Inkjet fabrication and characterization of calcium alginate microcapsules

Sodium Alginate as a Pharmaceutical Excipient: Novel Applications of a Well-known Polymer

Alginates are naturally occurring polymers revealing low toxicity, good biocompatibility and biodegradability, excellent gelling and thickening properties, as well as low production cost and good availability. One of the most important features typical for alginates is the ability to undergo ionotropic gelation which is gel formation process occurring upon the contact with cations. Because of their advantageous properties, alginates have been extensively utilized in food and pharmaceutical industries. In this review the current knowledge regarding the most recent studies involving both popularly applied dosage forms, like tablets or hydrogels, and novel advanced drug delivery systems applied in targeted therapies are summarized and discussed. The presented studies indicate that although sodium alginate is a well-established polymer, it is still widely applied as pharmaceutical excipient and the presented research studies indicate that there are still research areas that can be explored and provide innovation in drug delivery systems.

Multicomponent polysaccharide alginate-based bioinks

3D-Bioprinting has seen a rapid expansion in the last few years, with an increasing number of reported bioinks. Alginate is a natural biopolymer that forms hydrogels by ionic cross-linking with calcium ions. Due to its biocompatibility and ease of gelation, it is an ideal ingredient for bioinks. This review focuses on recent advances on bioink formulations based on the combination of alginate with other polysaccharides. In particular, the molecular weight of the alginate and its loading level have an impact on the material's performance, as well as the loading of the divalent metal salt and its solubility, which affects the cross-linking of the gel. Alginate is often combined with other polysaccharides that can sigificantly modify the properties of the gel, and can optimise alginate for use in different biological applications. It is also possible to combine alginate with sacrificial polymers, which can temporarily reinforce the 3D printed construct, but then be removed at a later stage. Other additives can be formulated into the gels to enhance performance, including nanomaterials that tune rheological properties, peptides to encourage cell adhesion, or growth factors to direct stem cell differentiation. The ease of formulating multiple components into alginate gels gives them considerable potential for further development. In summary, this review will facilitate the identification of different alginate-polysaccharide bioink formulations and their optimal applications, and help inform the design of second generation bioinks, allowing this relatively simple gel system to achieve more sophisticated control over biological processes.

Recyclable and biocompatible microgel-based supporting system for positive 3D freeform printing of silicone rubber

Additive manufacturing (AM) of biomaterials has evolved from a rapid prototyping tool into a viable approach for the manufacturing of patient-specific implants over the past decade. It can tailor to the unique physiological and anatomical criteria of the patient's organs or bones through precise controlling of the structure during the 3D printing. Silicone elastomers, which is a major group of materials in many biomedical implants, have low viscosities and can be printed with a special AM platform, known as freeform 3D printing systems. The freeform 3D printing systems are composed of a supporting bath and a printing material. Current supporting matrices that are either commercially purchased or synthesized were usually disposed of after retrieval of the printed part. In this work, we proposed a new and improved supporting matrix comprises of synthesized calcium alginate microgels produced via encapsulation which can be recycled, reused, and recovered for multiple prints, hence minimizing wastage and cost of materials. The dehydration tolerance of the calcium alginate microgels was improved through physical means by the addition of glycerol and chemical means by developing new calcium alginate microgels encapsulated with glycerol. The recyclability of the heated calcium alginate microgels was also enhanced by a rehydration step with sodium chloride solution and a recovery step with calcium chloride solution via the ion exchange process. We envisaged that our reusable and recyclable biocompatible calcium alginate microgels can save material costs, time, and can be applied in various freeform 3D printing systems.

Inkjet Bioprinting of Biomaterials

The inkjet technique has the capability of generating droplets in the picoliter volume range, firing thousands of times in a few seconds and printing in the noncontact manner. Since its emergence, inkjet technology has been widely utilized in the publishing industry for printing of text and pictures. As the technology developed, its applications have been expanded from two-dimensional (2D) to three-dimensional (3D) and even used to fabricate components of electronic devices. At the end of the twentieth century, researchers were aware of the potential value of this technology in life sciences and tissue engineering because its picoliter-level printing unit is suitable for depositing biological components. Currently inkjet technology has been becoming a practical tool in modern medicine serving for drug development, scaffold building, and cell depositing. In this article, we first review the history, principles and different methods of developing this technology. Next, we focus on the recent achievements of inkjet printing in the biological field. Inkjet bioprinting of generic biomaterials, biomacromolecules, DNAs, and cells and their major applications are introduced in order of increasing complexity. The current limitations/challenges and corresponding solutions of this technology are also discussed. A new concept, biopixels, is put forward with a combination of the key characteristics of inkjet printing and basic biological units to bring a comprehensive view on inkjet-based bioprinting. Finally, a roadmap of the entire 3D bioprinting is depicted at the end of this review article, clearly demonstrating the past, present, and future of 3D bioprinting and our current progress in this field.

Microencapsulation of Saccharomyces cerevisiae into Alginate Beads: A Focus on Functional Properties of Released Cells

Five yeast strains (four wild Saccharomyces cerevisiae strains and a collection strain-S. cerevisiae var. boulardii) were encapsulated in alginate beads. Encapsulation yield was at least 60% (100% for some strains) and yeasts survived in beads for 30 days at 4 °C, although the viability was strongly affected during storage at 25 °C (3 log reduction after 7 days). The kinetic of cell release was studied under static and dynamic conditions, but the results suggest that, after 48 h, beads contained a high number of yeasts. Thus, their use is advisable as re-usable carriers of starter cultures or as a vehicle of probiotics into the gut. Finally, some functional properties (biofilm formation, hydrophobicity, auto-aggregation, survival during the transit into the gut) were evaluated on yeasts released by beads to assess if microencapsulation could negatively affect these traits. The results showed that yeasts' entrapment in beads did not affect probiotic properties.

Enabling Free-Standing 3D Hydrogel Microstructures with Microreactive Inkjet Printing

Reactive inkjet printing holds great prospect as a multimaterial fabrication process because of its unique advantages involving customization, miniaturization, and precise control of droplets for patterning. For inkjet printing of hydrogel structures, a hydrogel precursor (or cross-linker) is printed onto a cross-linker (or precursor) bath or a substrate. However, the progress of patterning and design of intricate hydrogel structures using the inkjet printing technique is limited by the erratic interplay between gelation and motion control. Accordingly, microreactive inkjet printing (MRIJP) was applied to demonstrate a spontaneous 3D printing of hydrogel microstructures by using alginate as the model system. In addition, a printable window within the capillary number-Weber number for the MRIJP technique demonstrated the importance of velocity to realization of in-air binary droplet collision. Finally, systematic analysis shows that the structure and diffusion coefficient of hydrogels are important factors that affect the shape of printed hydrogels over time. Based on such a fundamental understanding of MRIJP of hydrogels, the fabrication process and the structure of hydrogels can be controlled and adapt for 2D/3D microstructure printing of any low-viscosity (<40 cP) reactive inks, with a representative tissue-mimicking structure of a ∼200 μm diameter hollow tube presented in this work.

Diffusion in and around alginate and chitosan films with embedded sub-millimeter voids

Hydrogel scaffolds from biopolymers have potential use in the controlled release of drugs, and as 3-D structure for the formation of tissue matrix. This article describes the solute release behavior of alginate and chitosan films with embedded voids of sub-millimeter dimensions. Nitrogen gas was bubbled in a fluidic arrangement to generate bubbles, prior to the crosslinking. The crosslinked gel was dried in a vacuum oven, and subsequently, soaked in Vitamin B-12 solution. The dimensions of the voids immediately after the cross-linking of gel, and also after complete drying were obtained using a digital microscope and scanning electron microscope respectively. The porosity of the gel was measured gravimetrically. The release of Vitamin B-12 in PBS buffer on a shaker was studied. The release experiments were repeated at an elevated temperature of 37°C in the presence of lysozyme. The diffusion coefficient within the gel layer and the mass transfer coefficient at the interface with the bulk-liquid were estimated using a mathematical model. For comparison, the experiment was repeated with a film that does not have any embedded void. The enhancement in diffusion coefficient due to the presence of voids is discussed in this article.

Microcapsule mechanics: from stability to function

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

Sphyga: a multiparameter open source tool for fabricating smart and tunable hydrogel microbeads

Hydrogel microbeads are used in many biological applications, particularly for cell, protein or drug encapsulation. Although there are several methods for fabricating microbeads with controlled shapes and dimensions, many are limited to a small range of materials or sizes. We describe a compact open source tool-the spherical hydrogel generator (Sphyga)-for the fabrication of highly reproducible hydrogel based microbeads with predictable shapes and diameters ranging from 100 to 2000 µm. The unique feature of the system is the ability to modulate multiple parameters independently, so as to create a wide range of working conditions for fabricating tailored microbeads. Hence, by combining the different fabrication parameters, hydrogel beads with chosen shapes, sizes and materials can be generated with Sphyga. A multiparameter working-window was obtained by fixing the concentration of the base material, alginate, and varying the viscosity of the solution along with Sphyga's fabrication parameters (needle size, external air pressure, and material outflow). To validate the multiparameter working window, components such as proteins, cells, dyes and nanoparticles were also used to fabricate composite microbeads. The results show that the architecture of hydrogel microbeads can be engineered by considering the viscosity of the initial solution, which depends principally on the pH and composition of alginate solution. Coupled with Sphyga's multiple working parameters, material viscosity can then be used to tune hydrogel domains and thereby generate complex biologically relevant microenvironments for many biomedical applications.

Process engineering of high voltage alginate encapsulation of mesenchymal stem cells

Encapsulation of stem cells in alginate beads is promising as a sophisticated drug delivery system in treatment of a wide range of acute and chronic diseases. However, common use of air flow encapsulation of cells in alginate beads fails to produce beads with narrow size distribution, intact spherical structure and controllable sizes that can be scaled up. Here we show that high voltage encapsulation (≥ 15 kV) can be used to reproducibly generate spherical alginate beads (200-400 μm) with narrow size distribution (± 5-7%) in a controlled manner under optimized process parameters. Flow rate of alginate solution ranged from 0.5 to 10 ml/h allowed producing alginate beads with a size of 320 and 350 μm respectively, suggesting that this approach can be scaled up. Moreover, we found that applied voltages (15-25 kV) did not alter the viability and proliferation of encapsulated mesenchymal stem cells post-encapsulation and cryopreservation as compared to air flow. We are the first who employed a comparative analysis of electro-spraying and air flow encapsulation to study the effect of high voltage on alginate encapsulated cells. This report provides background in application of high voltage to encapsulate living cells for further medical purposes. Long-term comparison and work on alginate-cell interaction within these structures will be forthcoming.

Microfluidic production of single micrometer-sized hydrogel beads utilizing droplet dissolution in a polar solvent

In this study, a microfluidic process is proposed for preparing monodisperse micrometer-sized hydrogel beads. This process utilizes non-equilibrium aqueous droplets formed in a polar organic solvent. The water-in-oil droplets of the hydrogel precursor rapidly shrunk owing to the dissolution of water molecules into the continuous phase. The shrunken and condensed droplets were then gelled, resulting in the formation of hydrogel microbeads with sizes significantly smaller than the initial droplet size. This study employed methyl acetate as the polar organic solvent, which can dissolve water at 8%. Two types of monodisperse hydrogel beads-Ca-alginate and chitosan-with sizes of 6-10 μm (coefficient of variation < 6%) were successfully produced. In addition, we obtained hydrogel beads with non-spherical morphologies by controlling the degree of droplet shrinkage at the time of gelation and by adjusting the concentration of the gelation agent. Furthermore, the encapsulation and concentration of DNA molecules within the hydrogel beads were demonstrated. The process presented in this study has great potential to produce small and highly concentrated hydrogel beads that are difficult to obtain by using conventional microfluidic processes.

Structural changes and imaging signatures of acoustically sensitive microcapsules under ultrasound

The ultrasound drug delivery field is actively designing new agents that would obviate the problems of just using microbubbles for drug delivery. Microbubbles have very short circulation time (minutes), low payload and large size (2-10μm), all of these aspects are not ideal for systemic drug delivery. However, microbubble carriers provide excellent image contrast and their use for image guidance can be exploited. In this paper, we suggest an alternative approach by developing acoustically sensitive microcapsule reservoirs that have future applications for treating large ischemic tumors through intratumoral therapy. We call these agents Acoustically Sensitized Microcapsules (ASMs) and these are not planned for the circulation. ASMs are very simple in their formulation, robust and reproducible. They have been designed to offer high payload (because of their large size), be acoustically sensitive and reactive (because of the Ultrasound Contrast Agents (UCAs) encapsulated) and mechanically robust for future injections/implantations within tumors. We describe three different aspects - (1) effect of therapeutic ultrasound; (2) mechanical properties and (3) imaging signatures of these agents. Under therapeutic ultrasound, the formation of a cavitational bubble was seen prior to rupture. The time to rupture was size dependent. Size dependency was also seen when measuring mechanical properties of these ASMs. % Alginate and permeability also affected the Young's modulus estimates. For study of imaging signatures of these agents, we show six schemes. For example, with harmonic imaging, tissue phantoms and controls did not generate higher harmonic components. Only ASM phantoms created a harmonic signal, whose sensitivity increased with applied acoustic pressure. Future work includes developing schemes combining both sonication and imaging to help detect ASMs before, during and after release of drug substance.

Remotely controlled diffusion from magnetic liposome microgels

The reversible, temperature-dependent change in the permeability of a phospholipid bilayer has been used for controlling the diffusion rate of encapsulated molecular payload from liposomes. Liposomes were preloaded with a fluorescent dye and immobilized in calcium alginate hydrogel microparticles that also contained iron oxide nanoparticles. The composite microparticles were produced by a drop-on-demand inkjet method. The ability of iron oxide nanoparticles to locally dissipate heat upon exposure to a radio-frequency (RF) alternating magnetic field was used to control the local temperature and therefore diffusion from the liposomes in a contactless way using an RF coil. Several different release patterns were realized, including repeated on-demand release. The internal structure of the composite alginate-liposome-magnetite microparticles was investigated, and the influence of microparticle concentration on the heating rate was determined. In order to achieve a temperature rise required for the liposome membrane melting, the concentration of alginate beads should be at least 25% of their maximum packing density for the nanoparticle concentration and specific absorption rate used.

Comparative diffusivity measurements for alginate-based atomized and inkjet-bioprinted artificial cells using fluorescence microscopy

Radial diffusivity profiles of atomized (MC, d = 1800 ± 200 µm) and inkjet-printed (MI, d = 40 ± 5 µm) alginate-based artificial cells have been generated using 2D Fluorescence Microscopy. The passive outward diffusion of FITC-Dextrans from MIs (0.5% LV alginate/15% CaCl2 coated with 0.5% Chitosan) and MCs (1.5% MV alginate/1.5% CaCl2) was measured and quantified using a Fickian model. As an expected outcome of miniaturization, the ratios of the outer layer diffusivities defined as D(MIout)/D(MCout) were 4.25 and 5.07 respectively for the 4 and 70 kDa markers, indicative of the enhanced diffusive potential of the miniaturized capsules.

Active targeting in a random porous medium by chemical swarm robots with secondary chemical signaling

The multibody dynamics of a system of chemical swarm robots in a porous environment is investigated. The chemical swarm robots are modeled as brownian particles capable of delivering an encapsulated chemical payload toward a given target location and releasing it in response to an external stimulus. The presence of chemical signals (chemo-attractant) in the system plays a crucial role in coordinating the collective movement of the particles via chemotaxis. For a number of applications, such as distributed chemical processing and targeted drug delivery, the understanding of factors that govern the collective behavior of the particles, especially their ability to localize a given target, is of immense importance. A hybrid modeling methodology based on the combination of the brownian dynamics method and diffusion problem coupled through the chemotaxis phenomena is used to analyze the impact of a varying signaling threshold and the strength of chemotaxis on the ability of the chemical robots to fulfill their target localization mission. The results demonstrate that the selected performance criteria (the localization half time and the success rate) can be improved when an appropriate signaling process is chosen. Furthermore, for an optimum target localization strategy, the topological complexity of the porous environment needs to be reflected.

High throughput miniaturization of artificial cells

In this study, inkjet bio-printing has been used to produce miniaturized alginate microcapsules. A parametric study using subsequent Taguchi L(18) (3(1) × 2(7)) and L(16) (4(5)) designs was performed to elucidate the effect of inkjet parameters on microcapsule size. A 120-minute pilot run using the optimal waveform parameters and 0.5% alginate ink yielded a throughput of 1.8×10(6) microcapsules/hr, averaging 40 μm in diameter. Real-time stable jetting conditions were confirmed visually by the generation of a single droplet with a straight trajectory and non-fluctuating Ohnesorge numbers. The rate of stirring of the cross-linking CaCl(2) solution determined scaffold vs. single vesicle formation.