RESOURCES

Abstract
This work reports the construction of a bicomponent scaffold co-loaded with both a prodrug and a drug (BiFp@Ht) as an efficient platform for wound dressing, by combining the electrospinning and 3D-printing technologies. The outer component consisted of a chitosan/polyethylene oxide-electrospun membrane loaded with the indomethacin–polyethylene glycol–indomethacin prodrug (Fp) and served as a support for printing the inner component, a gelatin methacryloyl/sodium alginate hydrogel loaded with tetracycline hydrochloride (Ht). The different architectural characteristics of the electrospun and 3D-printed layers were very well highlighted in a morphological analysis performed by Scanning Electron Microscopy (SEM). In vitro release profile studies demonstrated that both Fp and Ht layers were capable to release the loaded therapeutics in a controlled and sustained manner. According to a quantitative in vitro biological assessment, the bicomponent BiFp@Ht scaffold showed a good biocompatibility and no cytotoxic effect on HeLa cell cultures, while the highest proliferation level was noted in the case of HeLa cells seeded onto an Fp nanofibrous membrane. Furthermore, the BiFp@Ht scaffold presented an excellent antimicrobial activity against the E. coli and S. aureus bacterial strains, along with promising anti-inflammatory and proangiogenic activities, proving its potential to be used for wound dressing.

Abstract
The 3D printing of a multifunctional hydrogel biomaterial with bioactivity for tissue engineering, good mechanical properties and a biodegradability mediated by free and encapsulated cellulase was proposed. Bioinks of cellulase-laden and cellulose nanofiber filled chitosan viscous suspensions were used to 3D print enzymatic biodegradable and biocompatible cellulose nanofiber (CNF) reinforced chitosan (CHI) hydrogels. The study of the kinetics of CNF enzymatic degradation was studied in situ in fibroblast cell culture. To preserve enzyme stability as well as to guarantee its sustained release, the cellulase was preliminarily encapsulated in chitosan–caseinate nanoparticles, which were further incorporated in the CNF/CHI viscous suspension before the 3D printing of the ink. The incorporation of the enzyme within the CHI/CNF hydrogel contributed to control the decrease of the CNF mechanical reinforcement in the long term while keeping the cell growth-promoting property of chitosan. The hydrolysis kinetics of cellulose in the 3D printed scaffolds showed a slow but sustained degradation of the CNFs with enzyme, with approximately 65% and 55% relative activities still obtained after 14 days of incubation for the encapsulated and free enzyme, respectively. The 3D printed composite hydrogels showed excellent cytocompatibility supporting fibroblast cell attachment, proliferation and growth. Ultimately, the concomitant cell growth and biodegradation of CNFs within the 3D printed CHI/CNF scaffolds highlights the remarkable potential of CHI/CNF composites in the design of tissue models for the development of 3D constructs with tailored in vitro/in vivo degradability for biomedical applications.

Abstract
The advent of 3D-printing/additive manufacturing in biomedical engineering field has introduced great potential for the preparation of 3D structures that can mimic native tissues. This technology has accelerated the progress in numerous areas of regenerative medicine{,} especially led to a big wave of biomimetic functional scaffold developments for tissue engineering demands. In recent years{,} the introduction of smart bio-inks has created growing efforts to facilitate the preparation of complex and homogeneous living-cell-containing 3D constructs. In the past decade{,} a considerable body of literature has been created on identifying an ideal bioinspired-ink with excellent printability{,} cell viability{,} bioactivity{,} and mechanical properties. This state-of-the-art review article briefly outlines 3D-printing/bioprinting techniques applied for chitosan-based bio-inks{,} their resources{,} crosslinking methods{,} characteristics{,} reasons for their superiority over other bio-inks{,} and challenges of commercialization; this is followed by a comprehensive description of the full potential and the key indicators of success in terms of 3D bio-printing of such bio-inks as platforms for tissue regeneration{,} advanced biosensors{,} drug delivery{,} and wastewater treatment. Next{,} the restrictions and challenges of chitosan bio-inks are highlighted. In this work{,} we also discussed about developing a coherent research strategy based on combination of microfluidics-based lab-on-a-chip (organ-on-a-chip) platforms with 3D-bioprinting which enables designing of self-healing scaffolds. And finally{,} the potential of smart inks based on chitosan for 4D bioprinting of more detailed and practical engineered tissues and artificial organs is reviewed.

Abstract
Hydrogels underpin many applications in tissue engineering, cell encapsulation, drug delivery and bioelectronics. Methods improving control over gelation mechanisms and patterning are still needed. Here we explore a less-known gelation approach relying on sequential electrochemical-chemical-chemical (ECC) reactions. An ionic species and/or molecule in solution is oxidised over a conductive surface at a specific electric potential. The oxidation generates an intermediate species that reacts with a macromolecule, forming a hydrogel at the electrode-electrolyte interface. We introduce potentiostatic control over this process, allowing the selection of gelation reactions and control of hydrogel growth rate. In chitosan and alginate systems, we demonstrate precipitation, covalent and ionic gelation mechanisms. The method can be applied in the polymerisation of hybrid systems consisting of more than one polymer. We demonstrate concomitant deposition of the conductive polymer Poly(3,4-ethylenedioxythiophene) (PEDOT) and alginate. Deposition of the hydrogels occurs in small droplets held between a conductive plate (working electrode, WE), a printing nozzle (counter electrode, CE) and a pseudoreference electrode (reference electrode, RE). We install this setup on a commercial 3D printer to demonstrate patterning of adherent hydrogels on gold and flexible ITO foils. Electro-assisted printing may contribute to the integration of well-defined hydrogels on hybrid electronic-hydrogel devices for bioelectronics applications.

Abstract
Thermosensitive chitosan (CH)-based hydrogels prepared with a mix of sodium bicarbonate and β-glycerophosphate as gelling agents rapidly pass from a liquid at room temperature to a mechanically strong solid at body temperature without any crosslinker. They show excellent potential for tissue engineering applications and could be interesting candidates for bioprinting. Unfortunately, since gelation is not instantaneous, formulations compatible with cell encapsulation (chitosan concentrations around 2% or lower) lead to very poor resolution and fidelity due to filament spreading. Here, we investigate the FRESH bioprinting approach with a warm sacrificial support bath, to overcome these limitations and enhance their bioprintability. First, a support bath, made of Pluronic including sodium chloride salt as a rheology modifier agent, was designed to meet the specific physical state requirements (solid at 37 °C and liquid at room temperature) and rheological properties appropriate for bioprinting. This support bath presented yield stress of over 100 Pa, a shear thinning behavior, and fast self-healing during cyclic recovery tests. Three different chitosan hydrogels (CH2%w/v, CH3%w/v, and a mixture of CH and gelatin) were tested for their ability to form filament and 3D structures, with and without a support bath. Both the resolution and mechanical properties of the printed structure were drastically enhanced using the FRESH method, with an approximate four fold decrease of the filament diameter which is close to the needle diameter. The printed structures were easily harvested without altering their shape by cooling down the support bath, and do not swell when immersed in PBS. Live/dead assays confirmed that the viability of encapsulated mesenchymal stem cells was highest in CH2% and that the support bath-assisted bioprinting process did not adversely impact cell viability. This study demonstrates that using a warm FRESH-like approach drastically enhances the potential for bioprinting of the thermosensitive biodegradable chitosan hydrogels and opens up a wide range of applications for 3D models and tissue engineering.

Abstract
Dentoalveolar tissue engineering is an emerging yet challenging field, considering the lack of suitable materials and difficulty to produce patient-specific hydrogel scaffolds. The present paper aims to produce a 3D printable and tuneable biomaterial by copolymerizing a synthesized water-soluble chitosan derivative called maleic anhydride grafted chitosan (MA-C) with gelatin using genipin, a natural crosslinking agent. Development and testing of this material for 3D printing, degradation, and swelling demonstrated the ability to fabricate scaffolds with controlled physical properties based on pre-determined designs. The MA-C-gelatin copolymer demonstrated excellent biocompatibility, which was verified by analyzing the viability, growth and proliferation of human dental pulp stem cells seeded on MA-C-gelatin constructs through live/dead, alamar blue and DNA quantification assays. Based on the present findings, the proposed material might be a suitable candidate for dentoalveolar tissue engineering, while further research is required to achieve this goal.

Abstract
Efficient wound treatments to target specific events in the healing process of chronic wounds constitute a significant aim in regenerative medicine. In this sense, nanomedicine can offer new opportunities to improve the effectiveness of existing wound therapies. The aim of this study was to develop catechol bearing polymeric nanoparticles (NPs) and to evaluate their potential in the field of wound healing. Thus, NPs wound healing promoting activities, potential for drug encapsulation and controlled release, and further incorporation in a hydrogel bioink formulation to fabricate cell-laden 3D scaffolds are studied. NPs with 2 and 29 M % catechol contents (named NP2 and NP29) were obtained by nanoprecipitation and presented hydrodynamic diameters of 100 and 75 nm respectively. These nanocarriers encapsulated the hydrophobic compound coumarin-6 with 70% encapsulation efficiency values. In cell culture studies, the NPs had a protective effect in RAW 264.7 macrophages against oxidative stress damage induced by radical oxygen species (ROS). They also presented a regulatory effect on the inflammatory response of stimulated macrophages and promoted upregulation of the vascular endothelial growth factor (VEGF) in fibroblasts and endothelial cells. In particular, NP29 were used in a hydrogel bioink formulation using carboxymethyl chitosan and hyaluronic acid as polymeric matrices. Using a reactive mixing bioprinting approach, NP-loaded hydrogel scaffolds with good structural integrity, shape fidelity and homogeneous NPs dispersion, were obtained. The in vitro catechol NPs release profile of the printed scaffolds revealed a sustained delivery. The bioprinted scaffolds supported viability and proliferation of encapsulated L929 fibroblasts over 14 days. We envision that the catechol functionalized NPs and resulting bioactive bioink presented in this work offer promising advantages for wound healing applications, as they: 1) support controlled release of bioactive catechol NPs to the wound site; 2) can incorporate additional therapeutic functions by co-encapsulating drugs; 3) can be printed into 3D scaffolds with tailored geometries based on patient requirements.

Abstract
Soft tissues are commonly fiber-reinforced hydrogel composite structures, distinguishable from hard tissues by their low mineral and high water content. In this work, we proposed the development of 3D printed hydrogel constructs of the biopolymers chitosan (CHI) and cellulose nanofibers (CNFs), both without any chemical modification, which processing did not incorporate any chemical crosslinking. The unique mechanical properties of native cellulose nanofibers offer new strategies for the design of environmentally friendly high mechanical performance composites. In the here proposed 3D printed bioinspired CNF-filled CHI hydrogel biomaterials, the chitosan serves as a biocompatible matrix promoting cell growth with balanced hydrophilic properties, while the CNFs provide mechanical reinforcement to the CHI-based hydrogel. By means of extrusion-based printing (EBB), the design and development of 3D functional hydrogel scaffolds was achieved by using low concentrations of chitosan (2.0–3.0% (w/v)) and cellulose nanofibers (0.2–0.4% (w/v)). CHI/CNF printed hydrogels with good mechanical performance (Young’s modulus 3.0 MPa, stress at break 1.5 MPa, and strain at break 75%), anisotropic microstructure and suitable biological response, were achieved. The CHI/CNF composition and processing parameters were optimized in terms of 3D printability, resolution, and quality of the constructs (microstructure and mechanical properties), resulting in good cell viability. This work allows expanding the library of the so far used biopolymer compositions for 3D printing of mechanically performant hydrogel constructs, purely based in the natural polymers chitosan and cellulose, offering new perspectives in the engineering of mechanically demanding hydrogel tissues like intervertebral disc (IVD), cartilage, meniscus, among others.

Abstract
Hierarchical porous materials are widespread in nature and find an increasing number of applications as catalytic supports, biological scaffolds and lightweight structures. Recent advances in additive manufacturing and 3D printing technologies have enabled the digital fabrication of porous materials in the form of lattices, cellular structures and foams across multiple length scales. However, current approaches do not allow for the fast manufacturing of bulk porous materials featuring pore sizes that span broadly from macroscopic dimensions down to the nanoscale. Here, ink formulations are designed and investigated to enable 3D printing of hierarchical materials displaying porosity at the nano-, micro- and macroscales. Pores are generated upon removal of nanodroplets and microscale templates present in the initial ink. Using particles to stabilize the droplet templates is key to obtain Pickering nanoemulsions that can be 3D printed through direct ink writing. The combination of such self-assembled templates with the spatial control offered by the printing process allows for the digital manufacturing of hierarchical materials exhibiting thus far inaccessible multiscale porosity and complex geometries.

Abstract
3D printing via direct ink writing (DIW) is a versatile additive manufacturing approach applicable to a variety of materials ranging from ceramics over composites to hydrogels. Due to the mild processing conditions compared to other additive manufacturing methods{,} DIW enables the incorporation of sensitive compounds such as proteins or drugs into the printed structure. Although emulsified oil-in-water systems are commonly used vehicles for such compounds in biomedical{,} pharmaceutical{,} and cosmetic applications{,} printing of such emulsions into architectured soft materials has not been fully exploited and would open new possibilities for the controlled delivery of sensitive compounds. Here{,} we 3D print concentrated emulsions into soft materials{,} whose multiphase architecture allows for site-specific incorporation of both hydrophobic and hydrophilic compounds into the same structure. As a model ink{,} concentrated emulsions stabilized by chitosan-modified silica nanoparticles are studied{,} because they are sufficiently stable against coalescence during the centrifugation step needed to create a bridging network of droplets. The resulting ink is ideal for 3D printing as it displays high yield stress{,} storage modulus and elastic recovery{,} through the formation of networks of droplets as well as of gelled silica nanoparticles in the presence of chitosan. To demonstrate possible architectures{,} we print biocompatible soft materials with tunable hierarchical porosity containing an encapsulated hydrophobic compound positioned in specific locations of the structure. The proposed emulsion-based ink system offers great flexibility in terms of 3D shaping and local compositional control{,} and can potentially help address current challenges involving the delivery of incompatible compounds in biomedical applications.

Abstract
Bioprinting is a promising automated platform that enables the simultaneous deposition of multiple types of cells and biomaterials to fabricate complex three-dimensional (3D) tissue constructs. Most of the previous bioprinting works focused on collagen-based biomaterial, which has poor printability and long crosslinking time. This posed a immerse challenge to create a 3D construct with pre-determined shape and configuration. There is a need for a functional material with good printability in order to fabricate a 3D skin construct. Recently, the use of chitosan for wound healing applications has attracted huge attention due to its attractive traits such as its antimicrobial properties and ability to trigger hemostasis. In this paper, we report the modification of chitosan-based biomaterials for functional 3D bioprinting. Modification to the chitosan was carried out via the oppositely charged functional groups from chitosan and gelatin at a specific pH of ~pH 6.5 to form polyelectrolyte complexes. The polyelectrolyte hydrogels were evaluated in terms of chemical interactions within polymer blend, rheological properties (viscosities, storage and loss modulus), printing resolution at varying pressures and feed rates and biocompatibility. The chitosan-based hydrogels formulated in this work exhibited good printability at room temperature, high shape fidelity of the printed 3D constructs and good biocompatibility with fibroblast skin cells.