Nanoemulsion-induced enzymatic crosslinking of tyramine-functionalized polymer droplets

Enzymatic Dimerization-Induced Self-Assembly of Alanine-Tyramine Conjugates into Versatile, Uniform, Enzyme-Loaded Organic Nanoparticles

Herein, we report a novel enzymatic dimerization-induced self-assembly (e-DISA) procedure that converts alanine-tyramine conjugates into highly uniform enzyme-loaded nanoparticles (NPs) or nanocontainers by the action of horseradish peroxidase (HRP) in an aqueous medium under ambient conditions. The NP formation was possible with both enantiomers of alanine, and the average diameter could be varied from 150 nm to 250 nm (with a 5-12 % standard deviation of as-prepared samples) depending on the precursor concentration. About 60 % of the added HRP enzyme was entrapped within the NPs and was subsequently utilized for post-synthetic modification of the NPs with phenolic compounds such as tyramine or tannic acid. One-pot multi-enzyme entrapment of glucose oxidase (GOx) and peroxidase (HRP) within the NPs was also achieved. These GOx-HRP loaded NPs allowed multimodal detection of glucose, including that present in human saliva, with a limit of detection (LoD) of 740 nM through fluorimetry. The NPs exhibited good cytocompatibility and were stable to changes in pH (acidic to basic), temperature, ultrasonication, and even the presence of organic solvent (EtOH) to a certain extent, since they are stabilized by intermolecular hydrogen bonding, π-π, and CH-π interactions. The proposed e-DISA procedure can be widely expanded through the design of diverse enzyme-responsive precursors.

Primary human hepatocytes-laden scaffolds for the treatment of acute liver failure

Cell-based liver therapies based on retrieving and steadying failed metabolic function(s) for acute and chronic diseases could be a valuable substitute for liver transplants, even though they are limited by the low engraftment capability and reduced functional quality of primary human hepatocytes (PHH). In this paper we propose the use of gelatin-hyaluronic acid (Gel-HA) scaffolds seeded with PHH for the treatment of liver failure. We first optimized the composition using Gel-HA hydrogels, looking for the mechanical properties closer to the human liver and determining HepG2 cells functionality. Gel-HA scaffolds with interconnected porosity (pore size 102 μm) were prepared and used for PHH culture and evaluation of key hepatic functions. PHH cultured in Gel-HA scaffolds exhibited increased albumin and urea secretion and metabolic capacity (CYP and UGT activity levels) compared to standard monolayer cultures. The transplant of the scaffold containing PHH led to an improvement in liver function (transaminase levels, necrosis) and ameliorated damage in a mouse model of acetaminophen (APAP)-induced liver failure. The study provided a mechanistic understanding of APAP-induced liver injury and the impact of transplantation by analyzing cytokine production and oxidative stress induction to find suitable biomarkers of cell therapy effectiveness.

Steering Stem Cell Fate within 3D Living Composite Tissues Using Stimuli-Responsive Cell-Adhesive Micromaterials

Engineered living microtissues such as cellular spheroids and organoids have enormous potential for the study and regeneration of tissues and organs. Microtissues are typically engineered via self-assembly of adherent cells into cellular spheroids, which are characterized by little to no cell-material interactions. Consequently, 3D microtissue models currently lack structural biomechanical and biochemical control over their internal microenvironment resulting in suboptimal functional performance such as limited stem cell differentiation potential. Here, this work report on stimuli-responsive cell-adhesive micromaterials (SCMs) that can self-assemble with cells into 3D living composite microtissues through integrin binding, even under serum-free conditions. It is demonstrated that SCMs homogeneously distribute within engineered microtissues and act as biomechanically and biochemically tunable designer materials that can alter the composite tissue microenvironment on demand. Specifically, cell behavior is controlled based on the size, stiffness, number ratio, and biofunctionalization of SCMs in a temporal manner via orthogonal secondary crosslinking strategies. Photo-based mechanical tuning of SCMs reveals early onset stiffness-controlled lineage commitment of differentiating stem cell spheroids. In contrast to conventional encapsulation of stem cell spheroids within bulk hydrogel, incorporating cell-sized SCMs within stem cell spheroids uniquely provides biomechanical cues throughout the composite microtissues' volume, which is demonstrated to be essential for osteogenic differentiation.

Cytoprotection of Human Progenitor and Stem Cells through Encapsulation in Alginate Templated, Dual Crosslinked Silk and Silk-Gelatin Composite Hydrogel Microbeads

Susceptibility of mammalian cells against harsh processing conditions limit their use in cell transplantation and tissue engineering applications. Besides modulation of the cell microenvironment, encapsulation of mammalian cells within hydrogel microbeads attract attention for cytoprotection through physical isolation of the encapsulated cells. The hydrogel formulations used for cell microencapsulation are largely dominated by ionically crosslinked alginate (Alg), which suffer from low structural stability under physiological culture conditions and poor cell-matrix interactions. Here the fabrication of Alg templated silk and silk/gelatin composite hydrogel microspheres with permanent or on-demand cleavable enzymatic crosslinks using simple and cost-effective centrifugation-based droplet processing are demonstrated. The composite microbeads display structural stability under ion exchange conditions with improved mechanical properties compared to ionically crosslinked Alg microspheres. Human mesenchymal stem and neural progenitor cells are successfully encapsulated in the composite beads and protected against environmental factors, including exposure to polycations, extracellular acidosis, apoptotic cytokines, ultraviolet (UV) irradiation, anoikis, immune recognition, and particularly mechanical stress. The microbeads preserve viability, growth, and differentiation of encapsulated stem and progenitor cells after extrusion in viscous polyethylene oxide solution through a 27-gauge fine needle, suggesting potential applications in injection-based delivery and three-dimensional bioprinting of mammalian cells with higher success rates.

Cell-based biocomposite engineering directed by polymers

Biological cells such as bacterial, fungal, and mammalian cells always exploit sophisticated chemistries and exquisite micro- and nano-structures to execute life activities, providing numerous templates for engineering bioactive and biomorphic materials, devices, and systems. To transform biological cells into functional biocomposites, polymer-directed cell surface engineering and intracellular functionalization have been developed over the past two decades. Polymeric materials can be easily adopted by various cells through polymer grafting or in situ hydrogelation and can successfully bridge cells with other functional materials as interfacial layers, thus achieving the manufacture of advanced biocomposites through bioaugmentation of living cells and transformation of cells into templated materials. This review article summarizes the recent progress in the design and construction of cell-based biocomposites by polymer-directed strategies. Furthermore, the applications of cell-based biocomposites in broad fields such as cell research, biomedicine, and bioenergy are discussed. Last, we provide personal perspectives on challenges and future trends in this interdisciplinary area.

Tethering Cells via Enzymatic Oxidative Crosslinking Enables Mechanotransduction in Non-Cell-Adhesive Materials

Cell-matrix interactions govern cell behavior and tissue function by facilitating transduction of biomechanical cues. Engineered tissues often incorporate these interactions by employing cell-adhesive materials. However, using constitutively active cell-adhesive materials impedes control over cell fate and elicits inflammatory responses upon implantation. Here, an alternative cell-material interaction strategy that provides mechanotransducive properties via discrete inducible on-cell crosslinking (DOCKING) of materials, including those that are inherently non-cell-adhesive, is introduced. Specifically, tyramine-functionalized materials are tethered to tyrosines that are naturally present in extracellular protein domains via enzyme-mediated oxidative crosslinking. Temporal control over the stiffness of on-cell tethered 3D microniches reveals that DOCKING uniquely enables lineage programming of stem cells by targeting adhesome-related mechanotransduction pathways acting independently of cell volume changes and spreading. In short, DOCKING represents a bioinspired and cytocompatible cell-tethering strategy that offers new routes to study and engineer cell-material interactions, thereby advancing applications ranging from drug delivery, to cell-based therapy, and cultured meat.

Droplet microfluidics as a tool for production of bioactive calcium phosphate microparticles with controllable physicochemical properties

Affordable and therapeutically effective biomaterials are required for successful treatment of orthopaedic critical-size bone defects. Calcium phosphate (CaP) ceramics are widely used for bone repair and regeneration, however, further optimization of their properties and biological performance is still required. To improve the existing CaP bone graft substitutes, novel synthesis and production approaches are needed that provide a fine control over the chemical and physical properties and versatility in the delivery format. In this study, a microfluidic strategy for production of CaP microparticles with different sizes derived from highly monodisperse droplets is proposed for the controlled synthesis of bioactive CaP ceramics. Microfluidic droplets, that served as microreactors for CaP precipitation, allowed the production of different CaP phases, as well as strontium-substituted CaP. By varying the concentration of the precursor solution, microparticles with different porosity were obtained. The droplet microfluidic system allowed direct visualization and quantification of the reaction kinetics. Upon production and purification of the microparticles, the biocompatibility and bioactivity were tested in vitro using human mesenchymal stromal cells (hMSCs). Cell attachment was analysed by imaging of the cytoskeleton and focal adhesions Moreover, cell proliferation, metabolic activity, alkaline phosphatase activity and mRNA expression of a set of osteogenic markers were quantified. We demonstrated that droplet microfluidics is a functional technique for the synthesis of a range of bioactive CaP-based ceramics with controlled properties. STATEMENT OF SIGNIFICANCE: Calcium phosphate (CaP) ceramics are widely applied synthetic biomaterials for repair and regeneration of damaged bone; yet, CaP bone graft substitutes require further improvement to fully replace natural bone grafts in challenging clinical situations. To this end, novel synthesis and production approaches are needed that provide a fine control over the chemical and physical properties. Here, we developed a microfluidic platform for production of CaP microparticles with different size, composition and porosity, derived from monodisperse droplets. We demonstrated that CaP microparticles produced using this platform supported growth and differentiation of human mesenchymal stromal cells. This platform is a useful tool for developing a variety of CaPs in a controlled manner to study their physicochemical properties in relation to their bioactivity.

Microgels Based on Carboxymethylpullulan Grafted with Ferulic Acid Obtained by Enzymatic Crosslinking in Emulsion for Drug Delivery Systems

Carboxymethylpullulan (CMP) grafted with ferulic acid (FA) is crosslinked with laccase by the reverse water-in-oil emulsion technique (with sunflower oil) to obtain microgels with size from 40 to 200 µm. It is demonstrated that laccase activity and dispersion time have an impact on microgels' size. Fluorescence spectroscopy of different probes (e.g., pyrene, Nile red, and curcumin) shows the nonpolar characteristics of hydrophobic microdomains formed by the FA moieties and its dimers forming the crosslinking nodes. Encapsulation and release of curcumin or lidocaine used as drug models are studied in different buffers. Curcumin is well encapsulated but retained in microgels, while lidocaine is released at 65-70% in 2 h and 30 min in buffer simulating the gastrointestinal tract and at 75-85% in 1 h in acetate buffer pH 5.6 or phosphate-buffered saline (PBS) pH 6.9.

Crosslinking Strategies for the Microfluidic Production of Microgels

This article provides a systematic review of the crosslinking strategies used to produce microgel particles in microfluidic chips. Various ionic crosslinking methods for the gelation of charged polymers are discussed, including external gelation via crosslinkers dissolved or dispersed in the oil phase; internal gelation methods using crosslinkers added to the dispersed phase in their non-active forms, such as chelating agents, photo-acid generators, sparingly soluble or slowly hydrolyzing compounds, and methods involving competitive ligand exchange; rapid mixing of polymer and crosslinking streams; and merging polymer and crosslinker droplets. Covalent crosslinking methods using enzymatic oxidation of modified biopolymers, photo-polymerization of crosslinkable monomers or polymers, and thiol-ene “click” reactions are also discussed, as well as methods based on the sol−gel transitions of stimuli responsive polymers triggered by pH or temperature change. In addition to homogeneous microgel particles, the production of structurally heterogeneous particles such as composite hydrogel particles entrapping droplet interface bilayers, core−shell particles, organoids, and Janus particles are also discussed. Microfluidics offers the ability to precisely tune the chemical composition, size, shape, surface morphology, and internal structure of microgels by bringing multiple fluid streams in contact in a highly controlled fashion using versatile channel geometries and flow configurations, and allowing for controlled crosslinking.

Lab-On-Fiber Technology: A Roadmap toward Multifunctional Plug and Play Platforms

This review presents an overview of the “lab-on-fiber technology” vision and the main milestones set in the technological roadmap to achieve the ultimate objective of developing flexible, multifunctional plug and play fiber-optic platforms designed for specific applications. The main achievements, obtained with nanofabrication strategies for unconventional substrates, such as optical fibers, are discussed here. The perspectives and challenges that lie ahead are highlighted with a special focus on full spatial control at the nanoscale and high-throughput production scenarios. The rapid progress in the fabrication stage has opened new avenues toward the development of multifunctional plug and play platforms, discussed here with particular emphasis on new functionalities and unparalleled figures of merit, to demonstrate the potential of this powerful technology in many strategic application scenarios. The paper also analyses the benefits obtained from merging lab-on-fiber (LOF) technology objectives with the emerging field of optomechanics, especially at the microscale and the nanoscale. We illustrate the main advances at the fabrication level, describe the main achievements in terms of functionalities and performance, and highlight future directions and related milestones. All achievements reviewed and discussed clearly suggest that LOF technology is much more than a simple vision and could play a central role not only in scenarios related to diagnostics and monitoring but also in the Information and Communication Technology (ICT) field, where optical fibers have already yielded remarkable results.

Enzymatic outside-in cross-linking enables single-step microcapsule production for high-throughput three-dimensional cell microaggregate formation

Cell-laden hydrogel microcapsules enable the high-throughput production of cell aggregates, which are relevant for three-dimensional tissue engineering and drug screening applications. However, current microcapsule production strategies are limited by their throughput, multistep protocols, and limited amount of compatible biomaterials. We here present a single-step process for the controlled microfluidic production of single-core microcapsules using enzymatic outside-in cross-linking of tyramine-conjugated polymers. It was hypothesized that a physically, instead of the conventionally explored biochemically, controlled enzymatic cross-linking process would improve the reproducibility, operational window, and throughput of shell formation. Droplets were flown through a silicone delay line, which allowed for highly controlled diffusion of the enzymatic cross-linking initiator. The microcapsules' cross-linking density and shell thickness is strictly depended on the droplet's retention time in the delay line, which is predictably controlled by flow rate. The here presented hydrogel cross-linking method allows for facile and cytocompatible production of cell-laden microcapsules compatible with the formation and biorthogonal isolation of long-term viable cellular spheroids for tissue engineering and drug screening applications.

Microfluidic technologies to engineer mesenchymal stem cell aggregates-applications and benefits

Three-dimensional cell culture and the forming multicellular aggregates are superior over traditional monolayer approaches due to better mimicking of in vivo conditions and hence functions of a tissue. A considerable amount of attention has been devoted to devising efficient methods for the rapid formation of uniform-sized multicellular aggregates. Microfluidic technology describes a platform of techniques comprising microchannels to manipulate the small number of reagents with unique properties and capabilities suitable for biological studies. The focus of this review is to highlight recent studies of using microfluidics, especially droplet-based types for the formation, culture, and harvesting of mesenchymal stem cell aggregates and their subsequent application in stem cell biology, tissue engineering, and drug screening. Droplet-based microfluidics can be used to form microgels as carriers for delivering cells and to provide biological cues to the target tissue so as to be minimally invasive. Stem cell-laden microgels with a shape-forming property can be used as smart building blocks by injecting them into the injured tissue thereby constituting the cornerstone of tissue regeneration.

Enzymatically crosslinked silk and silk-gelatin hydrogels with tunable gelation kinetics, mechanical properties and bioactivity for cell culture and encapsulation

Silk fibroin (SF) was enzymatically crosslinked with tyramine-substituted silk fibroin (SF-TA) or gelatin (G-TA) to fabricate hybrid hydrogels with tunable gelation kinetics, mechanical properties and bioactivity. Horseradish peroxidase (HRP)/hydrogen peroxide (H2O2) mediated crosslinking of SF in physiological buffers results in slow gelation and limited mechanical properties. Moreover, SF lacks cell attachment sequences, leading to poor cell-material interactions. These shortcomings can limit the uses of enzymatically crosslinked silk hydrogels in injectable tissue fillings, 3D bioprinting or cell microencapsulation, where rapid gelation and high bioactivity are desired. Here SF/SF-TA and SF/G-TA composite hydrogels were characterized for hydrogel properties and the influence of conjugated cyclic arginine-glycine-aspartic acid (RGD) peptide or G-TA content on bioactivity was explored. Both SF-TA and G-TA significantly increased gelation kinetics, improved mechanical properties and delayed enzymatic degradation in a concentration-dependent manner. β-Sheet formation and hydrogel stiffening were accelerated by SF-TA content but delayed by G-TA. Both cyclic RGD and G-TA significantly improved morphology and metabolic activity of human mesenchymal stem cells (hMSCs) cultured on or encapsulated in composite hydrogels. The hydrogel formulations introduced in this study provide improved control of gel formation and properties, along with biocompatible systems that can be utilized in tissue engineering and cell delivery applications.

On-the-fly exchangeable microfluidic nozzles for facile production of various monodisperse micromaterials

Microfluidic manufacturing platforms have advanced the production of monodisperse, shape-controlled, and chemically defined micromaterials. However, conventional microfabrication platforms are typically designed and fabricated as single-purpose and single-use tools, which limits their efficiency, versatility, and overall potential. We here present an on-the-fly exchangeable nozzle concept that operates in a transparent, 3D, and reusable microfluidic device produced without cleanroom technology. The facile exchange and repositioning of the nozzles readily enables the production of monodisperse water-in-oil and oil-in-water emulsions, solid and core-shell microspheres, microfibers, and even Janus type micromaterials with controlled diameters ranging from 10 to 1000 μm using a single microfluidic device.

Single-Cell Microgels: Technology, Challenges, and Applications

Single-cell-laden microgels effectively act as the engineered counterpart of the smallest living building block of life: a cell within its pericellular matrix. Recent breakthroughs have enabled the encapsulation of single cells in sub-100-μm microgels to provide physiologically relevant microniches with minimal mass transport limitations and favorable pharmacokinetic properties. Single-cell-laden microgels offer additional unprecedented advantages, including facile manipulation, culture, and analysis of individual cell within 3D microenvironments. Therefore, single-cell microgel technology is expected to be instrumental in many life science applications, including pharmacological screenings, regenerative medicine, and fundamental biological research. In this review, we discuss the latest trends, technical challenges, and breakthroughs, and present our vision of the future of single-cell microgel technology and its applications.