Bacteriophages are essentially bionanoparticles with a protein coat, the composition of which can be controlled with atomic precision via genetic engineering, a property that makes them superior to synthetic nanoparticles as building blocks for bottom-up synthesis of multifunctional materials with advanced properties. We report hierarchically structured hydrogels of self-organized M13 bacteriophage bundles, composed of hundreds of M13 nanofilaments, which exhibit both longrange and micron-scale order, are visible in electron micrographs of the cross-linked state, and can adsorb up to 16× their weight in water. We further demonstrate that these hierarchical hydrogels of M13 exhibit advanced properties at room temperature, namely, self-healing under biological conditions, autofluorescence in three channels, which decays through biodegradation, potentiating non-destructive imaging capability, and bioactivity in the cross-linked state toward the host bacteria. The latter is, in particular, a powerful property, allowing the development of hydrogels with tunable bioactivity when combined with the phage display and/or recombinant DNA technology. Filamentous phage M13 has garnered significant attention in the past decade for the development of functional materials, ranging from tissue engineering scaffolds to batteries. Our investigation reveals the ability of these nanofilaments to self-organize into hierarchically structured soft matter, highlighting the power of self-organized M13 structures as building blocks for bottom-up synthesis.

Biological hydrogels play important physiological roles in the body. These hydrogels often contain ordered subdomains that provide mechanical toughness and other tissuespecific functionality. Filamentous bacteriophages are nanofilaments with a high aspect ratio that can self-assemble into liquid crystalline domains that could be designed to mimic ordered biological hydrogels and can thus find applications in biomedical engineering. We have previously reported hydrogels of pure crosslinked liquid crystalline filamentous phage formed at very high concentrations exhibiting a tightly packed microstructure and high stiffness. In this work, we report a method for inducing selfassembly of filamentous phage into liquid crystalline hydrogels at concentrations that are several orders of magnitude below that of lyotropic liquid crystal formation, thus creating structural order but a less densely packed microstructure. Hybrid hydrogels of M13 phage and bovine serum albumin (0.25 w/v%) were formed and shown to adsorb up to 16× their weight in water. Neither component gelled on its own at the low concentrations used, suggesting synergistic action between the two components in the formation of the hydrogel. The hybrid hydrogels exhibited repetitive self-healing under physiological conditions and at room temperature, autofluorescence in three channels, and antibacterial activity toward Escherichia coli host cells. Furthermore, the hybrid hydrogels exhibited a more than 2× higher ability to pack water compared to BSA-only hydrogels and 2× lower compression modulus compared to tightly packed M13-only hydrogels, suggesting that our method could be used to create hydrogels with tunable mechanical properties and pore structure through the addition of globular proteins, while maintaining bioactivity and microscale structural order.

Filamentous bacteriophages (bacterial viruses) are semiflexible proteinous nanofilaments with high aspect ratios for which the surface chemistry can be controlled with atomic precision via genetic engineering. That, in addition to their ability to self-propagate and replicate a nearly monodisperse batch of biologically and chemically identical nanofilaments, makes these bionanofilaments superior to most synthetic nanoparticles and thus a powerful tool in the bioengineers’ toolbox. Furthermore, filamentous phages form liquid crystalline structures at high concentrations; these ordered assemblies create hierarchically ordered macro-, micro-, and nanostructures that, once cross-linked, can form hierarchically ordered hydrogels, hydrated soft material with a variety of physical and chemical properties suitable for biomedical applications (e.g., wound dressings and tissue engineering scaffolds) as well as biosensing, diagnostic assays. We provide a critical review of these hydrogels of filamentous phage, and their physical, mechanical, chemical, and biological properties and current applications, as well as an overview of limitations and challenges and outlook for future applications. In addition, we present a list of design parameters for filamentous phage hydrogels to serve as a guide for the (bio)­engineer and (bio)­chemist interested in utilizing these powerful bionanofilaments for designing smart, bioactive materials and devices.

Biological hydrogels play important physiological roles in the body. These hydrogels often contain ordered subdomains that provide mechanical toughness and other tissue-specific functionality. Filamentous bacteriophages are nanofilaments with a high aspect ratio that can self-assemble into liquid crystalline domains that could be designed to mimic ordered biological hydrogels and can thus find application in biomedical engineering. We have previously reported hydrogels of pure crosslinked liquid crystalline filamentous phage formed at very high concentrations exhibiting a tightly packed microstructure and high stiffness. In this work, we report a method for inducing self-assembly of filamentous phage into liquid crystalline hydrogels at concentrations that are several orders of magnitude below that of lyotropic liquid crystal formation, thus creating structural order, but a less densely packed hydrogel. Hybrid hydrogels of M13 phage and bovine serum albumin (0.25 w/v%) were formed and shown to adsorb up to 16 its weight in water. Neither component gelled on its own at the low concentrations used, suggesting synergistic action between the two components in forming the hydrogel. The hybrid hydrogels exhibited repetitive self-healing under physiological conditions and at room temperature, autofluorescence in three channels, and antibacterial activity towards <i>Escherichia coli</i> host cells. Furthermore, the hybrid hydrogels exhibited more than 2 higher ability to pack water compared to BSA-only hydrogels and 2 higher flexibility (lower compression modulus) compared to tightly packed M13-only hydrogels, suggesting that our method could be used to create hydrogels with tunable mechanical properties through the addition of globular proteins, while maintaining structural order at the microscale.

Biological hydrogels play important physiological roles in the body. These hydrogels often contain ordered subdomains that provide mechanical toughness and other tissue-specific functionality. Filamentous bacteriophages are nanofilaments with a high aspect ratio that can self-assemble into liquid crystalline domains that could be designed to mimic ordered biological hydrogels and can thus find application in biomedical engineering. We have previously reported hydrogels of pure crosslinked liquid crystalline filamentous phage formed at very high concentrations exhibiting a tightly packed microstructure and high stiffness. In this work, we report a method for inducing self-assembly of filamentous phage into liquid crystalline hydrogels at concentrations that are several orders of magnitude below that of lyotropic liquid crystal formation, thus creating structural order, but a less densely packed hydrogel. Hybrid hydrogels of M13 phage and bovine serum albumin (0.25 w/v%) were formed and shown to adsorb up to 16 its weight in water. Neither component gelled on its own at the low concentrations used, suggesting synergistic action between the two components in forming the hydrogel. The hybrid hydrogels exhibited repetitive self-healing under physiological conditions and at room temperature, autofluorescence in three channels, and antibacterial activity towards <i>Escherichia coli</i> host cells. Furthermore, the hybrid hydrogels exhibited more than 2 higher ability to pack water compared to BSA-only hydrogels and 2 higher flexibility (lower compression modulus) compared to tightly packed M13-only hydrogels, suggesting that our method could be used to create hydrogels with tunable mechanical properties through the addition of globular proteins, while maintaining structural order at the microscale.