Rheology of peptide‐ and protein‐based physical hydrogels: Are everyday measurements just scratching the surface?

Abstract

Rheological characterization of physically crosslinked peptide- and protein-based hydrogels is widely reported in the literature. In this review, we focus on solid injectable hydrogels, which are commonly referred to as 'shear-thinning and rehealing' materials. This class of what sometimes also are called 'yield-stress' materials holds exciting promise for biomedical applications that require well-defined morphological and mechanical properties after delivery to a desired site through a shearing process (e.g., syringe or catheter injection). In addition to the review of recent studies using common rheometric measurements on peptide- and protein-based, physically crosslinked hydrogels, we provide experimentally obtained visual evidence, using a rheo-confocal microscope, of the fracture and subsequent flow of physically crosslinked β-hairpin peptide hydrogels under steady-state shear mimicking commonly conducted experimental conditions using bench-top rheometers. The observed fracture demonstrates that the supposed bulk shear-thinning and rehealing behavior of physical gels can be limited to the yielding of a hydrogel layer close to the shearing surface with the bulk of the hydrogel below experiencing negligible shear. We suggest some measures to be taken while acquiring and interpreting data using bench-top rheometers with a particular focus on physical hydrogels. In particular, the use of confocal-rheometer assembly is intended to inspire studies on yielding behavior of hydrogels perceived as shear-thinning and rehealing materials. A deeper insight into their yielding behavior will lead to the development of yield-stress, injectable, solid biomaterials, and hopefully inspire the design of new shear-thinning and rehealing hydrogels and more thorough physical characterization of such systems. Finally, more examples of bulk fracture in some physical hydrogels based on peptides and proteins are explored in the light of their behavior as yield-stress materials.

The impact of damage accumulation on the kinetics of network strength recovery for a physical polymer gel subjected to shear deformation

“…16 Several methods for characterizing the fracture-healing behavior of viscoelastic gels have been proposed in literature using shear rheological experiments. 13,[17][18][19][20] These methods typically use oscillation-based measurements to evaluate postdeformation strength recovery and are similar to the tests used to measure network gelation kinetics. 21,22 For "shear thinning" injectable materials, the material's storage modulus (G 0 ) is the critical parameter used to quantify selfhealing behavior because it describes the elastic character of Additional Supporting Information may be found in the online version of this article.…”

“…is typically observed to increase over time to match its original value, which is thought to indicate full strength recovery or "healing" of the material. 17,23 In our previous work, fracture-healing behavior was investigated by rotational constant shear rate experiments to probe fracture and shear banding of physically associating gels. 24 The shear stress response of the network was measured as a function of strain, and simultaneous flow visualization was performed to directly correlate the measured overshoots in the shear stress response with shear-induced strain localization taking place within the gels in the form of macroscale fracture planes and shear bands.…”

“…25 This "shear-rest-shear" measurement protocol is different from the oscillatory techniques used by others, 13,[17][18][19][20] in which the gel was always exposed to some small amount of deformation during the period of recovery. The applied shear rate (1 s 21 ) is also much lower than those used to shear thin materials discussed previously for the oscillation time sweeps.…”

Protein based therapeutic delivery agents: Contemporary developments and challenges

“…Schneider and co-workers have produced a β-hairpin peptide MAX1 containing a tetrapeptide type II’ β-turn (V D P L PT) flanked by alternating V and K [337–341]. MAX1 and its derivatives are designed to fold while adhering to the negatively charged cell membrane; the folding is promoted by intramolecular hydrogen bonds caused by the amphiphilic β-hairpin [342]. When one K is replaced by E in the peptide MAX8, more rapid and stable self-assembly is observed [333].…”

“…The growing plethora of protein constructs of controllable physicochemical properties at a nanoscopic and supramolecular scale enables the field with a toolkit of solutions to address the gamut of existing therapeutic (macro) molecules [10, 13, 34, 70, 76, 345]. The particular challenges of bioavailability and pharmacokinetics [3, 4, 17–20] have been shown to be addressed with the widest range of protein-based delivery vehicles, including proteins with defined secondary and tertiary structures like HSA [34, 61, 62, 65, 120], coiled-coils [12, 126, 189], cage proteins [13, 100, 160, 163, 165, 242–244], ferritin [199]/apoferritin [200], and vaults [245, 246], as well as macroscopically amorphous structural proteins such as ELPs [30, 76, 91, 144, 149, 150] and gelatin [9, 31, 86], and rationally designed peptides for self-assembly covering peptide amphiphiles [332, 336], MAX family [333, 342], dock-and-lock (DnL) system [334], and the SHIELD network [335]. The examples presented in this review further demonstrate the applicably of these constructs for both passive and targeted delivery modalities.…”