Layer-by-Layer Printing of Photopolymers in 3D: How Weak is the Interface?

Additive manufacturing or, as also called, three-dimensional (3D) printing is considered as a game-changer in replacing traditional processing methods in numerous applications; yet, it has one intrinsic potential weakness related to bonding of layers formed during the printing process. Prior to finding solutions for improvement, a thorough quantitative understanding of the mechanical properties of the interface is needed. Here, a quantitative analysis of the nanomechanical properties in 3D printed photopolymers formed by digital light processing (DLP) stereolithography (SLA) is shown. Mapping of the contact Young's modulus across the layered structure is performed by atomic force microscopy (AFM) with a submicrometer resolution. The peakforce quantitative nanomechanical mapping (PF-QNM) mode was employed in the AFM experiments. The layered specimens were obtained from an acrylate-based resin (PR48, Autodesk), containing also a light-absorbing dye. We observed local depressions with values up to 30% of the maximum stiffness at the interface between the consecutively deposited layers, indicating local depletion of molecular cross-link density. The thickness values of the interfacial layers were approximately 11 µm, which corresponds to ∼22% of the total layer thickness (50 µm). We attribute this to heterogeneities of the photopolymerization reaction, related to (1) atmospheric oxygen inhibition and (2) molecular diffusion across the interface. Additionally, a pronounced stiffness decay was observed across each individual layer with a skewed profile. This behavior was rationalized by a spatial variation of the polymer cross-link density related to the variations of light absorption within the layers. This is caused by the presence of light absorbers in the printed material, resulting in a spatial decay of light intensity during photopolymerization.

Poly(ferrocenylsilane) electrolytes as a gold nanoparticle foundry: "two-in-one" redox synthesis and electrosteric stabilization, and sensing applications.

Gold nanoparticles (AuNPs) coated with responsive polymers gained considerable interest due to their controllable size, good stability, and fast environmental response suitable for biological applications and sensing. Here we report on a simple and efficient method for the synthesis of stable and redox responsive AuNPs using organometallic polyelectrolytes in aqueous solutions of HAuCl4. In the redox reaction, positively or negatively charged poly(ferrocenylsilanes) (PFS+/PFS-) served as reducing agents, and also as stabilizing polymers. Due to their unique tunable electrostatic and electrosteric protection, AuNPs coated with PFS-, (PFS+)@AuNPs, possess high redox sensitivity, with reversible, repetitive, sustainable color switching between the assembled (purple color) and disassembled (red color) states as evidenced by UV-Vis absorption and TEM measurements. Feasibility studies reported here indicate that the particles described can be applied as a colorimetric probe for the detection of redox molecules, e.g. vitamin C, in a controlled and facile manner.

PEG stabilized DNA - poly(ferrocenylsilane) polyplexes for gene delivery.

Polycationic poly(ferrocenylsilane)s (PFS) with tunable amounts of PEG side chains were used for the condensation of DNA into polyplexes of 110 nm in 5.0 mM HEPES. The PFS-PEG/DNA polyplexes showed negligible aggregation, a strongly reduced protein adsorption, transfection activities comparable with linear polyethyleneimine and an excellent cytocompatibility.

Redox-responsive organometallic hydrogels for in situ metal nanoparticle synthesis.

A new class of redox active hydrogels composed of poly(ferrocenylsilane) polyanion and poly(ethylene glycol) chains was assembled, using a copper-free azide-alkyne Huisgen cycloaddition reaction. These organometallic hydrogels displayed reversible collapse and reswelling upon chemical oxidation and reduction, respectively, and formed relatively well-defined, unaggregated Pd(0) nanoparticles (8.2 ± 2.2 nm) from K2PdCl4 salts.