Fractal Structure of Hydrogels Modulates Stem Cell Behavior

Fractal dimension (<i>D</i>_f) is an index to describe the irregular continuous structure by quantifying the complexity. The concept of fractals has been employed to describe the complicated structure of polymer gel and human tissue. This study examined the effect of <i>D</i>_f on cell proliferation and stem cell differentiation in six polymer hydrogels with <i>D</i>_f ranging from 1.2 to 2.1. It was observed that fibroblasts and mesenchymal stem cells (MSCs) grew faster in hydrogels with higher <i>D</i>_f. Moreover, hydrogels with a fractal structure of <i>D</i>_f ≤ 1.4, ≥1.6, and ≥1.8 promoted the neural, osteogenic, and chondrogenic differentiation of MSCs, respectively. The fractal structure of gel can modulate cell proliferation and fate, which provides an insight into designing the appropriate fractal and molecular structure of polymer hydrogel for biomedical applications

Biodegradable Water-Based Polyurethane Shape Memory Elastomers for Bone Tissue Engineering

Shape memory polymers (SMPs) are polymers with the shape memory effect. The biodegradable SMPs are candidate materials for making biomedical devices and scaffolds for tissue engineering. Superparamagnetic iron oxide nanoparticles (SPIO NPs) have recently been reported to promote the osteogenic induction of human mesenchymal stem cells (hMSCs). In this study, we synthesized water-based biodegradable shape memory polyurethane (PU) as the main component of the 3D printing ink for fabricating bone scaffolds. The 3D printing ink contained 500 ppm of SPIO NPs to promote osteogenic induction and shape fixity, and it also contained polyethylene oxide (PEO) or gelatin for the improvement of printability. Scaffolds were printed by the microextrusion-based low-temperature fuse deposition manufacturing (LFDM) platform. Both PU–PEO and PU–gelatin ink showed excellent printability. Shape memory properties were evaluated in 50 °C air and 37 °C water. PU–PEO scaffolds showed better shape fixity and recovery than PU–gelatin scaffolds, while the shape memory properties in water were better than those in air. hMSCs were seeded for evaluation of bone regeneration. The proliferation of the hMSCs in PU/gelatin and PU/gelatin/SPIO scaffolds was greater than that in PU/PEO and PU/PEO/SPIO scaffolds, confirming the better compatibility of gelatin vs PEO as the viscosity enhancer of the ink. The gradual release of SPIO NPs from the scaffolds promoted the osteogenesis of seeded hMSCs. With SPIO in the scaffolds, the osteogenesis increased 2.7 times for PU/PEO and 1.5 times for PU/gelatin scaffolds based on the collagen content. Meanwhile, SPIO release from PU/PEO/SPIO scaffolds was faster than that from PU/gelatin/SPIO scaffolds at 14 days, consistent with the better osteogenesis observed in PU/PEO/SPIO scaffolds. We concluded that 3D printed PU scaffolds with shape memory properties, biodegradability, and osteogenic effect may be employed to the minimally invasive surgical procedures as customized-bone substitutes for bone tissue engineering

Morphologies of Self-Organizing Regioregular Conjugated Polymer/Fullerene Aggregates in Thin Film Solar Cells

In this study, we used simultaneous synchrotron grazing incidence X-ray scattering and diffraction to elucidate the overall morphologies of bulk heterojunction (BHJ) thin film (ca. 100 nm) solar cells containing phase-separated poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) domains. Specifically, the dimensions and orientation of the P3HT crystallites and the sizes of the PCBM aggregates in BHJ thin films were determined. The appropriate PCBM aggregate size and density required for an optimum performance of the film in the photovoltaics device resulted in deteriorated ordering in the out-of-plane direction, but improved the in-plane packing of the P3HT lamellae. When the P3HT crystallites and PCBM aggregates had comparable domain sizes and number densities, the interpercolated networks for electron- and hole-transport were optimized in the film. This new understanding of the underlying mechanism of carrier mobility in BHJ thin films might be crucial in improving the efficiency of future solar cells

Structural Evolution of Poly(styrene-<i>b</i>-4-vinylpyridine) Diblock Copolymer/Gold Nanoparticle Mixtures from Solution to Solid State

We used in situ annealing small-angle X-ray scattering to monitor the structural evolution of a spherical poly(styrene-b-4-vinylpyridine) diblock copolymer (PS-b-P4VP)/2-phenylethanethiol-coated Au nanoparticle (AuSC2Ph) mixture in the solid state during its thermal annealing. We found that the Au nanoparticles (NPs) that existed initially in a random state with some cluster packing in the PS domain diffused to the interface of the amphiphilic PS-b-P4VP diblock copolymer within 4 h at 170 °C under vacuum to form NP-filled shell-like assemblies, as further evidenced from transmission electron microscopy imaging. From the X-ray photoelectron spectroscopy data, we speculate that this interfacial activity of AuSC2Ph results from the fact that the initially hydrophobic Au NP surfaces became increasingly hydrophilic as most of the 2-phenylethanethiol ligands had evaporated off. The Au NP nanoshell assemblies located at the interface between PS and P4VP were quite stable even after redissolving in toluene; they remained in the form of PS−Au−P4VP core/shell/corona onion micelles, as evidenced from solution state small-angle X-ray scattering data

A Biomimetic Bilayer Hydrogel Actuator Based on Thermoresponsive Gelatin Methacryloyl–Poly(<i>N</i>‑isopropylacrylamide) Hydrogel with Three-Dimensional Printability

Development of hydrogel-based actuators with programmable deformation is an important topic that arouses much attention in fundamental and applied research. Most of these actuators are nonbiodegradable or work under nonphysiological conditions. Herein, a temperature-responsive and biodegradable gelatin methacryloyl (GelMA)–poly(N-isopropylacrylamide) hydrogel (i.e., GN hydrogel) network was explored as the active layer of a bilayer actuator. Small-angle X-ray scattering (SAXS) revealed that the GN hydrogel formed a mesoglobular structure (∼230 Å) upon a thermally induced phase transition. Rheological data supported that the GN hydrogel possessed 3D printability and tunable mechanical properties. A bilayer hydrogel actuator composed of active GN and passive GelMA layers was optimized by varying the layer thickness and compositions to achieve large, reproducible, and anisotropic bending with a curvature of ∼5.5 cm–1. Different patterns of the active layer were designed for actuation in programmable control. The 3D printed GN hydrogel constructs showed significant volume reduction (∼25–60% depending on construct design) at 37 °C with the resolution enhanced by the thermo-triggered actuation, while they were able to fully reswell at room temperature. A more intricate 3D printed butterfly actuator demonstrated the ability to mimic the wing movement through thermoresponsiveness. Furthermore, myoblasts laden in the GN hydrogel exhibited significant proliferation of ∼376% in 14 days. This study provides a new fabrication approach for developing biomimetic devices, artificial muscles, and soft robotics for biomedical applications

Characterization of Biodegradable Polyurethane Nanoparticles and Thermally Induced Self-Assembly in Water Dispersion

Waterborne polyurethanes (PU) with different compositions of biodegradable oligodiols as the soft segment were synthesized as nanoparticles (NPs) in this study. Using dynamic light scattering (DLS), multiangle light scattering (MALS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS), we demonstrated that these NPs were compact spheres with different shape factors. The temperature-dependent swelling of the PU NPs in water was distinct. In particular, PU NPs with 80 mol % polycaprolactone (PCL) diol and 20 mol % poly­(l-lactide) (PLLA) diol as the soft segment had significant swelling (∼450%) at 37 °C. This was accompanied by a sol–gel transition observed in about 2 min for the NP dispersion. The thermally induced swelling and self-assembly of these NPs were associated with the secondary force (mainly hydrogen bonding) and degree of crystallinity, which depended on the soft segment compositions. The thermo-responsiveness of the PU NPs with mixed biodegradable oligodiols may be employed to design smart biodegradable carriers for delivery of cells or drugs near body temperature

A Biomimetic Bilayer Hydrogel Actuator Based on Thermoresponsive Gelatin Methacryloyl–Poly(<i>N</i>‑isopropylacrylamide) Hydrogel with Three-Dimensional Printability

Development of hydrogel-based actuators with programmable deformation is an important topic that arouses much attention in fundamental and applied research. Most of these actuators are nonbiodegradable or work under nonphysiological conditions. Herein, a temperature-responsive and biodegradable gelatin methacryloyl (GelMA)–poly(N-isopropylacrylamide) hydrogel (i.e., GN hydrogel) network was explored as the active layer of a bilayer actuator. Small-angle X-ray scattering (SAXS) revealed that the GN hydrogel formed a mesoglobular structure (∼230 Å) upon a thermally induced phase transition. Rheological data supported that the GN hydrogel possessed 3D printability and tunable mechanical properties. A bilayer hydrogel actuator composed of active GN and passive GelMA layers was optimized by varying the layer thickness and compositions to achieve large, reproducible, and anisotropic bending with a curvature of ∼5.5 cm–1. Different patterns of the active layer were designed for actuation in programmable control. The 3D printed GN hydrogel constructs showed significant volume reduction (∼25–60% depending on construct design) at 37 °C with the resolution enhanced by the thermo-triggered actuation, while they were able to fully reswell at room temperature. A more intricate 3D printed butterfly actuator demonstrated the ability to mimic the wing movement through thermoresponsiveness. Furthermore, myoblasts laden in the GN hydrogel exhibited significant proliferation of ∼376% in 14 days. This study provides a new fabrication approach for developing biomimetic devices, artificial muscles, and soft robotics for biomedical applications

Functionalized Nanoporous Gyroid SiO₂ with Double-Stimuli-Responsive Properties as Environment-Selective Delivery Systems

Herein, we aim to fabricate nanoporous gyroid SiO₂ from templated sol–gel reaction using degradable block copolymer with gyroid-forming nanostructure as a template and then to functionalize the nanoporous materials using “smart” polymer, poly­(2-(dimethylamino)­ethyl methacrylate) (PDMAEMA), brushes via the “grafting from” method to give double-stimuli-responsive properties. By taking advantage of the responses to environmental stimuli, both thermal and pH, the pore features can be well-defined by the stretching and recoiling of PDMAEMA brushes because of their adjustable chain conformations with reversible character. The responsive properties with respect to environmental stimuli can be successfully traced by temperature-resolved small-angle X-ray scattering (SAXS) in aqueous environment. Owing to the high specific surface area and porosity, 3D pore network, biocompatibility, and environmental responses, the functionalized nanoporous gyroid SiO₂ is further demonstrated as a stimuli-responsive controlled release system

Pseudo-Single-Crystalline Self-Assembled Structure Formed from Hydrophilic CdSe and Hydrophobic Au Nanoparticles in the Polystyrene and Poly(4-vinylpyridine) Blocks, Respectively, of a Polystyrene-<i>b</i>-poly(4-vinylpyridine) Diblock Copolymer

Pseudo-Single-Crystalline Self-Assembled Structure Formed from Hydrophilic CdSe and Hydrophobic Au Nanoparticles in the Polystyrene and Poly(4-vinylpyridine) Blocks, Respectively, of a Polystyrene-b-poly(4-vinylpyridine) Diblock Copolyme