RESOURCES

Abstract
The global prevalence of respiratory diseases caused by infectious pathogens has resulted in an increased demand for realistic in-vitro alveolar lung models to serve as suitable disease models. This demand has resulted in the fabrication of numerous two-dimensional (2D) and three-dimensional (3D) in-vitro alveolar lung models. The ability to fabricate these 3D in-vitro alveolar lung models in an automated manner with high repeatability and reliability is important for potential scalable production. In this study, we reported the fabrication of human triple-layered alveolar lung models comprising of human lung epithelial cells, human endothelial cells, and human lung fibroblasts using the drop-on-demand (DOD) 3D bioprinting technique. The polyvinylpyrrolidone-based bio-inks and the use of a 300 mm nozzle diameter improved the repeatability of the bioprinting process by achieving consistent cell output over time using different human alveolar lung cells. The 3D bioprinted human triple-layered alveolar lung models were able to maintain cell viability with relative similar proliferation profile over time as compared to non-printed cells. This DOD 3D bioprinting platform offers an attractive tool for highly repeatable and scalable fabrication of 3D in-vitro human alveolar lung models.

Abstract
Abstract Extrusion additive manufacturing is widely used to fabricate polymer-based 3D bone scaffolds. However, the insight views of crystal growths, scaffold features and eventually cell-scaffold interactions are still unknown. In this work, melt and solvent extrusion additive manufacturing techniques are used to produce scaffolds considering highly analogous printing conditions. Results show that the scaffolds produced by these two techniques present distinct physiochemical properties, with melt-printed scaffolds showing stronger mechanical properties and solvent-printed scaffolds showing rougher surface, higher degradation rate, and faster stress relaxation. These differences are attributed to the two different crystal growth kinetics, temperature-induced crystallization (TIC) and strain-induced crystallization (SIC), forming large/integrated spherulite-like and a small/fragmented lamella-like crystal regions respectively. The stiffer substrate of melt-printed scaffolds contributes to higher ratio of nuclear Yes-associated protein (YAP) allocation, favoring cell proliferation and differentiation. Faster relaxation and degradation of solvent-printed scaffolds result in dynamic surface, contributing to an early-stage faster osteogenesis differentiation.

Abstract
This research investigates the accelerated hydrolytic degradation process of both anatomically designed bone scaffolds with a pore size gradient and a rectangular shape (biomimetically designed scaffolds or bone bricks). The effect of material composition is investigated considering poly-ε-caprolactone (PCL) as the main scaffold material, reinforced with ceramics such as hydroxyapatite (HA), β-tricalcium phosphate (TCP) and bioglass at a concentration of 20 wt%. In the case of rectangular scaffolds, the effect of pore size (200 μm, 300 μm and 500 μm) is also investigated. The degradation process (accelerated degradation) was investigated during a period of 5 days in a sodium hydroxide (NaOH) medium. Degraded bone bricks and rectangular scaffolds were measured each day to evaluate the weight loss of the samples, which were also morphologically, thermally, chemically and mechanically assessed. The results show that the PCL/bioglass bone brick scaffolds exhibited faster degradation kinetics in comparison with the PCL, PCL/HA and PCL/TCP bone bricks. Furthermore, the degradation kinetics of rectangular scaffolds increased by increasing the pore size from 500 μm to 200 μm. The results also indicate that, for the same material composition, bone bricks degrade slower compared with rectangular scaffolds. The scanning electron microscopy (SEM) images show that the degradation process was faster on the external regions of the bone brick scaffolds (600 μm pore size) compared with the internal regions (200 μm pore size). The thermal gravimetric analysis (TGA) results show that the ceramic concentration remained constant throughout the degradation process, while differential scanning calorimetry (DSC) results show that all scaffolds exhibited a reduction in crystallinity (Xc), enthalpy (Δm) and melting temperature (Tm) throughout the degradation process, while the glass transition temperature (Tg) slightly increased. Finally, the compression results show that the mechanical properties decreased during the degradation process, with PCL/bioglass bone bricks and rectangular scaffolds presenting higher mechanical properties with the same design in comparison with the other materials.

Abstract
The development of advanced biomaterials and manufacturing processes to fabricate biologically and mechanically appropriate scaffolds for bone tissue is a significant challenge. Polycaprolactone (PCL) is a biocompatible and degradable polymer used in bone tissue engineering, but it lacks biofunctionalization. Bioceramics, such as hydroxyapatite (HA) and β tricalcium phosphate (β-TCP), which are similar chemically to native bone, can facilitate both osteointegration and osteoinduction whilst improving the biomechanics of a scaffold. Carbon nanotubes (CNTs) display exceptional electrical conductivity and mechanical properties. A major limitation is the understanding of how PCL-based scaffolds containing HA, TCP, and CNTs behave in vivo in a bone regeneration model. The objective of this study was to evaluate the use of three-dimensional (3D) printed PCL-based composite scaffolds containing CNTs, HA, and β-TCP during the initial osteogenic and inflammatory response phase in a critical bone defect rat model. Gene expression related to early osteogenesis, the inflammatory phase, and tissue formation was evaluated using quantitative real-time PCR (RT-qPCR). Tissue formation and mineralization were assessed by histomorphometry. The CNT+HA/TCP group presented higher expression of osteogenic genes after seven days. The CNT+HA and CNT+TCP groups stimulated higher gene expression for tissue formation and mineralization, and pro- and anti-inflammatory genes after 14 and 30 days. Moreover, the CNT+TCP and CNT+HA/TCP groups showed higher gene expressions related to M1 macrophages. The association of CNTs with ceramics at 10wt% (CNT+HA/TCP) showed lower expressions of inflammatory genes and higher osteogenic, presenting a positive impact and balanced cell signaling for early bone formation. The association of CNTs with both ceramics promoted a minor inflammatory response and faster bone tissue formation.

Abstract
Critical bone defects are the most difficult challenges in the area of tissue repair. Polycaprolactone (PCL) scaffolds, associated with hydroxyapatite (HA) and tricalcium phosphate (TCP), are reported to have an enhanced bioactivity. Moreover, the use of electrical stimulation (ES) has overcome the lack of bioelectricity at the bone defect site and compensated the endogenous electrical signals. Such treatments could modulate cells and tissue signaling pathways. However, there is no study investigating the effects of ES and bioceramic composite scaffolds on bone tissue formation, particularly in the view of cell signaling pathway. This study aims to investigate the application of HA/TCP composite scaffolds and ES and their effects on the Wingless-related integration site (Wnt) pathway in critical bone repair. Critical bone defects (25 mm2) were performed in rats, which were divided into four groups: PCL, PCL + ES, HA/TCP and HA/TCP + ES. The scaffolds were grafted at the defect site and applied with the ES application twice a week using 10 µA of current for 5 min. Bone samples were collected for histomorphometry, immunohistochemistry and molecular analysis. At the Wnt canonical pathway, HA/TCP and HA/TCP + ES groups showed higher Wnt1 and β-catenin gene expression levels, especially HA/TCP. Moreover, HA/TCP + ES presented higher Runx2, Osterix and Bmp-2 levels. At the Wnt non-canonical pathway, HA/TCP group showed higher voltage-gated calcium channel (Vgcc), calmodulin-dependent protein kinase II, and Wnt5a genes expression, while HA/TCP + ES presented higher protein expression of VGCC and calmodulin (CaM) at the same period. The decrease in sclerostin and osteopontin genes expressions and the lower bone sialoprotein II in the HA/TCP + ES group may be related to the early bone remodeling. This study shows that the use of ES modulated the Wnt pathways and accelerated the osteogenesis with improved tissue maturation.

Abstract
Critical bone defects are a major clinical challenge in reconstructive bone surgery. Polycaprolactone (PCL) mixed with bioceramics, such as hydroxyapatite (HA) and tricalcium phosphate (TCP), create composite scaffolds with improved biological recognition and bioactivity. Electrical stimulation (ES) aims to compensate the compromised endogenous electrical signals and to stimulate cell proliferation and differentiation. We investigated the effects of composite scaffolds (PCL with HA; and PCL with β-TCP) and the use of ES on critical bone defects in Wistar rats using eight experimental groups: untreated, ES, PCL, PCL/ES, HA, HA/ES, TCP, and TCP/ES. The investigation was based on histomorphometry, immunohistochemistry, and gene expression analysis. The vascular area was greater in the HA/ES group on days 30 and 60. Tissue mineralization was greater in the HA, HA/ES, and TCP groups at day 30, and TCP/ES at day 60. Bmp-2 gene expression was higher in the HA, TCP, and TCP/ES groups at day 30, and in the TCP/ES and PCL/ES groups at day 60. Runx-2, Osterix, and Osteopontin gene expression were also higher in the TCP/ES group at day 60. These results suggest that scaffolds printed with PCL and TCP, when paired with electrical therapy application, improve bone regeneration.

Abstract
Polycaprolactone (PCL) is one of the most recognized polymeric materials used for bone tissue engineering scaffold fabrication. This study aims to evaluate the effects of the molecular weight (Mn) of PCL on the degradation kinematics, surface, microstructural, thermal, mechanical, and biological properties of 3D printed bone scaffolds. Surface properties were investigated considering water-in-air contact angle and nanoindentation tests, while morphological characteristics and degradation kinematics (accelerated degradation tests) were examined using scanning electron microscopy (SEM), pairing with thermal and mechanical properties monitored at each considered time point. A set of mathematical equations describing the variation of fiber diameter, porosity, mechanical properties, and weight, as a function of molecular weight and degradation time, were obtained based on the experimental results. Human adipose-derived stem cells (hADSCs) proliferation and differentiation tests were also conducted using in vitro colorimetric assay. All results indicated that molecular weight had impacts on the surface, mechanical and biological properties of PCL scaffolds, while no significant effects were observed on the degradation rate. Scaffolds with lower molecular weight presented better bio-mechanical properties. These findings provide useful information for the design of polymeric bone tissue engineering scaffolds.

Abstract
The design of scaffolds with optimal biomechanical properties for load-bearing applications is an important topic of research. Most studies have addressed this problem by focusing on the material composition and not on the coupled effect between the material composition and the scaffold architecture. Polymer–bioglass scaffolds have been investigated due to the excellent bioactivity properties of bioglass, which release ions that activate osteogenesis. However, material preparation methods usually require the use of organic solvents that induce surface modifications on the bioglass particles, compromising the adhesion with the polymeric material thus compromising mechanical properties. In this paper, we used a simple melt blending approach to produce polycaprolactone/bioglass pellets to construct scaffolds with pore size gradient. The results show that the addition of bioglass particles improved the mechanical properties of the scaffolds and, due to the selected architecture, all scaffolds presented mechanical properties in the cortical bone region. Moreover, the addition of bioglass indicated a positive long-term effect on the biological performance of the scaffolds. The pore size gradient also induced a cell spreading gradient.

Abstract
Solid adsorbents have been actively developed for energy-efficient gas separations including carbon capture and air purification. However, conventional particulate adsorbents often show ineffective mass transfer and significant pressure drop in practical operations, leading to a limited overall performance. As a potential solution to these issues, the development of three-dimensionally (3D) structured adsorbents has been proposed. Herein, we report a novel approach to design 3D monolithic adsorbents for CO2 separation via 3D printing of a processible polymer, which in turn can be transformed into a functional porous material via hypercrosslinking and amine-grafting. Importantly, such structure can be realized without an aid from binders or mechanical supports. Our adsorbents demonstrated a promising CO2 adsorption performance without experiencing any pressure drop under dynamic flow condition. The stability and regenerability, which are also important requirements for practical operations, were also successfully demonstrated through a repetitive adsorption-desorption cycling test in the presence of water vapor. We envisage that our approach can be applied in the development of structurally versatile adsorbents for various gas separation processes.

Abstract
The weak mechanical properties of hydrogels due to the inefficient dissipation of energy in the intrinsic structures limit their practical applications. Here, a double-network (DN) hydrogel has been developed by integrating an ionically cross-linked agar network, a covalently cross-linked acrylic acid (AAC) network, and the dynamic and reversible ionically cross-linked coordination between the AAC chains and Fe3+ ions. The proposed model reveals the mechanisms of the improved mechanical performances in the DN agar/AAC-Fe3+ hydrogel. The hydrogen-bond cross-linked double helices of agar and ionic-coordination interactions of AAC-Fe3+ can be temporarily sacrificed during large deformation to readily dissipate the energy, whereas the reversible AAC-Fe3+ interactions can be regenerated after stress relief, which greatly increases the material toughness. The developed DN hydrogel demonstrates a remarkable stretchability with a break strain up to 3174.3%, high strain sensitivity with the gauge factor being 0.83 under a strain of 1000%, and good 3D printability, making the material a desirable candidate for fabricating flexible strain sensors, electronic skin, and soft robots.
The weak mechanical properties of hydrogels due to the inefficient dissipation of energy in the intrinsic structures limit their practical applications. Here, a double-network (DN) hydrogel has been developed by integrating an ionically cross-linked agar network, a covalently cross-linked acrylic acid (AAC) network, and the dynamic and reversible ionically cross-linked coordination between the AAC chains and Fe3+ ions. The proposed model reveals the mechanisms of the improved mechanical performances in the DN agar/AAC-Fe3+ hydrogel. The hydrogen-bond cross-linked double helices of agar and ionic-coordination interactions of AAC-Fe3+ can be temporarily sacrificed during large deformation to readily dissipate the energy, whereas the reversible AAC-Fe3+ interactions can be regenerated after stress relief, which greatly increases the material toughness. The developed DN hydrogel demonstrates a remarkable stretchability with a break strain up to 3174.3%, high strain sensitivity with the gauge factor being 0.83 under a strain of 1000%, and good 3D printability, making the material a desirable candidate for fabricating flexible strain sensors, electronic skin, and soft robots.

Abstract
Abstract The development of multifunctional 3D printing materials from sustainable natural resources is a high priority in additive manufacturing. Using an eco-friendly method to transform hard pollen grains into stimulus-responsive microgel particles, we engineered a pollen-derived microgel suspension that can serve as a functional reinforcement for composite hydrogel inks and as a supporting matrix for versatile freeform 3D printing systems. The pollen microgel particles enabled the printing of composite inks and improved the mechanical and physiological stabilities of alginate and hyaluronic acid hydrogel scaffolds for 3D cell culture applications. Moreover, the particles endowed the inks with stimulus-responsive controlled release properties. The suitability of the pollen microgel suspension as a supporting matrix for freeform 3D printing of alginate and silicone rubber inks was demonstrated and optimized by tuning the rheological properties of the microgel. Compared with other classes of natural materials, pollen grains have several compelling features, including natural abundance, renewability, affordability, processing ease, monodispersity, and tunable rheological features, which make them attractive candidates to engineer advanced materials for 3D printing applications.

Abstract
Sensors are crucial in the understanding of machines working under high temperatures and high-pressure conditions. Current devices utilize polymeric materials as electrical insulators which pose a challenge in the device’s lifespan. Ceramics, on the other hand, is robust and able to withstand high temperature and pressure. For such applications, a co-fired ceramic device which can provide both electrical conductivity and insulation is beneficial and acts as a superior candidate for sensor devices. In this paper, we propose a novel fabrication technique of complex multi-ceramics structures via 3D printing. This fabrication methodology increases both the geometrical complexity and the device’s shape precision. Structural ceramics (alumina) was employed as the electrical insulator whilst providing mechanical rigidity while a functional ceramic (alumina-doped zinc oxide) was employed as the electrically conductive material. The addition of sintering additives, tailoring the printing pastes’ solid loadings and heat treatment profile resolves multi-materials printing challenges such as shrinkage disparity and densification matching. Through high-temperature co-firing of ceramics (HTCC) technology, dense high quality functional multi-ceramics structures are achieved. The proposed fabrication methodology paves the way for multi-ceramics sensors to be utilized in high temperature and pressure systems in the near future.