingly, we observed a significant decrease in the gelation temperature ( T gel ) with increasing azide content, likely due to an increase in the polymer hydrophobicity and the number of charged units along the polymer, an analogous observation to increasing the salt concentration of PIC solutions. 40,42 Simultaneously, G 0 consistently increases in conjunction with the increasing azide content from 235 Pa in PIC-0 ( L = 145 nm), to 563 Pa in PIC-6.0 ( L = 146 nm), despite the consistent polymer length. To further explore this observation, we conducted small-angle neutron scattering (SANS) experiments that indicated an increase in the polymer-bundle diameter correlating with the increase in azide content (Table 3).…”