Victor Breedveld

Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles

Protein-based hydrogels are used for many applications, ranging from food and cosmetic thickeners to support matrices for drug delivery and tissue replacement. These materials are usually prepared using proteins extracted from natural sources, which can give rise to inconsistent properties unsuitable for medical applications. Recent developments have utilized recombinant DNA methods to prepare artificial protein hydrogels with specific association mechanisms and responsiveness to various stimuli. Here we synthesize diblock copolypeptide amphiphiles containing charged and hydrophobic segments. Dilute solutions of these copolypeptides would be expected to form micelles; instead, they form hydrogels that retain their mechanical strength up to temperatures of about 90 °C and recover rapidly after stress. The use of synthetic materials permits adjustment of copolymer chain length and composition, which we varied to study their effect on hydrogel formation and properties. We find that gelation depends not only on the amphiphilic nature of the polypeptides, but also on chain conformations—α-helix, β-strand or random coil. Indeed, shape-specific supramolecular assembly is integral to the gelation process, and provides a new class of peptide-based hydrogels with potential for applications in biotechnology.

Microrheology as a tool for high-throughput screening

Microrheology can be used for high-throughput screening of the rheological properties of sample libraries of complex fluids. Two passive techniques are particularly suitable: video microscopy and diffusing-wave spectroscopy. The techniques complement each other very well and can be applied to samples that offer different experimental challenges. We offer a thorough analysis of the strengths and limitations of microrheology with the emphasis on high-throughput applications. To illustrate the potential of microrheology, results are presented for two representative cases: the rheological screening of aqueous solutions of a block copolypeptide library and the rheological phase diagram of a water/surfactant/salt system.

Tunability of the Adhesion of Water Drops on a Superhydrophobic Paper Surface via Selective Plasma Etching

We report the fabrication of a sticky superhydrophobic paper surface with extremely high contact angle hysteresis: advancing contact angle ∼150° (superhydrophobic) and receding contact angle ∼10° (superhydrophilic). In addition, we report the controlled tunability of the contact angle hysteresis from 149.8 ± 5.8° to 3.5 ± 1.1°, while maintaining superhydrophobicity, as defined through an advancing contact angle above 150°. The hysteresis was tuned through the controlled fabrication of nano-scale features on the paper fibers via selective plasma etching. The variations in contact angle hysteresis are attributed to a transition of the liquid–surface interaction from a Wenzel state to a Cassie state on the nano-scale, while maintaining a Cassie state on the micro-scale. Superhydrophobic cellulosic surfaces with tunable stickiness or adhesion have potential applications in the control of aqueous drop mobility and the transfer of drops on inexpensive, renewable substrates.

Design and Fabrication of Superamphiphobic Paper Surfaces

Cellulose-based paper remains a vital component of modern day society; however, its use is severely limited in certain applications because of hydrophilic and oleophilic properties. In this manuscript we present a novel method to create superamphiphobic paper by combining the control of fiber size and structure with plasma etching and fluoropolymer deposition. The heterogeneous nature of the paper structure is drastically different from that of artificially created superamphiphobic surfaces. By refining the wood fibers, smaller diameter fibers (fibrils) are created to support fluid droplets. After oxygen plasma etching and deposition of a fluoropolymer film, paper samples are able to support motor oil contact angles of 149 ± 3°, although these structures readily absorb n-hexadecane. Exchange of water in the pulp solution with sec-butanol provides additional control over fiber spacing to create superamphiphobic substrates with contact angles >150° for water, ethylene glycol, motor oil, and n-hexadecane.

Graded membrane supports produced by centrifugal casting of a slightly polydisperse suspension

Tubular structures of a continuous particle size gradient are formed if a hollow cylindrical mold filled with a suspension of dispersed powder with a size distribution is centrifuged around its center axis. The mean particle size in the final structure increases gradually with increasing radial coordinate. Because the bulk properties can be optimized simultaneously with the surface composition, this process has advantages for the production of porous tubular ceramic membrane supports in case subsequent membrane layers are coated on the inner surface of the support. Particle velocities and concentrations in the suspension, as well as the compact profile, are numerically analyzed for completely filled molds. Using the analysis the composition at each location in the compact can be predicted, which can be used to calculate the permeance (flux per unit pressure difference), as well as the particle composition of the inner and outer surfaces.

Modulating mechanical properties of self-assembled polymer networks by multi-functional complementary hydrogen bonding

The mechanical properties of reversible polymer networks have been modulated successfully at room temperature with a high degree of control over a large magnitude exclusively by altering the complementary hydrogen bonding interactions used for the inter-chain crosslinking process. For these studies, norbornene-based copolymers have been synthesized with multiple functional side-chains that offer different hydrogen bonding motifs. By adding small molecule crosslinking agents with complementary motifs to solutions of these copolymers, self-assembled polymer networks with tunable mechanical properties were obtained. The hydrogen bonding motifs utilized in this study are based on thymine/2,4-diaminotriazine and cyanuric acid/Hamilton wedge pairs. It was found that the mechanical properties of the self-assembled polymer networks strongly depend upon the type of hydrogen bonding motif used for the inter-chain crosslinking as well as the concentration of crosslinking agent. We were able to modulate the rheological properties of the networks from highly viscous to highly elastic and vary the dynamic moduli over five orders of magnitude at room temperature. This degree of control over the networks mechanical properties was achieved without changing the copolymer backbone architecture. Finally, competitive hydrogen bonding of various motifs was used to de-crosslink and re-crosslink the network at room temperature through the selective addition of various crosslinking agents. In addition to the more common thermal responsiveness of hydrogen bonded networks, competitive binding offers an additional parameter to control the mechanical properties of the self-assembled polymer networks at ambient temperature.

Physical Aging and Phase Behavior of Multiresponsive Microgel Colloidal Dispersions

Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following approximately 1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pK(a), the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pK(a), repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.

Transient rheology of solvent-responsive complex fluids by integrating microrheology and microfluidics

A new microrheology set-up which allows us to quantitatively measure the transient rheological properties of solvent-responsive complex fluids was constructed by integrating particle tracking microrheology and microfluidics. The dialysis cell consists of a reservoir, porous dialysis membrane, and sample chamber. Solvent molecules can freely diffuse between the reservoir and the sample chamber while macromolecular sample components are trapped in the sample chamber with a rigid semipermeable dialysis membrane. The design enables manipulation of the solvent composition in the sample chamber by simply switching the fluid composition in the reservoir. Validation experiments for solvent diffusion in the dialysis cell showed good agreement with numerical solutions of the transport equations and confirmed that the solvent composition in a sample can be changed in a controlled and predictable fashion within a few minutes due to the small device dimensions. For aqueous solutions of sodium alginate and sodium polys...

Tunable attractive and repulsive interactions between pH-responsive microgels

We report direct measurements of the pairwise interparticle potential between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) colloidal microgels as a function of pH, as determined through Ornstein–Zernike analysis of the pair distribution function of quasi-2D dilute colloidal suspensions. The interaction potential ranges from purely repulsive at high pH due to electrosteric interactions to weakly attractive at low pH due to hydrogen bonding, which explains previous observations on the unique phase behavior of these particles in concentrated suspensions.

Microscopic Structural Relaxation in a Sheared Supercooled Colloidal Liquid

The rheology of dense amorphous materials under large shear strain is not fully understood, partly due to the difficulty of directly viewing the microscopic details of such materials. We use a colloidal suspension to simulate amorphous materials and study the shear-induced structural relaxation with fast confocal microscopy. We quantify the plastic rearrangements of the particles in several ways. Each of these measures of plasticity reveals spatially heterogeneous dynamics, with localized regions where many particles are strongly rearranging by these measures. We examine the shapes of these regions and find them to be essentially isotropic, with no alignment in any particular direction. Furthermore, individual particles are equally likely to move in any direction other than the overall bias imposed by the strain.