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

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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. WIREs Nanomed Nanobiotechnol 2015, 7:34–68. doi: 10.1002/wnan.1299 This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Biology-Inspired Nanomaterials > Peptide-Based Structures Biology-Inspired Nanomaterials > Protein and Virus-Based Structures
Oscillatory frequency sweep measurement of a 0.5% (w/v) hydrogel at pH 9 (125 mM Boric Acid 10 mM NaCl), showing relative independence of G′ (Pa) to applied angular frequency (rad/second) indicating solid‐like character of MAX1 0.5% (w/v) hydrogel. G′ (Pa) indicated by solid squares, G″ (Pa) indicated by hollow circles (G′ ≫ G″). The hydrogel sample shows G ′≫ G″ for all frequencies, particularly the lower frequencies.
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Schematic representation of a particle‐tracking velocimetry apparatus. (Reprinted with permission from Ref . Copyright 2012 American Chemical Society)
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Schematic showing results obtained from confocal‐rheometer compound assembly. The hydrogel undergoes shear rate‐dependent fracture in a layer of certain thickness away from the upper plate of the rheometer geometry, while the rest of the hydrogel undergoes no or negligible flow. The square schematics indicate volumes across the cross section of the hydrogel sample between the rheometer plates when the sample is subject to 5/second, 50/second, 250/second, 400/second, and 1000/second rate of steady‐state shear. The light blue and dark blue layers indicate the fractured and consequently flowing layers and stationary layers of the MAX8 hydrogel, respectively. Tf indicates the % thickness of the fractured layer.
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Visualization of the flow profile in capillary flow of a PC10P hydrogel labeled with fluorescent microsphere tracers. Time‐dependent line scans along the diameter of a capillary (1.93 ms/scan) show that the transit time for a particle to cross the plane of the line scan is approximately the same for all distances from the center of the capillary, indicative of a plug‐flow‐type profile. This implies yielding in the gel layer adjacent to the capillary wall, consistent with the shear‐banding mechanism. (Reprinted with permission from Ref . Copyright 2010 American Chemical Society)
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(a) Flow profile of a cell suspension and cells encapsulated in MAX8 gels. Solid symbols: one‐dimensional flow velocity of living MG 63 cells against lateral position when suspended in aqueous buffer (pH 7.4, 25 mM HEPES, 37°C). Open symbols: one‐dimensional flow velocity of living MG 63 cells when encapsulated in 0.75 wt % MAX8 gels, pH 7.4, 25 mM HEPES, 37°C). (b) Three‐dimensional confocal microscope images showing live‐dead assays of MG63 cells encapsulated in 0.5 wt % MAX8 hydrogel after injection. (Reprinted with permission from Ref . Copyright 2012 American Chemical Society)
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Oscillatory time sweep measurement (6 rad/second and 1% strain) from 0 to ∼80 min after mixing peptide solution with buffer solution to form a 2% (w/v) MAX1 peptide hydrogel at pH 9 (125 mM Boric Acid 10 mM NaCl) showing evolution of storage modulus of 2% (w/v) pH 9 (125 mM Boric Acid 10 mM NaCl) to ∼2200 Pa followed by steady‐state shear of amplitude 1000/second for 2 min indicated by the dotted line. The shear step is followed by another oscillatory time sweep measurement probing rehealing of the hydrogel to G′ value close to the initial G′ value. G′ (Pa) indicated by solid squares, G″ (Pa) indicated by hollow circles (G′ ≫ G″), indicating gel‐like rheological behavior from all samples at all times.
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Schematic of MAX1 self‐assembly (a) MAX1 random coil conformation at low to neutral pH and low temperature, the pink side chains represent lysine side chains and the blue side chains represent valine side chains. (b) β‐Hairpin conformation induced by rise in the pH, temperature, and/or ionic strength of the peptide solution. (c) Subsequent to intramolecular folding, facial hydrophobic collapse of two hairpins leading to formation of a bilayer type structure. (d) Direction of lateral hydrophobic interactions among multiple bilayer type structures (e) hierarchically assembled branched fibril of MAX1. (f) Cryogenic transmission electron microscopy showing fibrillar structure of MAX1.
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Dynamic Time Sweep (1% strain, 6 rad/second) of 2 wt % MAX1 [(VK)4‐VDPPT‐(KV)4] solution with 20 mM (G′, filled circles), 150 mM (G′, filled triangles and 400 mM (G′, filled squares) NaCl at 20°C. For all three samples (G ′≫ G″) indicating gel‐like rheological behavior from all samples at all times. (Reprinted with permission from Ref . Copyright 2004 American Chemical Society)
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Biology-Inspired Nanomaterials > Protein and Virus-Based Structures
Nanotechnology Approaches to Biology > Nanoscale Systems in Biology
Biology-Inspired Nanomaterials > Peptide-Based Structures

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