High carrier mobility along the [111] orientation in Cu2O photoelectrodes.

Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight1,2. Following a decade of advancement, Cu2O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials3-5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance6. Here we demonstrate performance of Cu2O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu2O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu2O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu2O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm-2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.

Improving the photovoltage of Cu2O photocathodes with dual buffer layers.

Cuprous oxide (Cu2O) is a promising oxide material for photoelectrochemical water splitting (PEC), and increasing its photovoltage is the key to creating efficient overall PEC water-splitting devices. Previous reports are mostly focused on optimizing the energy band alignment between Cu2O and the n-type buffer layer to improve the photovoltage of Cu2O photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. In this work, Cu2O photocathodes with a single buffer layer (Ga2O3) and dual buffer layers (Ga2O3/ZnGeOx) are fabricated, and their PEC performances are compared. Results show that after inserting the second buffer layer (ZnGeOx), the onset potential of the Cu2O photocathode increases by 0.16 V. Operando electrochemical impedance spectroscopy measurements and analysis of the energy-level diagrams of each layer show that an energy level gradient between Ga2O3 and TiO2 is created when ZnGeOx is introduced, which eliminates the potential barrier at the interface of Ga2O3/TiO2 and improves the photovoltage of the Cu2O photocathode. Our work provides an effective approach to improve the photovoltage of photoelectrodes for solar water splitting by introducing dual buffer layers.

Full-space dual-helicity decoupled metasurface for a high-efficiency multi-folded reflective antenna.

The independent tailoring of electromagnetic waves with different circular-polarized (CP) wavefront in both reflection and transmission channels is of broad scientific and technical interest, offering ultimate degrees of freedom in designing advanced devices with the merits of functionality integration and spatial exploitation. However, most metasurfaces only provide dependent wavefront control of dual-helicity in a single channel, restricting their applications to limited practical scenarios. Herein, we propose a full-space dual-helicity decoupled metasurface and apply it to assemble a multi-folded reflective antenna (MFRA) in the microwave regime. A multilayered chiral meta-atom is designed and optimized to reflect a particular helical wave while allowing the orthogonal helical wave to penetrate through, with simultaneous full span of phase modulations in both channels. When a uniform reflection and a hyperbolic transmission phase profile is imposed simultaneously on the metasurface in a polarization-selective manner, it can be engineered to conduct specular reflection for one helical wave and convergent transmission of the other helical wave. Combining the proposed metasurface with a metallic plate as a bottom reflector and an integrated microstrip patch antenna in the center of metasurface as a feed, a MFRA is realized with a low profile, high efficiency, and high polarization purity in a broad frequency band. The proposed design method of the dual-helicity decoupled metasurface and its antenna application provide opportunities for high-performance functional devices, promising more potential in future communication and detection systems.

Elucidating the Role of Hypophosphite Treatment in Enhancing the Performance of BiVO4 Photoanode for Photoelectrochemical Water Oxidation.

Slow water oxidation kinetics and poor charge transport restrict the development of efficient BiVO4 photoanodes for photoelectrochemical (PEC) water splitting. Oxygen vacancy as an effective strategy can significantly enhance charge transport and improve conductivity in semiconductor photoanodes. Herein, we obtained BiVO4 photoanodes with appropriate oxygen vacancy by treating them with hypophosphite, which significantly improved the PEC performance. The synthesized photoanode exhibits a remarkable photocurrent density of 3.37 mA/cm2 at 1.23 V vs reversible hydrogen electrode with excellent stability. Interestingly, the performance improvement mainly originates from the oxygen vacancy rather than P doping. Our study provides insights in understanding the role of oxygen vacancy in PEC water splitting and strategies for designing more efficient photoelectrodes.

Human airway-like multilayered tissue on 3D-TIPS printed thermoresponsive elastomer/collagen hybrid scaffolds.

Developing a biologically representative complex tissue of the respiratory airway is challenging, however, beneficial for treatment of respiratory diseases, a common medical condition representing a leading cause of death in the world. This in vitro study reports a successful development of synthetic human tracheobronchial epithelium based on interpenetrated hierarchical networks composed of a reversely 3D printed porous structure of a thermoresponsive stiffness-softening elastomer nanohybrid impregnated with collagen nanofibrous hydrogel. Human bronchial epithelial cells (hBEpiCs) were able to attach and grow into an epithelial monolayer on the hybrid scaffolds co-cultured with either human bronchial fibroblasts (hBFs) or human bone-marrow derived mesenchymal stem cells (hBM-MSCs), with substantial enhancement of mucin expression, ciliation, well-constructed intercellular tight junctions and adherens junctions. The multi-layered co-culture 3D scaffolds consisting of a top monolayer of differentiated epithelium, with either hBFs or hBM-MSCs proliferating within the hyperelastic nanohybrid scaffold underneath, created a tissue analogue of the upper respiratory tract, validating these 3D printed guided scaffolds as a platform to support co-culture and cellular organization. In particular, hBM-MSCs in the co-culture system promoted an overall matured physiological tissue analogue of the respiratory system, a promising synthetic tissue for drug discovery, tracheal repair and reconstruction. STATEMENT OF SIGNIFICANCE: Respiratory diseases are a common medical condition and represent a leading cause of death in the world. However, the epithelium is one of the most challenging tissues to culture in vitro, and suitable tracheobronchial models, physiologically representative of the innate airway, remain largely elusive. This study presents, for the first time, a systematic approach for the development of functional multilayered epithelial synthetic tissue in vitro via co-culture on a 3D-printed thermoresponsive elastomer interpenetrated with a collagen hydrogel network. The viscoelastic nature of the scaffold with stiffness softening at body temperature provide a promising matrix for soft tissue engineering. The results presented here provide new insights about the epithelium at different surfaces and interfaces of co-culture, and pave the way to offer a customizable reproducible technology to generate physiologically relevant 3D biomimetic systems to advance our understanding of airway tissue regeneration.

Thermoresponsive Stiffness Softening of Hierarchically Porous Nanohybrid Membranes Promotes Niches for Mesenchymal Stem Cell Differentiation.

Despite the attention given to the development of novel responsive implants for regenerative medicine applications, the lack of integration with the surrounding tissues and the mismatch with the dynamic mechanobiological nature of native soft tissues remain in the current products. Hierarchical porous membranes based on a poly (urea-urethane) (PUU) nanohybrid have been fabricated by thermally induced phase separation (TIPS) of the polymer solution at different temperatures. Thermoresponsive stiffness softening of the membranes through phase transition from the semicrystalline phase to rubber phase and reverse self-assembly of the quasi-random nanophase structure is characterized at body temperature near the melting point of the crystalline domains of soft segments. The effects of the porous structure and stiffness softening on proliferation and differentiation of human bone-marrow mesenchymal stem cells (hBM-MSCs) are investigated. The results of immunohistochemistry, histological, ELISA, and qPCR demonstrate that hBM-MSCs maintain their lineage commitment during stiffness relaxation; chondrogenic differentiation is favored on the soft and porous scaffold, while osteogenic differentiation is more prominent on the initial stiff one. Stiffness relaxation stimulates more osteogenic activity than chondrogenesis, the latter being more influenced by the synergetic coupling effect of softness and porosity.

Development data associated with effects of stiffness softening of 3D-TIPS elastomer nanohybrid scaffolds on tissue ingrowth, vascularization and inflammation in vivo.

This DiB article contains data related to the research article entitled "Cellular responses to thermoresponsive stiffness memory elastomer nanohybrid scaffolds by 3D-TIPS" (Wu et al., 2018). Thermoresponsive poly (urea-urethane) nanohybrid elastomer (PUU-POSS) scaffolds were implanted in rats for up to 3 months. The porous structure and tensile mechanical properties of the scaffolds are listed and compared before and after in vitro and in vivo tests. The details of the histological analysis of the explants with different initial stiffness and porous structures at various time points are presented. The images and data presented support the conclusion about the coupled effects of stiffness softening and the hierarchical porous structure modulating tissue ingrowth, vascularization and macrophage polarization in the article (Wu et al., 2018).

Cellular responses to thermoresponsive stiffness memory elastomer nanohybrid scaffolds by 3D-TIPS.

Increasing evidence suggests the contribution of the dynamic mechanical properties of the extracellular matrix (ECM) to regulate tissue remodeling and regeneration. Following our recent study on a family of thermoresponsive 'stiffness memory' elastomeric nanohybrid scaffolds manufactured via an indirect 3D printing guided thermally-induced phase separation process (3D-TIPS), this work reports in vitro and in vivo cellular responses towards these scaffolds with different initial stiffness and hierarchically interconnected porous structure. The viability of mouse embryonic dermal fibroblasts in vitro and the tissue responses during the stiffness softening of the scaffolds subcutaneously implanted in rats for three months were evaluated by immunohistochemistry and histology. Scaffolds with a higher initial stiffness and a hierarchical porous structure outperformed softer ones, providing initial mechanical support to cells and surrounding tissues before promoting cell and tissue growth during stiffness softening. Vascularization was guided throughout the digitally printed interconnected networks. All scaffolds exhibited polarization of the macrophage response from a macrophage phenotype type I (M1) towards a macrophage phenotype type II (M2) and down-regulation of the T-cell proliferative response with increasing implantation time; however, scaffolds with a more pronounced thermo-responsive stiffness memory mechanism exerted higher inflammo-informed effects. These results pave the way for personalized and biologically responsive soft tissue implants and implantable device with better mechanical matches, angiogenesis and tissue integration. Statement of Significance This work reports cellular responses to a family of 3D-TIPS thermoresponsive nanohybrid elastomer scaffolds with different stiffness softening both in vitro and in vivo rat models. The results, for the first time, have revealed the effects of initial stiffness and dynamic stiffness softening of the scaffolds on tissue integration, vascularization and inflammo-responses, without coupling chemical crosslinking processes. The 3D printed, hierarchically interconnected porous structures guide the growth of myofibroblasts, collagen fibers and blood vessels in real 3D scales. In vivo study on those unique smart elastomer scaffolds will help pave the way for personalized and biologically responsive soft tissue implants and implantable devices with better mechanical matches, angiogenesis and tissue integration.

Stiffness memory of indirectly 3D-printed elastomer nanohybrid regulates chondrogenesis and osteogenesis of human mesenchymal stem cells.

The cellular microenvironment is dynamic, remodeling tissues lifelong. The biomechanical properties of the extracellular matrix (ECM) influence the function and differentiation of stem cells. While conventional artificial matrices or scaffolds for tissue engineering are primarily static models presenting well-defined stiffness, they lack the responsive changes required in dynamic physiological settings. Engineering scaffolds with varying elastic moduli is possible, but often lead to stiffening and chemical crosslinking of the molecular structure with limited control over the scaffold architecture. A family of indirectly 3D printed elastomeric nanohybrid scaffolds with thermoresponsive mechanical properties that soften by reverse self-assembling at body temperature have been developed recently. The initial stiffness and subsequent stiffness relaxation of the scaffolds regulated proliferation and differentiation of human bone-marrow derived mesenchymal stem cells (hBM-MSCs) towards the chondrogenic and osteogenic lineages over 4 weeks, as measured by immunohistochemistry, histology, ELISA and qPCR. hBM-MSCs showed enhanced chondrogenic differentiation on softer scaffolds and osteogenic differentiation on stiffer ones, with similar relative expression to that of human femoral head tissue. Overall, stiffness relaxation favored osteogenic activity over chondrogenesis in vitro.

Data of a stiffness softening mechanism effect on proliferation and differentiation of a human bone marrow derived mesenchymal stem cell line towards the chondrogenic and osteogenic lineages.

This article contains data related to the research article entitled "Stiffness memory of indirectly 3D-printed elastomer nanohybrid regulates chondrogenesis and osteogenesis of human mesenchymal stem cells" [1] (Wu et al., 2018). Cells respond to the local microenvironment in a context dependent fashion and a continuous challenge is to provide a living construct that can adapt to the viscoelasticity changes of surrounding tissues. Several materials are attractive candidates to be used in tissue engineering, but conventional manufactured scaffolds are primarily static models with well-defined and stable stiffness that lack the dynamic biological nature required to undergo changes in substrate elasticity decisive in several cellular processes key during tissue development and wound healing. A family of poly (urea-urethane) (PUU) elastomeric nanohybrid scaffolds (PUU-POSS) with thermoresponsive mechanical properties that soften by reverse self-assembling at body temperature had been developed through a 3D thermal induced phase transition process (3D-TIPS) at various thermal conditions: cryo-coagulation (CC), cryo-coagulation and heating (CC + H) and room temperature coagulation and heating (RTC + H). The stiffness relaxation and stiffness softening of these scaffolds suggest regulatory effects in proliferation and differentiation of human bone-marrow derived mesenchymal stem cells (hBM-MSCs) towards the chondrogenic and osteogenic lineages.

Stiffness memory nanohybrid scaffolds generated by indirect 3D printing for biologically responsive soft implants.

Cell and tissue stiffness is an important biomechanical signalling parameter for dynamic biological processes; responsive polymeric materials conferring responsive functionality are therefore appealing for in vivo implants. We have developed thermoresponsive poly(urea-urethane) nanohybrid scaffolds with 'stiffness memory' through a versatile 3D printing-guided thermally induced phase separation (3D-TIPS) technique. 3D-TIPS, a combination of 3D printing with phase separation, allows uniform phase-separation and phase transition of the polymer solution at a large interface of network within the printed sacrificial preform, leading to the creation of full-scale scaffolds with bespoke anatomical complex geometry. A wide range of hyperelastic mechanical properties of the soft elastomer scaffolds with interconnected pores at multi-scale, controlled porosity and crystallinity have been manufactured, not previously achievable via direct printing techniques or phase-separation alone. Semi-crystalline polymeric reverse self-assembly to a ground-stated quasi-random nanophase structure, throughout a hierarchical structure of internal pores, contributes to gradual stiffness relaxation during in vitro cell culture with minimal changes to shape. This 'stiffness memory' provides initial mechanical support to surrounding tissues before gradually softening to a better mechanical match, raising hopes for personalized and biologically responsive soft tissue implants which promote human fibroblast cells growth as model and potential scaffold tissue integration. STATEMENT OF SIGNIFICANCE: Biological processes are dynamic in nature, however current medical implants are often stronger and stiffer than the surrounding tissue, with little adaptability in response to biological and physical stimuli. This work has contributed to the development of a range of thermoresponsive nanohybrid elastomer scaffolds, with tuneable stiffness and hierarchically interconnected porous structure, manufactured by a versatile indirect 3D printing technique. For the first time, stiffness memory of the scaffold was observed to be driven by phase transition and a reverse self-assembly from a semicrystalline phase to a quasi-random nanostructured rubber phase. Early insight into cell response during the stiffness relaxation of the scaffolds in vitro holds promise for personalized biologically responsive soft implants.