Rheological behavior of alginate solutions for biomanufacturing

Developing a transcatheter injectable nanoclay- alginate gel for minimally invasive procedures

Shear-thinning materials have held considerable promise as embolic agents due to their capability of transition between solid and liquid state. In this study, a laponite nanoclay (NC)/alginate gel embolic agent was developed, characterized, and studied for transcatheter based minimally invasive procedures. Both NC and alginate are biocompatible and FDA-approved. Due to electrostatic interactions, the NC/alginate gels exhibit shear-thinning properties that are desirable for transcatheter delivery. The unique shear-thinning nature of the NC/alginate gel allows it to function as a fluid-like substance during transcatheter delivery and as a solid-like embolic agent once deployed. To ensure optimal performance and safety in clinical applications, the rheological characteristics were thoroughly investigated to optimize the mechanical properties of the NC/alginate gel, including storage modulus, yield stress/strain, and thixotropy. To improve physicians' experience and enhance the predictability of gel delivery, a combination of experimental and theoretical approaches was used to assess the injection force required for successful delivery of the gel through clinically employed catheters. Overall, NC/alginate gel exhibited excellent stability and tunable injectability by optimizing the composition of each component. These findings highlight the gel's potential as a robust embolic agent for a wide range of minimally invasive procedures.

Effects of starch-fatty acid complexes with different fatty acid chain lengths and degrees of saturation on the rheological and 3D printing properties of corn starch

The effect of corn starch-fatty (CS-FA) complexes from varying carbon chain length and degree of unsaturation on the rheological and 3D printing properties of corn CS-FA complex gels. The CS-FA complexes with longer carbon chain lengths and lower saturation enhanced the ability of gels to bind water, promoting the formation of intermolecular hydrogen bonds. The CS-FA complexes inhibit retrogradation and increase the amount of bound water, thereby reducing the structural integrity and transforming the original skeleton structure into a flake-like structure. These changes in gel structure led to lower flow stress and storage modulus for CS-FA gels containing FAs with shorter carbon chain lengths and lower saturation, resulting in reduced "extrusion swelling" of the material and facilitating its extrusion. The decreased "extrusion swelling" of gel improved print line width and printing performance. The CS-FA complex gel-printed product with a 12-carbon chain FA has the greatest printing accuracy, thanks to its moderate G', flow stress, and viscosity. This study provides important information for the CS-FA complexes for the preparation of starch-based 3D printing materials.

Protective Effect of Alginate Microcapsules with Different Rheological Behavior on Lactiplantibacillus plantarum 299v

Alginate encapsulation is a well-known technique used to protect microorganisms from adverse conditions. However, it is also known that the viscosity of the alginate is dependent on its composition and degree of polymerization and that thermal treatments, such as pasteurization and sterilization, can affect the structure of the polymer and decrease its protection efficiency. The goal of this study was to evaluate the protective effect of encapsulation, using alginates of different viscosities treated at different temperatures, on Lactiplantibacillus plantarum 299v under in vitro gastrointestinal conditions and cold storage at 4 °C and -15 °C, respectively. Steady- and dynamic-shear rheological tests were used to characterize the polymers. Thermal treatments profoundly affected the rheological characteristics of alginates with high and low viscosity. However, the solutions and gels of the low-viscosity alginate were more affected at a temperature of 117 °C. The capsules elaborated with high-viscosity alginate solution and pasteurized at 63 °C for 30 min provided better protection to the cells of L. plantarum 299v under simulated gastrointestinal and cold storage conditions.

Multiscale investigation of viscoelastic properties of aqueous solutions of sodium alginate and evaluation of their biocompatibility

In order to get better knowledge of mechanical properties from microscopic to macroscopic scale of biopolymers, viscoelastic bulk properties of aqueous solutions of sodium alginate were studied at different scales by combining macroscopic shear rheology (Hz), diffusing-wave spectroscopy microrheology (kHz-MHz) and Brillouin spectroscopy (GHz). Structural properties were also directly probed by small-angle X-ray scattering (SAXS). The results demonstrate a change from polyelectrolyte behavior to neutral polymer behavior by increasing polymer concentration with the determination of characteristic sizes (persistence length, correlation length). The viscoelastic properties probed at the phonon wavelength much higher than the ones obtained at low frequency reflect the variation of microscopic viscosity. First experiments obtained by metabolic activity assays with mouse embryonic fibroblasts showed biocompatibility of sodium alginate aqueous solutions in the studied range of concentrations (2.5-10 g L-1) and consequently their potential biomedical applications.

Effects of κ-carrageenan gum on 3D printability and rheological properties of pork pastes

The effects of κ-carrageenan gum (KG) on the 3D printability and rheological properties of pork pastes were investigated in this study. There were five groups with different levels of KG (0, 2, 4, 6, and 8 g/kg) named as KG-0, KG-2, KG-4, KG-6, and KG-8, respectively. The addition of KG increased the yield stress, viscosity, shear stress, recovery percentage, storage modulus, loss modulus, and initial and average flow forces (P < 0.05). The results of low-field nuclear magnetic resonance analysis revealed that addition of KG reduced T₂₁ and T₂₂ (P < 0.05). The best printing parameters were obtained by accuracy and stability results: printing filling percent, 90%; printing speed, 35 mm⋅s^-1; layer height, 2 mm; nozzle diameter, 1.55 mm, and KG addition level, 6 g/kg. KG addition improved the hardness, springiness, chewiness, cohesiveness, adhesiveness, and density, respectively (P < 0.05). The results suggested that KG addition improved the rheological properties and 3D printability of the pork pastes.

Printable Alginate Hydrogels with Embedded Network of Halloysite Nanotubes: Effect of Polymer Cross-Linking on Rheological Properties and Microstructure

Rapidly growing 3D printing of hydrogels requires network materials which combine enhanced mechanical properties and printability. One of the most promising approaches to strengthen the hydrogels consists of the incorporation of inorganic fillers. In this paper, the rheological properties important for 3D printability were studied for nanocomposite hydrogels based on a rigid network of percolating halloysite nanotubes embedded in a soft alginate network cross-linked by calcium ions. Particular attention was paid to the effect of polymer cross-linking on these properties. It was revealed that the system possessed a pronounced shear-thinning behavior accompanied by a viscosity drop of 4-5 orders of magnitude. The polymer cross-links enhanced the shear-thinning properties and accelerated the viscosity recovery at rest so that the system could regain 96% of viscosity in only 18 s. Increasing the cross-linking of the soft network also enhanced the storage modulus of the nanocomposite system by up to 2 kPa. Through SAXS data, it was shown that at cross-linking, the junction zones consisting of fragments of two laterally aligned polymer chains were formed, which should have provided additional strength to the hydrogel. At the same time, the cross-linking of the soft network only slightly affected the yield stress, which seemed to be mainly determined by the rigid percolation network of nanotubes and reached 327 Pa. These properties make the alginate/halloysite hydrogels very promising for 3D printing, in particular, for biomedical purposes taking into account the natural origin, low toxicity, and good biocompatibility of both components.

Impact of drying on the sodium alginate obtained after polyphenols ultrasound-assisted extraction from Ascophyllum nodosum seaweeds

Ultrasound-assisted extraction (UAE) of polyphenols from the brown seaweeds Ascophyllum nodosum leaves a solid phase where alginates can be extracted. This study characterizes alginates extracted after the UAE process, with and without an intermediate drying stage at different temperatures (50 and 90 °C) producing sequentially two bioactive compounds from a unique raw material. FT-IR and 1H NMR analyses showed the high purity of alginates with features in the range of commercial alginates. Drying at high temperature decreased average block length and viscosity average molecular weight (Mv) of alginate from 428 to 133 kg/mol. Steady-shear curves (shear-thinning behaviour) and viscoelasticity (liquid like character) features depended clearly on Mv. Solutions of alginates with high Mv were more viscous and the elastic character was more relevant. Cox-Merz rule was only accomplished within the semi-dilute regimes of alginate concentration. Tested process conditions allow the production of alginates with different properties.

3D Printing of Alginate-Natural Clay Hydrogel-Based Nanocomposites

Biocompatibility, biodegradability, shear tinning behavior, quick gelation and an easy crosslinking process makes alginate one of the most studied polysaccharides in the field of regenerative medicine. The main purpose of this study was to obtain tissue-like materials suitable for use in bone regeneration. In this respect, alginate and several types of clay were investigated as components of 3D-printing, nanocomposite inks. Using the extrusion-based nozzle, the nanocomposites inks were printed to obtain 3D multilayered scaffolds. To observe the behavior induced by each type of clay on alginate-based inks, rheology studies were performed on composite inks. The structure of the nanocomposites samples was examined using Fourier Transform Infrared Spectrometry and X-ray Diffraction (XRD), while the morphology of the 3D-printed scaffolds was evaluated using Electron Microscopy (SEM, TEM) and Micro-Computed Tomography (Micro-CT). The swelling and dissolvability of each composite scaffold in phosfate buffer solution were followed as function of time. Biological studies indicated that the cells grew in the presence of the alginate sample containing unmodified clay, and were able to proliferate and generate calcium deposits in MG-63 cells in the absence of specific signaling molecules. This study provides novel information on potential manufacturing methods for obtaining nanocomposite hydrogels suitable for 3D printing processes, as well as valuable information on the clay type selection for enabling accurate 3D-printed constructs. Moreover, this study constitutes the first comprehensive report related to the screening of several natural clays for the additive manufacturing of 3D constructs designed for bone reconstruction therapy.

Rheological Properties of Aqueous Sodium Alginate Slurries for LTO Battery Electrodes

Rheological properties of electrode slurries have been intensively studied for manifold different combinations of active materials and binders. Standardly, solvent-based systems are under use, but a trend towards water-based electrode manufacturing is becoming more and more important. The different solvent is beneficial in terms of sustainability and process safety but is also accompanied by some disadvantages such as extraction of residual humidity and a higher complexity concerning slurry stability. Li4Ti5O12 (LTO) active material provides good long-term stability and can be processed in aqueous solutions. Combining the LTO active material with sodium alginate (SA) as a promising biobased polymer binder reveals good electrochemical properties but suffers from bad slurry stability. In this work, we present a comprehensive rheological study on material interactions in anode slurries consisting of LTO and SA, based on a complex interaction of differentially sized materials. The use of two different surfactants-namely, an anionic and non-ionic one, to enhance slurry stability, compared with surfactant-free slurry.

Towards the Experimentally-Informed In Silico Nozzle Design Optimization for Extrusion-Based Bioprinting of Shear-Thinning Hydrogels

Research in bioprinting is booming due to its potential in addressing several manufacturing challenges in regenerative medicine. However, there are still many hurdles to overcome to guarantee cell survival and good printability. For the 3D extrusion-based bioprinting, cell viability is amongst one of the lowest of all the bioprinting techniques and is strongly influenced by various factors including the shear stress in the print nozzle. The goal of this study is to quantify, by means of in silico modeling, the mechanical environment experienced by the bioink during the printing process. Two ubiquitous nozzle shapes, conical and blunted, were considered, as well as three common hydrogels with material properties spanning from almost Newtonian to highly shear-thinning materials following the power-law behavior: Alginate-Gelatin, Alginate and PF127. Comprehensive in silico testing of all combinations of nozzle geometry variations and hydrogels was achieved by combining a design of experiments approach (DoE) with a computational fluid dynamics (CFD) of the printing process, analyzed through a machine learning approach named Gaussian Process. Available experimental results were used to validate the CFD model and justify the use of shear stress as a surrogate for cell survival in this study. The lower and middle nozzle radius, lower nozzle length and the material properties, alone and combined, were identified as the major influencing factors affecting shear stress, and therefore cell viability, during printing. These results were successfully compared with those of reported experiments testing viability for different nozzle geometry parameters under constant flow rate or constant pressure. The in silico 3D bioprinting platform developed in this study offers the potential to assist and accelerate further development of 3D bioprinting.

3D bioprinting of an electroactive and self-healing polysaccharide hydrogels

Polysaccharide hydrogels including alginate, agarose, hyaluronic acid and chitosan have been widely used as scaffolds in 3D bio-printing field. Konjac glucomannan (KGM) exhibits excellent properties of water solubility, biocompatibility and biodegradability. Herein composite hydrogels were prepared via schiff-base reaction between the aldehyde group of oxidized konjac glucomannan (OKGM) and the amino group of branched polyethyleneimine (PEI). The OKGM/PEI composite hydrogel displayed self-healing ability and pH sensitivity and showed shear thinning capability, which is suitable for 3D bio-printing technology. Furthermore, the OKGM/PEI electroactive composite hydrogel was obtained by adding carbon nanotubes (CNTs). Then the rheological behavior and morphology of OKGM/PEI electroactive hydrogels were thoroughly characterized. The conductivities of OKGM/PEI electroactive composite hydrogels increased with increasing the content of carbon nanotubes. The rheological behavior and 3D bio-printability of OKGM/PEI electroactive hydrogels were also tested. It was found that carbon nanotubes can also improve the bio-printability of OKGM/PEI electroactive hydrogels. Thus, the OKGM/PEI electroactive hydrogels could be employed as scaffolds for muscle and cardiac nerve tissue regeneration. This article is protected by copyright. All rights reserved.

Impact of the Type of Crosslinking Agents on the Properties of Modified Sodium Alginate/Poly(vinyl Alcohol) Hydrogels

Here, we report on studies on the influence of different crosslinking methods (ionic and chemical) on the physicochemical (swelling ability and degradation in simulated body fluids), structural (FT-IR spectra analysis) and morphological (SEM analysis) properties of SA/PVA hydrogels containing active substances of natural origin. First, an aqueous extract of Echinacea purpurea was prepared using a Soxhlet apparatus. Next, a series of modified SA/PVA-based hydrogels were obtained through the chemical crosslinking method using poly(ethylene glycol) diacrylate (PEGDA, Mn = 700 g/mol) as a crosslinking agent and, additionally, the ionic reaction in the presence of a 5% w/v calcium chloride solution. The compositions of SA/PVA/E. purpurea-based hydrogels contained a polymer of natural origin-sodium alginate (SA, 1.5% solution)-and a synthetic polymer-poly(vinyl alcohol) (PVA, Mn = 72,000 g/mol, 10% solution)-in the ratio 2:1, and different amounts of the aqueous extract of E. purpurea-5, 10, 15 or 20% (v/v). Additionally, the release behavior of echinacoside from the polymeric matrix was evaluated in phosphate-buffered saline (PBS) at 37 °C. The results indicate that the type of the crosslinking method has a direct impact on the release profile. Consequently, it is possible to design a system that delivers an active substance in a way that depends on the application.

Rapid Fabrication of Cell-Laden Microfibers for Construction of Aligned Biomimetic Tissue

Bottom-up engineering of tissue constructs is being rapidly developed and broadly applied in biomanufacturing. As one type of building block, cell-laden microfibers are promising for reconstruction of oriented structures and functions of linear tissues, such as skeletal muscles, myocardia, and spinal cord tissues. Herein, we propose wet-spinning method with agitating collection, wherein alginate-based material is extruded into an agitated CaCl2 bath with a magnetic rotor acting as the microfiber collector. By applying this method, we achieve rapid fabrication and oriented collection of hydrogel microfibers with diameters ranging from 100 to 400 μm. In addition, we encapsulate myoblasts in the hydrogel to form cell-laden microfibers, which show a high viability (more than 94%) during in vitro culture. Moreover, the method allows to fabricate of cell-laden core-sheath microfibers and hollow microfibers. We also fabricate 3D constructs using various methods of microfiber assembly like weaving and braiding. The assembling results suggest that the proposed method is a promising technology for bottom-up engineering of aligned biomimetic tissue constructs.

Extracellular-Matrix-Reinforced Bioinks for 3D Bioprinting Human Tissue

Recent advances in 3D bioprinting allow for generating intricate structures with dimensions relevant for human tissue, but suitable bioinks for producing translationally relevant tissue with complex geometries remain unidentified. Here, a tissue-specific hybrid bioink is described, composed of a natural polymer, alginate, reinforced with extracellular matrix derived from decellularized tissue (rECM). rECM has rheological and gelation properties beneficial for 3D bioprinting while retaining biologically inductive properties supporting tissue maturation ex vivo and in vivo. These bioinks are shear thinning, resist cell sedimentation, improve viability of multiple cell types, and enhance mechanical stability in hydrogels derived from them. 3D printed constructs generated from rECM bioinks suppress the foreign body response, are pro-angiogenic and support recipient-derived de novo blood vessel formation across the entire graft thickness in a murine model of transplant immunosuppression. Their proof-of-principle for generating human tissue is demonstrated by 3D bioprinting human airways composed of regionally specified primary human airway epithelial progenitor and smooth muscle cells. Airway lumens remained patent with viable cells for one month in vitro with evidence of differentiation into mature epithelial cell types found in native human airways. rECM bioinks are a promising new approach for generating functional human tissue using 3D bioprinting.

Continuous Based Direct Ink Write for Tubular Cardiovascular Medical Devices

Bioresorbable cardiovascular applications are increasing in demand as fixed medical devices cause episodes of late restenosis. The autologous treatment is, so far, the gold standard for vascular grafts due to the similarities to the replaced tissue. Thus, the possibility of customizing each application to its end user is ideal for treating pathologies within a dynamic system that receives constant stimuli, such as the cardiovascular system. Direct Ink Writing (DIW) is increasingly utilized for biomedical purposes because it can create composite bioinks by combining polymers and materials from other domains to create DIW-printable materials that provide characteristics of interest, such as anticoagulation, mechanical resistance, or radiopacity. In addition, bioinks can be tailored to encounter the optimal rheological properties for the DIW purpose. This review delves into a novel emerging field of cardiovascular medical applications, where this technology is applied in the tubular 3D printing approach. Cardiovascular stents and vascular grafts manufactured with this new technology are reviewed. The advantages and limitations of blending inks with cells, composite materials, or drugs are highlighted. Furthermore, the printing parameters and the different possibilities of designing these medical applications have been explored.

Custom-made rheological setup for in situ real-time fast alginate-Ca2+ gelation

There is a growing interest in the in situ gelation of the alginate-Ca²⁺ system due to its remarkable applications. In this work, we record and evaluate the fast gelation kinetics of alginate-Ca²⁺ using a custom-made rheometric setup. This enables us to inject CaCl₂ into the alginate while we perform the rheological measurements. We successfully measure the in situ gelation reaction from the early stages. As the alginate concentration is increased up to 3 wt.%, we observe a systematic increase of the elastic modulus, G'. Similarly, higher concentrations and injected volumes of CaCl₂ increase the magnitude and initial growth rate of G'. At longer times, the growth rate of G' is lower. It decreases further very slowly, indicating that the chemical reaction requires quite a considerable amount of time to be completed. Finally, from the rheometric data, we estimate the average rates of the elastic modulus during the initial and quasi-steady-state stages.

Biomaterials for Bioprinting Microvasculature

Microvasculature functions at the tissue and cell level, regulating local mass exchange of oxygen and nutrient-rich blood. While there has been considerable success in the biofabrication of large- and small-vessel replacements, functional microvasculature has been particularly challenging to engineer due to its size and complexity. Recently, three-dimensional bioprinting has expanded the possibilities of fabricating sophisticated microvascular systems by enabling precise spatiotemporal placement of cells and biomaterials based on computer-aided design. However, there are still significant challenges facing the development of printable biomaterials that promote robust formation and controlled 3D organization of microvascular networks. This review provides a thorough examination and critical evaluation of contemporary biomaterials and their specific roles in bioprinting microvasculature. We first provide an overview of bioprinting methods and techniques that enable the fabrication of microvessels. We then offer an in-depth critical analysis on the use of hydrogel bioinks for printing microvascularized constructs within the framework of current bioprinting modalities. We end with a review of recent applications of bioprinted microvasculature for disease modeling, drug testing, and tissue engineering, and conclude with an outlook on the challenges facing the evolution of biomaterials design for bioprinting microvasculature with physiological complexity.

Emulating Human Tissues and Organs: A Bioprinting Perspective Toward Personalized Medicine

The lack of in vitro tissue and organ models capable of mimicking human physiology severely hinders the development and clinical translation of therapies and drugs with higher in vivo efficacy. Bioprinting allow us to fill this gap and generate 3D tissue analogues with complex functional and structural organization through the precise spatial positioning of multiple materials and cells. In this review, we report the latest developments in terms of bioprinting technologies for the manufacturing of cellular constructs with particular emphasis on material extrusion, jetting, and vat photopolymerization. We then describe the different base polymers employed in the formulation of bioinks for bioprinting and examine the strategies used to tailor their properties according to both processability and tissue maturation requirements. By relating function to organization in human development, we examine the potential of pluripotent stem cells in the context of bioprinting toward a new generation of tissue models for personalized medicine. We also highlight the most relevant attempts to engineer artificial models for the study of human organogenesis, disease, and drug screening. Finally, we discuss the most pressing challenges, opportunities, and future prospects in the field of bioprinting for tissue engineering (TE) and regenerative medicine (RM).

Physical and Chemical Factors Influencing the Printability of Hydrogel-based Extrusion Bioinks

Bioprinting researchers agree that "printability" is a key characteristic for bioink development, but neither the meaning of the term nor the best way to experimentally measure it has been established. Furthermore, little is known with respect to the underlying mechanisms which determine a bioink's printability. A thorough understanding of these mechanisms is key to the intentional design of new bioinks. For the purposes of this review, the domain of printability is defined as the bioink requirements which are unique to bioprinting and occur during the printing process. Within this domain, the different aspects of printability and the factors which influence them are reviewed. The extrudability, filament classification, shape fidelity, and printing accuracy of bioinks are examined in detail with respect to their rheological properties, chemical structure, and printing parameters. These relationships are discussed and areas where further research is needed, are identified. This review serves to aid the bioink development process, which will continue to play a major role in the successes and failures of bioprinting, tissue engineering, and regenerative medicine going forward.

Silk fibroin reactive inks for 3D printing crypt-like structures

A reactive silk fibroin ink formulation designed for extrusion three-dimensional (3D) printing of protein-based hydrogels at room temperature is reported. This work is motivated by the need to produce protein hydrogels that can be printed into complex shapes with long-term stability using extrusion 3D printing at ambient temperature without the need for the addition of nanocomposites, synthetic polymers, or sacrifical templates. Silk fibroin from the Bombyx mori silkworm was purified and synthesized into reactive inks by enzyme-catalyzed dityrosine bond formation. Rheological and printing studies showed that tailoring the peroxide concentration in the reactive ink enables the silk to be extruded as a filament and printed into hydrogel constructs, supporting successive printed layers without flow of the construct or loss of desired geometry. To enable success of longer-term in vitro studies, 3D printed silk hydrogels were found to display excellent shape retention over time, as evidenced by no change in construct dimensions or topography when maintained for nine weeks in culture medium. Caco-2 (an intestinal epithelial cell line) attachment, proliferation, and tight junction formation on the printed constructs was not found to be affected by the geometry of the constructs tested. Intestinal myofibroblasts encapsulated within reactive silk inks were found to survive shearing during printing and proliferate within the hydrogel constructs. The work here thus provides a suitable route for extrusion 3D printing of protein hydrogel constructs that maintain their shape during printing and culture, and is expected to enable longer-term cellular studies of hydrogel constructs that require complex geometries and/or varying spatial distributions of cells on demand via digital printing.

Programming Diffusion and Localization of DNA Signals in 3D-Printed DNA-Functionalized Hydrogels

Additive manufacturing enables the generation of 3D structures with predefined shapes from a wide range of printable materials. However, most of the materials employed so far are static and do not provide any intrinsic programmability or pattern-forming capability. Here, a low-cost 3D bioprinting approach is developed, which is based on a commercially available extrusion printer that utilizes a DNA-functionalized bioink, which allows to combine concepts developed in dynamic DNA nanotechnology with additive patterning techniques. Hybridization between diffusing DNA signal strands and immobilized anchor strands can be used to tune diffusion properties of the signals, or to localize DNA strands within the gel in a sequence-programmable manner. Furthermore, strand displacement mechanisms can be used to direct simple pattern formation processes and to control the availability of DNA sequences at specific locations. To support printing of DNA-functionalized gel voxels at arbitrary positions, an open source python script that generates machine-readable code (GCODE) from simple vector graphics input is developed.

Effects of Autoclaving, EtOH, and UV Sterilization on the Chemical, Mechanical, Printability, and Biocompatibility Characteristics of Alginate

Sterilization is a necessary step during the processing of biomaterials, but it can affect the materials' functional characteristics. This study characterizes the effects of three commonly used sterilization processes-autoclaving (heat-based), ethanol (EtOH; chemical-based), and ultraviolet (UV; radiation-based)-on the chemical, mechanical, printability, and biocompatibility properties of alginate, a widely used biopolymer for drug delivery, tissue engineering, and other biomedical applications. Sterility assessment tests showed that autoclaving was effective against Gram-positive and Gram-negative bacteria at loads up to 10⁸ CFU/mL, while EtOH was the least effective. Nuclear magnetic-resonance spectroscopy showed that the sterilization processes did not affect the monomeric content in the alginate solutions. The differences in compressive stiffness of the three sterilized hydrogels were also not significant. However, autoclaving significantly reduced the molecular weight and polydispersity index, as determined via gel permeation chromatography, as well as the dynamic viscosity of alginate. Printability analyses showed that the sterilization process as well as the extrusion pressure and speed affected the number of discontinuities and spreading ratio in printed and cross-linked strands. Finally, human adipose-derived stem cells demonstrated over 90% viability in all sterilized hydrogels over 7 days, but the differences in cellular metabolic activity in the three groups were significant. Taken together, the autoclaving process, while demonstrating broad spectrum sterility effectiveness, also resulted in most notable changes in alginate's key properties. In addition to the specific results with the three sterilization processes and alginate, this study serves as a roadmap to characterize the interrelationships between sterilization processes, fundamental chemical properties, and resulting functional characteristics and processability of hydrogels.

3D bioprinting using hollow multifunctional fiber impedimetric sensors

3D bioprinting is an emerging biofabrication process for the production of adherent cell-based products, including engineered tissues and foods. While process innovations are rapidly occurring in the area of process monitoring, which can improve fundamental understanding of process-structure-property relations as well as product quality by closed-loop control techniques, in-line sensing of the bioink composition remains a challenge. Here, we report that hollow multifunctional fibers enable in-line impedimetric sensing of bioink composition and exhibit selectivity for real-time classification of cell type, viability, and state of differentiation during bioprinting. Continuous monitoring of the fiber impedance magnitude and phase angle response from 102 to 106 Hz during microextrusion 3D bioprinting enabled compositional and quality analysis of alginate bioinks that contained fibroblasts, neurons, or mouse embryonic stem cells (mESCs). Fiber impedimetric responses associated with the bioinks that contained differentiated mESCs were consistent with differentiation marker expression characterized by immunocytochemistry. 3D bioprinting through hollow multifunctional fiber impedimetric sensors enabled classification of stem cells as stable or randomly differentiated populations. This work reports an advance in monitoring 3D bioprinting processes in terms of in-line sensor-based bioink compositional analysis using fiber technology and provides a non-invasive sensing platform for achieving future quality-controlled bioprinted tissues and injectable stem-cell therapies.

A Novel Simple Approach to Material Parameters from Commonly Accessible Rheometer Data

When characterizing the viscoelastic properties of polymers, shear rheological measurements are commonly the method of choice. These properties are known to affect extrusion and nozzle-based processes such as fiber melt spinning, cast film extrusion and 3D-printing. However, an adequate characterization of shear thinning polymers can be challenging and still insufficient to not only describe but predict process relevant influences. Furthermore, the evaluation of rheological model systems in literature is mostly based on stress–relaxation experiments, which are rarely available for various polymeric materials. Therefore, a simple approach is presented, that can be used to evaluate and benchmark a wide range of rheological model systems based on commonly accessible frequency sweep data. The approach is validated by analyzing alginate PH176 solutions of various concentrations, a thermoplastic poly-urethane (TPU) Elastollan 1180A melt, the liquid silicon rubber Elastosil 7670 and a polycaprolactone (PCL) fiber-alginate composite system. The used rheological model systems, consisting of simple springs and dashpots, are suitable for the description of complex, viscoelastic material properties that can be observed for polymer solutions and gel-like systems. After revealing a suitable model system for describing those material properties, the determination and evaluation of relevant model parameters can take place. We present a detailed guideline for the systematic parameter revelation using alginate solutions of different concentrations as example. Furthermore, a starting point for future correlations of strut spreading in 3D-bioprinting and model parameters is revealed. This work establishes the basis for a better understanding and potential predictability of key parameters for various fabrication techniques.

Bioprinting 101: Design, Fabrication, and Evaluation of Cell-Laden 3D Bioprinted Scaffolds

3D bioprinting is an additive manufacturing technique that recapitulates the native architecture of tissues. This is accomplished through the precise deposition of cell-containing bioinks. The spatiotemporal control over bioink deposition permits for improved communication between cells and the extracellular matrix, facilitates fabrication of anatomically and physiologically relevant structures. The physiochemical properties of bioinks, before and after crosslinking, are crucial for bioprinting complex tissue structures. Specifically, the rheological properties of bioinks determines printability, structural fidelity, and cell viability during the printing process, whereas postcrosslinking of bioinks are critical for their mechanical integrity, physiological stability, cell survival, and cell functions. In this review, we critically evaluate bioink design criteria, specifically for extrusion-based 3D bioprinting techniques, to fabricate complex constructs. The effects of various processing parameters on the biophysical and biochemical characteristics of bioinks are discussed. Furthermore, emerging trends and future directions in the area of bioinks and bioprinting are also highlighted. Graphical abstract [Figure: see text] Impact statement Extrusion-based 3D bioprinting is an emerging additive manufacturing approach for fabricating cell-laden tissue engineered constructs. This review critically evaluates bioink design criteria to fabricate complex tissue constructs. Specifically, pre- and post-printing evaluation approaches are described, as well as new research directions in the field of bioink development and functional bioprinting are highlighted.

Incorporation of probiotics (Bifidobacterium animalis subsp. Lactis) into 3D printed mashed potatoes: Effects of variables on the viability

3D printing is an emerging technology with the potential to revolutionize people's eating habits. This study firstly optimized the mashed potatoes (MP) formulation and correlated its 3D performance with rheological properties. Yield stress and consistency index (K) were closely related with MP's extrusion behavior, and too high values of them (like 2558 Pa and 2794 Pa·sⁿ) caused the difficulty in extrusion process. Yield stress and elastic modulus (G') were critical to MP's self-supporting performance and too low values of them resulted in the deformation of printed parts during storage. The feasibility of incorporation of probiotics (Bifidobacterium animalis subsp. Lactis BB-12) into 3D printed mashed potatoes (MP) was then studied. MP with probiotics was printed with different nozzle diameter (0.6, 1.0 and 1.4 mm), printing temperature (25, 35, 45 and 55 °C) and evaluated for survival during extrusion and storage at 5 °C. It was found only the small nozzle diameter (0.6 mm) resulted in the reduction of probiotic viability from 9.93 log CFU/g to 9.74 log CFU/g. Greater reduction of viable counts of probiotics (from 10.07 log CFU/g to 7.99 log CFU/g) was found when the MP was held in a heating nozzle barrel at 55 °C for 45 min. No significant difference of probiotics viability in 3D printed samples was found during 12-day storage period at 5 °C. This study provides a new dimension on the development of functional foods by 3D printing.

A new pre-gelatinized starch preparing by gelatinization and spray drying of rice starch with hydrocolloids

Rice starch with hydrocolloids (pectin, xanthan gum, sodium alginate or ι-carrageenan) was gelatinized and subsequently spray dried to prepare pre-gelatinized rice starch (PRS) with hydrocolloids (PRS-H). The PRS-H displayed concave granular shape with amorphous structure, indicating rice starch in PRS-H was completely gelatinized. Cold paste viscosity of PRS-H was enhanced in comparison with that of PRS. Especially, xanthan and ι-carrageenan increased cold paste viscosity of PRS-H more than pectin and alginate did. Cold paste viscosity of physically mixed PRS and hydrocolloids (PRS+H), and flow behavior of hydrocolloids themselves as well as gelatinized starch-hydrocolloids without spray drying (GRS-H) indicated interactions existed between starch and hydrocolloids during the preparation. Swelling power, water solubility index, and dynamic viscoelastic properties of PRS-H were also adjusted by different hydrocolloids. These results showed that premixing hydrocolloids with starch before gelatinization in method of spray drying would be a suitable methodology for manufacture PRS with altered properties.

Endoscopically Injectable Shear-Thinning Hydrogels Facilitating Polyp Removal

Submucosal elevation, the process of instilling material in the submucosal space for separation of the surface mucosa and deeper muscularis layer, is a significant aspect of the endoscopic mucosal resection of large lesions performed to facilitate lesion removal and maximize safety. Submucosal injection, when applied, has historically been performed with normal saline, though this is limited by its rapid dissipation; solutions ideally need to be easily injectable, biocompatible, and provide a long-lasting submucosal cushion with a desirable height. Here, reported is a new set of materials, endoscopically injectable shear-thinning hydrogels, meeting these requirements because of their biocompatible components and ability to form a solid hydrogel upon injection. These findings are supported by evaluation in a large animal model and ultimately demonstrate the potential of these shear-thinning hydrogels to serve as efficient submucosal injection fluids for cushion development. Given these unique characteristics, their broad application in mucosal resection techniques is anticipated.

Experimental characterization and computational modeling of hydrogel cross-linking for bioprinting applications

Alginate-based hydrogels are extensively used to create bioinks for bioprinting, due to their biocompatibility, low toxicity, low costs, and slight gelling. Modeling of bioprinting process can boost experimental design reducing trial-and-error tests. To this aim, the cross-linking kinetics for the chemical gelation of sodium alginate hydrogels via calcium chloride diffusion is analyzed. Experimental measurements on the absorbed volume of calcium chloride in the hydrogel are obtained at different times. Moreover, a reaction-diffusion model is developed, accounting for the dependence of diffusive properties on the gelation degree. The coupled chemical system is solved using finite element discretizations which include the inhomogeneous evolution of hydrogel state in time and space. Experimental results are fitted within the proposed modeling framework, which is thereby calibrated and validated. Moreover, the importance of accounting for cross-linking-dependent diffusive properties is highlighted, showing that, if a constant diffusivity property is employed, the model does not properly capture the experimental evidence. Since the analyzed mechanisms highly affect the evolution of the front of the solidified gel in the final bioprinted structure, the present study is a step towards the development of reliable computational tools for the in silico optimization of protocols and post-printing treatments for bioprinting applications.

3D Printed Polymeric Hydrogels for Nerve Regeneration

The human nervous system lacks an inherent ability to regenerate its components upon damage or diseased conditions. During the last decade, this has motivated the development of a number of strategies for nerve regeneration. However, most of those approaches have not been used in clinical applications till today. For instance, although biomaterial-based scaffolds have been extensively used for nerve reparation, the lack of more customized structures have hampered their use in vivo. This highlight focuses mainly on how 3D bioprinting technology, using polymeric hydrogels as bio-inks, can be used for the development of new nerve guidance channels or devices for peripheral nerve cell regeneration. In this concise contribution, some of the most recent and representative examples are highlighted to discuss the challenges involved in various aspects of 3D bioprinting for nerve cell regeneration, specifically when using polymeric hydrogels.

Thiol-Ene Alginate Hydrogels as Versatile Bioinks for Bioprinting

Bioprinting is a powerful technique that allows precise and controlled 3D deposition of biomaterials in a predesigned, customizable, and reproducible manner. Cell-laden hydrogel ("bioink") bioprinting is especially advantageous for tissue engineering applications as multiple cells and biomaterial compositions can be selectively dispensed to create spatially well-defined architectures. Despite this promise, few hydrogel systems are easily available and suitable as bioinks, with even fewer systems allowing for molecular design of mechanical and biological properties. In this study, we report the development of a norbornene functionalized alginate system as a cell-laden bioink for extrusion-based bioprinting, with a rapid UV-induced thiol-ene cross-linking mechanism that avoids acrylate kinetic chain formation. The mechanical and swelling properties of the hydrogels are tunable by varying the concentration, length, and structure of dithiol PEG cross-linkers and can be further modified by postprinting secondary cross-linking with divalent ions such as calcium. The low concentrations of alginate needed (<2 wt %), coupled with their rapid in situ gelation, allow both the maintenance of high cell viability and the ability to fabricate large multilayer or multibioink constructs with identical bioprinting conditions. The modularity of this bioink platform design enables not only the rational design of materials properties but also the gel's biofunctionality (as shown via RGD attachment) for the expected tissue-engineering application. This modularity enables the creation of multizonal and multicellular constructs utilizing a chemically similar bioink platform. Such tailorable bioink platforms will enable increased complexity in 3D bioprinted constructs.

Essential steps in bioprinting: From pre- to post-bioprinting

An increasing demand for directed assembly of biomaterials has inspired the development of bioprinting, which facilitates the assembling of both cellular and acellular inks into well-arranged three-dimensional (3D) structures for tissue fabrication. Although great advances have been achieved in the recent decade, there still exist issues to be addressed. Herein, a review has been systematically performed to discuss the considerations in the entire procedure of bioprinting. Though bioprinting is advancing at a rapid pace, it is seen that the whole process of obtaining tissue constructs from this technique involves multiple-stages, cutting across various technology domains. These stages can be divided into three broad categories: pre-bioprinting, bioprinting and post-bioprinting. Each stage can influence others and has a bearing on the performance of fabricated constructs. For example, in pre-bioprinting, tissue biopsy and cell expansion techniques are essential to ensure a large number of cells are available for mass organ production. Similarly, medical imaging is needed to provide high resolution designs, which can be faithfully bioprinted. In the bioprinting stage, compatibility of biomaterials is needed to be matched with solidification kinetics to ensure constructs with high cell viability and fidelity are obtained. On the other hand, there is a need to develop bioprinters, which have high degrees of freedom of movement, perform without failure concerns for several hours and are compact, and affordable. Finally, maturation of bioprinted cells are governed by conditions provided during the post-bioprinting process. This review, for the first time, puts all the bioprinting stages in perspective of the whole process of bioprinting, and analyzes their current state-of-the art. It is concluded that bioprinting community will recognize the relative importance and optimize the parameter of each stage to obtain the desired outcomes.

Shear-Thinning and Thermo-Reversible Nanoengineered Inks for 3D Bioprinting

Three-dimensional (3D) printing is an emerging approach for rapid fabrication of complex tissue structures using cell-loaded bioinks. However, 3D bioprinting has hit a bottleneck in progress because of the lack of suitable bioinks that are printable, have high shape fidelity, and are mechanically resilient. In this study, we introduce a new family of nanoengineered bioinks consisting of kappa-carrageenan (κCA) and two-dimensional (2D) nanosilicates (nSi). κCA is a biocompatible, linear, sulfated polysaccharide derived from red algae and can undergo thermo-reversible and ionic gelation. The shear-thinning characteristics of κCA were tailored by nanosilicates to develop a printable bioink. By tuning κCA-nanosilicate ratios, the thermo-reversible gelation of the bioink can be controlled to obtain high printability and shape retention characteristics. The unique aspect of the nanoengineered κCA-nSi bioink is its ability to print physiologically-relevant-scale tissue constructs without requiring secondary supports. We envision that nanoengineered κCA-nanosilicate bioinks can be used to 3D print complex, large-scale, cell-laden tissue constructs with high structural fidelity and tunable mechanical stiffness for regenerative medicine.

Laponite as a rheology modifier of alginate solutions: Physical gelation and aging evolution

The rheological behavior of alginate and Laponite/alginate solutions was studied. It was observed that the Cross viscosity model successfully describes the steady-state shear behavior of this polysaccharide. The scaling behavior analyzed for the entangled regime is in good agreement with polyelectrolyte solutions (G_e∼c_p^3/2), with interactions generated between the alginate and the charged surfaces of the Laponite platelets. Therefore, the effect of Laponite as a rheology modifier is influenced by the alginate concentration. Higher alginate concentrations hindered the formation of the house of cards microstructure. Frequency sweep tests were performed to analyze the transition from solid-like to liquid-like behavior in a solid-like dominated domain. Soft physical gels were obtained at low alginate concentrations. The gel point was determined (1.65wt.% of alginate and 2wt.% of Laponite) through the Kramers-Krönig damping factor, and time sweep tests revealed the evolution of the storage (G') and loss modulus (G″) as functions of the waiting time (t_w). The growing elasticity revealed that Laponite/alginate solutions undergo aging.

New strategy for enhancing in situ cell viability of cell-printing process via piezoelectric transducer-assisted three-dimensional printing

Tissue engineering has become one of the great applications of three-dimensional cell printing because of the possibility of fabricating complex cell-laden scaffolds. Three typical methods (inkjet, micro-extrusion, and laser-assisted bio-printing) have been used to fabricate structures. Of these, micro-extrusion is a comparatively easy method, but has some drawbacks such as low in situ cell viability after fabricating cell-laden structures because of the high wall shear stress in micro-sized nozzles. To overcome this shortcoming, we suggest an innovative cell printing method, which is assisted by a piezoelectric transducer (PZT). The PZT assistance in the dispensing process enhances the printing efficiency and cell viability by decreasing the wall shear stress within a nozzle because the PZT effect can lower the shear viscosity of the bioink via micro-scale vibration. In this study, 5 wt% cell-laden alginate was used as a bioink, and various PZT conditions (frequencies up to ∼400 Hz and amplitudes up to ∼40.5 μm) were simultaneously applied to the cell-printing process to examine the effectiveness of the PZT. The PZT-assisted cell-printing method was found to be highly effective in direct cell printing and could achieve cell-laden structures with high in situ cell viability.

Controllable generation and encapsulation of alginate fibers using droplet-based microfluidics

Herein we demonstrate the segmentation of alginate solution streams to generate alginate fibers of precisely controllable lengths between 200 and 1000 μm. Moreover, we demonstrate the subsequent encapsulation of the formed fibers within pL-volume microdroplets, produced within the same microfluidic device, in a direct manner. Finally, we show immediate and complete on-chip gelation of alginate fibers in a rapid and reproducible fashion.

25th anniversary article: Engineering hydrogels for biofabrication

With advances in tissue engineering, the possibility of regenerating injured tissue or failing organs has become a realistic prospect for the first time in medical history. Tissue engineering - the combination of bioactive materials with cells to generate engineered constructs that functionally replace lost and/or damaged tissue - is a major strategy to achieve this goal. One facet of tissue engineering is biofabrication, where three-dimensional tissue-like structures composed of biomaterials and cells in a single manufacturing procedure are generated. Cell-laden hydrogels are commonly used in biofabrication and are termed "bioinks". Hydrogels are particularly attractive for biofabrication as they recapitulate several features of the natural extracellular matrix and allow cell encapsulation in a highly hydrated mechanically supportive three-dimensional environment. Additionally, they allow for efficient and homogeneous cell seeding, can provide biologically-relevant chemical and physical signals, and can be formed in various shapes and biomechanical characteristics. However, despite the progress made in modifying hydrogels for enhanced bioactivation, cell survival and tissue formation, little attention has so far been paid to optimize hydrogels for the physico-chemical demands of the biofabrication process. The resulting lack of hydrogel bioinks have been identified as one major hurdle for a more rapid progress of the field. In this review we summarize and focus on the deposition process, the parameters and demands of hydrogels in biofabrication, with special attention to robotic dispensing as an approach that generates constructs of clinically relevant dimensions. We aim to highlight this current lack of effectual hydrogels within biofabrication and initiate new ideas and developments in the design and tailoring of hydrogels. The successful development of a "printable" hydrogel that supports cell adhesion, migration, and differentiation will significantly advance this exciting and promising approach for tissue engineering.