Increasing the Microfabrication Performance of Synthetic Hydrogel Precursors through Molecular Design.

Implementation of hydrogel precursors in two-photon polymerization (2PP) technology provides promising opportunities in the tissue engineering field thanks to their soft characteristics and similarity to extracellular matrix. Most of the hydrogels, however, are prone to post-fabrication deformations, leading to a mismatch between the computer-aided design and the printed structure. In the present work, we have developed novel synthetic hydrogel precursors to overcome the limitations associated with 2PP processing of conventional hydrogel precursors such as post-processing deformations and a narrow processing window. The precursors are based on a poly(ethylene glycol) backbone containing urethane linkers and are, on average, functionalized with six acrylate terminal groups (three on each terminal group). As a benchmark material, we exploited a precursor with an identical backbone and urethane linkers, albeit functionalized with two acrylate groups, that were reported as state-of-the-art. An in-depth characterization of the hexafunctional precursors revealed a reduced swelling ratio (<0.7) and higher stiffness (>36 MPa Young's modulus) compared to their difunctional analogs. The superior physical properties of the newly developed hydrogels lead to 2PP-based fabrication of stable microstructures with excellent shape fidelity at laser scanning speeds up to at least 90 mm s-1, in contrast with the distorted structures of conventional difunctional precursors. The hydrogel films and microscaffolds revealed a good cell interactivity after functionalization of their surface with a gelatin methacrylamide-based coating. The proposed synthesis strategy provides a one-pot and scalable synthesis of hydrogel building blocks that can overcome the current limitations associated with 2PP fabrication of hydrogel microstructures.

Indirect Solid Freeform Fabrication of an Initiator-Free Photocrosslinkable Hydrogel Precursor for the Creation of Porous Scaffolds.

In the present work, a photopolymerized urethane-based poly(ethylene glycol) hydrogel is applied as a porous scaffold material using indirect solid freeform fabrication (SFF). This approach combines the benefits of SFF with a large freedom in material selection and applicable concentration ranges. A sacrificial 3D poly(ε-caprolactone) structure is generated using fused deposition modeling and used as template to produce hydrogel scaffolds. By changing the template plotting parameters, the scaffold channel sizes vary from 280 to 360 µm, and the strut diameters from 340 to 400 µm. This enables the production of scaffolds with tunable mechanical properties, characterized by an average hardness ranging from 9 to 43 N and from 1 to 6 N for dry and hydrated scaffolds, respectively. Experiments using mouse calvaria preosteoblasts indicate that a gelatin methacrylamide coating of the scaffolds results in an increased cell adhesion and proliferation with improved cell morphology.

Probing polymer colloids by 129Xe NMR.

Model aqueous dispersions of polystyrene, poly(methyl methacrylate), poly(n-butyl acrylate) and a statistical copolymer poly(n-butyl acrylate-co-methyl methacrylate) were studied using xenon NMR spectroscopy. The (129)Xe NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the (129)Xe spectral time-scale and a single (129)Xe signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the (129)Xe spectral time-scale and two (129)Xe NMR signals can be observed. If the signal of sorbed (129)Xe is observed, its chemical shift, line width and integral relative to the integral of free (129)Xe can be used for the characterization of the particle core. The line width of free (129)Xe provides the residence time of xenon outside the particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle surface.

Probing the surface of polymer colloids by conductometric surfactant titration.

This work revisits the use of surfactant titrations for the characterization of latex particle surfaces. Experiments were performed to study the effect of comonomer composition and the effect of acid comonomers, and the technique is applied to the characterization of particle morphology in composite latices for several different systems. It is confirmed that the packing density of surfactant on a polymer surface is a linear function of copolymer composition. Inclusion of acid comonomers has the expected effect of decreasing the amount of surfactant adsorbed on the polymer surfaces. The usefulness of the technique in the determination of particle morphology is demonstrated, in particular toward the detection of thin layers of either seed or second-stage polymer on the particle surface which are not easily detected by other techniques such as transmission electron microscopy (TEM). Finally, it is shown that the use of acid comonomers in composite particles greatly reduces the usefulness of the surfactant titration technique for morphology characterization. A possible explanation for this effect is proposed.