Microengineered 3D Collagen Gels with Independently Tunable Fiber Anisotropy and Directionality

Cancer-on-chip models for metastasis: importance of the tumor microenvironment

Cancer-on-chip (CoC) models, based on microfluidic chips harboring chambers for 3D tumor-cell culture, enable us to create a controlled tumor microenvironment (TME). CoC models are therefore increasingly used to systematically study effects of the TME on the various steps in cancer metastasis. Moreover, CoC models have great potential for developing novel cancer therapies and for predicting patient-specific response to cancer treatments. We review recent developments in CoC models, focusing on three main TME components: (i) the anisotropic extracellular matrix (ECM) architectures, (ii) the vasculature, and (iii) the immune system. We aim to provide guidance to biologists to choose the best CoC approach for addressing questions about the role of the TME in metastasis, and to inspire engineers to develop novel CoC technologies.

Microengineering 3D Collagen Matrices with Tumor-Mimetic Gradients in Fiber Alignment

Collagen fibers in the 3D tumor microenvironment (TME) exhibit complex alignment landscapes that are critical in directing cell migration through a process called contact guidance. Previous in vitro work studying this phenomenon has focused on quantifying cell responses in uniformly aligned environments. However, the TME also features short-range gradients in fiber alignment that result from cell-induced traction forces. Although the influence of graded biophysical taxis cues is well established, cell responses to physiological alignment gradients remain largely unexplored. In this work, fiber alignment gradients in biopsy samples are characterized and recreated using a new microfluidic biofabrication technique to achieve tunable sub-millimeter to millimeter scale gradients. This study represents the first successful engineering of continuous alignment gradients in soft, natural biomaterials. Migration experiments on graded alignment show that HUVECs exhibit increased directionality, persistence, and speed compared to uniform and unaligned fiber architectures. Similarly, patterned MDA-MB-231 aggregates exhibit biased migration toward increasing fiber alignment, suggesting a role for alignment gradients as a taxis cue. This user-friendly approach, requiring no specialized equipment, is anticipated to offer new insights into the biophysical cues that cells interpret as they traverse the extracellular matrix, with broad applicability in healthy and diseased tissue environments.

Biofabrication of engineered blood vessels for biomedical applications

To successfully engineer large-sized tissues, establishing vascular structures is essential for providing oxygen, nutrients, growth factors and cells to prevent necrosis at the core of the tissue. The diameter scale of the biofabricated vasculatures should range from 100 to 1,000 µm to support the mm-size tissue while being controllably aligned and spaced within the diffusion limit of oxygen. In this review, insights regarding biofabrication considerations and techniques for engineered blood vessels will be presented. Initially, polymers of natural and synthetic origins can be selected, modified, and combined with each other to support maturation of vascular tissue while also being biocompatible. After they are shaped into scaffold structures by different fabrication techniques, surface properties such as physical topography, stiffness, and surface chemistry play a major role in the endothelialization process after transplantation. Furthermore, biological cues such as growth factors (GFs) and endothelial cells (ECs) can be incorporated into the fabricated structures. As variously reported, fabrication techniques, especially 3D printing by extrusion and 3D printing by photopolymerization, allow the construction of vessels at a high resolution with diameters in the desired range. Strategies to fabricate of stable tubular structures with defined channels will also be discussed. This paper provides an overview of the many advances in blood vessel engineering and combinations of different fabrication techniques up to the present time.

Transforming Static Barrier Tissue Models into Dynamic Microphysiological Systems

Microphysiological systems are miniaturized cell culture platforms used to mimic the structure and function of human tissues in a laboratory setting. However, these platforms have not gained widespread adoption in bioscience laboratories where open-well, membrane-based approaches serve as the gold standard for mimicking tissue barriers, despite lacking fluid flow capabilities. This issue can be primarily attributed to the incompatibility of existing microphysiological systems with standard protocols and tools developed for open-well systems. Here, we present a protocol for creating a reconfigurable membrane-based platform with an open-well structure, flow enhancement capability, and compatibility with conventional protocols. This system utilizes a magnetic assembly approach that enables reversible switching between open-well and microfluidic modes. With this approach, users have the flexibility to begin an experiment in the open-well format using standard protocols and add or remove flow capabilities as needed. To demonstrate the practical usage of this system and its compatibility with standard techniques, an endothelial cell monolayer was established in an open-well format. The system was reconfigured to introduce fluid flow and then switched to the open-well format to conduct immunostaining and RNA extraction. Due to its compatibility with conventional open-well protocols and flow enhancement capability, this reconfigurable design is expected to be adopted by both engineering and bioscience laboratories.

Novel Bioinspired Nerve Scaffold with High Synchrony between Biodegradation and Nerve Regeneration for Repair of Peripheral Nerve Injury

The morphological structure reconstruction and functional recovery of long-distance peripheral nerve injury (PNI) are global medical challenges. Biodegradable nerve scaffolds that provide mechanical support for the growth and extension of neurites are a desired way to repair long-distance PNI. However, the synchrony of scaffold degradation and nerve regeneration is still challenging. Here, a novel bioinspired multichannel nerve guide conduit (MNGC) with topographical cues based on silk fibroin and ε-polylysine modification was constructed. This conduit (SF(A) + PLL MNGC) exhibited sufficient mechanical strength, excellent degradability, and favorable promotion of cell growth. Peripheral nerve repairing was evaluated by an in vivo 10 mm rat sciatic model. In vivo evidence demonstrated that SF(A) + PLL MNGC was completely biodegraded in the body within 4 weeks after providing sufficient physical support and guide for neurite extension, and a 10 mm sciatic nerve defect was effectively repaired without scar formation, indicating a high synchronous effect of scaffold biodegradation and nerve regeneration. More importantly, the regenerated nerve of the SF(A) + PLL MNGC group showed comparable morphological reconstruction and functional recovery to that of autologous nerve transplantation. This work proved that the designed SF(A) + PLL MNGC has potential for application in long-distance PNI repair in the clinic.

Microengineering 3D Collagen Matrices with Tumor-Mimetic Gradients in Fiber Alignment

In the tumor microenvironment (TME), collagen fibers facilitate tumor cell migration through the extracellular matrix. Previous studies have focused on studying the responses of cells on uniformly aligned or randomly aligned collagen fibers. However, the in vivo environment also features spatial gradients in alignment, which arise from the local reorganization of the matrix architecture due to cell-induced traction forces. Although there has been extensive research on how cells respond to graded biophysical cues, such as stiffness, porosity, and ligand density, the cellular responses to physiological fiber alignment gradients have been largely unexplored. This is due, in part, to a lack of robust experimental techniques to create controlled alignment gradients in natural materials. In this study, we image tumor biopsy samples and characterize the alignment gradients present in the TME. To replicate physiological gradients, we introduce a first-of-its-kind biofabrication technique that utilizes a microfluidic channel with constricting and expanding geometry to engineer 3D collagen hydrogels with tunable fiber alignment gradients that range from sub-millimeter to millimeter length scales. Our modular approach allows easy access to the microengineered gradient gels, and we demonstrate that HUVECs migrate in response to the fiber architecture. We provide preliminary evidence suggesting that MDA-MB-231 cell aggregates, patterned onto a specific location on the alignment gradient, exhibit preferential migration towards increasing alignment. This finding suggests that alignment gradients could serve as an additional taxis cue in the ECM. Importantly, our study represents the first successful engineering of continuous gradients of fiber alignment in soft, natural materials. We anticipate that our user-friendly platform, which needs no specialized equipment, will offer new experimental capabilities to study the impact of fiber-based contact guidance on directed cell migration.

Electropatterning-Contemporary developments for selective particle arrangements employing electrokinetics

The selective positioning and arrangement of distinct types of multiscale particles can be used in numerous applications in microfluidics, including integrated circuits, sensors and biochips. Electrokinetic (EK) techniques offer an extensive range of options for label-free manipulation and patterning of colloidal particles by exploiting the intrinsic electrical properties of the target of interest. EK-based techniques have been widely implemented in many recent studies, and various methodologies and microfluidic device designs have been developed to achieve patterning two- and three-dimensional (3D) patterned structures. This review provides an overview of the progress in electropatterning research during the last 5 years in the microfluidics arena. This article discusses the advances in the electropatterning of colloids, droplets, synthetic particles, cells, and gels. Each subsection analyzes the manipulation of the particles of interest via EK techniques such as electrophoresis and dielectrophoresis. The conclusions summarize recent advances and provide an outlook on the future of electropatterning in various fields of application, especially those with 3D arrangements as their end goal.

Magnetically Guided Synthesis of Anisotropic Porous Carbons toward Efficient CO2 Capture and Magnetic Separation of Oil

Conventional synthetic strategies do not allow one to impart structural anisotropy into porous carbons, thus leading to limited control over their textural properties. While structural anisotropy alters the mechanical properties of materials, it also introduces an additional degree of directionality to increase the pore connectivity and thus the flux in the designed direction. Accordingly, in this work the structure of porous carbons prepared from resorcinol-formaldehyde gels has been rendered anisotropic by integrating superparamagnetic colloids to the sol-gel precursor solution and by applying a uniform magnetic field during the sol-gel transition, which enables the self-assembly of magnetic colloids into chainlike structures to template the growth of the gel phase. Notably, the anisotropic pore structure is maintained upon pyrolysis of the gel, leading to hierarchically porous carbon monoliths with tunable structure and porosities. With an advantage granted to anisotropic materials, these porous carbons showed higher porosity, a higher CO₂ uptake capacity of 3.45 mmol g^-1 at 273 K at 1.1 bar, and faster adsorption kinetics compared to the ones synthesized in the absence of magnetic field. Moreover, these materials were also used as magnetic sorbents with fast adsorption kinetics for efficient oil-spill cleanup and retrieved easily by using an external magnetic field.

The Modular µSiM Reconfigured: Integration of Microfluidic Capabilities to Study In Vitro Barrier Tissue Models under Flow

Microfluidic tissue barrier models have emerged to address the lack of physiological fluid flow in conventional "open-well" Transwell-like devices. However, microfluidic techniques have not achieved widespread usage in bioscience laboratories because they are not fully compatible with traditional experimental protocols. To advance barrier tissue research, there is a need for a platform that combines the key advantages of both conventional open-well and microfluidic systems. Here, a plug-and-play flow module is developed to introduce on-demand microfluidic flow capabilities to an open-well device that features a nanoporous membrane and live-cell imaging capabilities. The magnetic latching assembly of this design enables bi-directional reconfiguration and allows users to conduct an experiment in an open-well format with established protocols and then add or remove microfluidic capabilities as desired. This work also provides an experimentally-validated flow model to select flow conditions based on the experimental needs. As a proof-of-concept, flow-induced alignment of endothelial cells and the expression of shear-sensitive gene targets are demonstrated, and the different phases of neutrophil transmigration across a chemically stimulated endothelial monolayer under flow conditions are visualized. With these experimental capabilities, it is anticipated that both engineering and bioscience laboratories will adopt this reconfigurable design due to the compatibility with standard open-well protocols.

Alginate microspheres-collagen hydrogel, as a novel 3D culture system, enhanced skin wound healing of hUCMSCs in rats model

While stem cell transplantation has emerged as a promising approach to improving wound healing outcomes, the application of stem cells to date has been limited by the poor survival and retention of these cells once transplanted. The survival, development, and migratory activity of transplanted cells can be improved through the use of three-dimensional (3D) culture systems. Here, a novel alginate microsphere-collage hydrogel (AMS-Col gel) 3D culture system was developed and found to improve human umbilical cord mesenchymal stem cell (hUCMSC) survival, permitting their sustained release so as to promote wound healing. Through hematoxylin and eosin staining and Masson's trichrome staining, the prepared hUCMSCs-AMS-Col gel was found to exhibit wound healing activity. On day 7 following the hUCMSCs-AMS-Col gel treatment of model wounds, improved collagen fiber deposition and re-epithelialization were evident, with complete epithelial regeneration as of day 14 and near-total wound healing was evident as of day 21. This hUCMSCs-AMS-Col gel was also associated with increased VEGF and FGF2 expression. Together, these data indicate that AMS-Col gels are a promising and novel form of 3D cell culture system capable of improving hUCMSC-mediated wound healing, highlighting the potential clinical utility of this regenerative strategy.

Microfluidic-driven mixing of high molecular weight polymeric complexes for precise nanoparticle downsizing

Chitosan (CHIT) and hyaluronic acid (HA) are two polysaccharides (PSs) with high value in several biomedical applications. In this study, we present a microfluidic method to synthetize CHIT-HA NPs to overcome the disadvantages of the dropwise approach generally used for nanoprecipitation of polyelectrolyte complexes. The proposed microfluidic approach enables to generate monodisperse suspensions of NPs with ≈100 nm of size compared to the dropwise method that generated ≈2 times bigger NPs. Finally, we evaluated the potential of obtained NPs in an inflammatory scenario. The treatment with NPs led to the reduction of the main inflammatory molecules produced by macrophages (PGE₂, IL-6, IL-8, MCAF and TNF-α) and fibroblasts (IL-1 α, PGE₂, TNF-α) stimulated with lipopolysaccharide or conditioned medium, respectively. This study demonstrates that our approach can be used to enhance the synthesis of nanocarriers based on bioactive macromolecules.

A miniaturized 3D printed pressure regulator (µPR) for microfluidic cell culture applications

Well-defined fluid flows are the hallmark feature of microfluidic culture systems and enable precise control over biophysical and biochemical cues at the cellular scale. Microfluidic flow control is generally achieved using displacement-based (e.g., syringe or peristaltic pumps) or pressure-controlled techniques that provide numerous perfusion options, including constant, ramped, and pulsed flows. However, it can be challenging to integrate these large form-factor devices and accompanying peripherals into incubators or other confined environments. In addition, microfluidic culture studies are primarily carried out under constant perfusion conditions and more complex flow capabilities are often unused. Thus, there is a need for a simplified flow control platform that provides standard perfusion capabilities and can be easily integrated into incubated environments. To this end, we introduce a tunable, 3D printed micro pressure regulator (µPR) and show that it can provide robust flow control capabilities when combined with a battery-powered miniature air pump to support microfluidic applications. We detail the design and fabrication of the µPR and: (i) demonstrate a tunable outlet pressure range relevant for microfluidic applications (1-10 kPa), (ii) highlight dynamic control capabilities in a microfluidic network, (iii) and maintain human umbilical vein endothelial cells (HUVECs) in a multi-compartment culture device under continuous perfusion conditions. We anticipate that our 3D printed fabrication approach and open-access designs will enable customized µPRs that can support a broad range of microfluidic applications.

Local extensional flows promote long-range fiber alignment in 3D collagen hydrogels

Randomly oriented type I collagen (COL1) fibers in the extracellular matrix are reorganized by biophysical forces into aligned domains extending several millimeters and with varying degrees of fiber alignment. These aligned fibers can transmit traction forces, guide tumor cell migration, facilitate angiogenesis, and influence tissue morphogenesis. To create aligned COL1 domains in microfluidic cell culture models, shear flows have been used to align thin COL1 matrices (<50µm in height) in a microchannel. However, there has been limited investigation into the role of shear flows in aligning 3D hydrogels (>130µm). Here, we show that pure shear flows do not induce fiber alignment in 3D atelo COL1 hydrogels, but the simple addition of local extensional flow promotes alignment that is maintained across several millimeters, with a degree of alignment directly related to the extensional strain rate. We further advance experimental capabilities by addressing the practical challenge of accessing a 3D hydrogel formed within a microchannel by introducing a magnetically coupled modular platform that can be released to expose the microengineered hydrogel. We demonstrate the platform's capability to pattern cells and fabricate multi-layered COL1 matrices using layer-by-layer fabrication and specialized modules. Our approach provides an easy-to-use fabrication method to achieve advanced hydrogel microengineering capabilities that combine fiber alignment with biofabrication capabilities.

Engineering fiber anisotropy within natural collagen hydrogels

It is well known that biophysical properties of the extracellular matrix (ECM), including stiffness, porosity, composition, and fiber alignment (anisotropy), play a crucial role in controlling cell behavior in vivo. Type I collagen (collagen I) is a ubiquitous structural component in the ECM and has become a popular hydrogel material that can be tuned to replicate the mechanical properties found in vivo. In this review article, we describe popular methods to create 2-D and 3-D collagen I hydrogels with anisotropic fiber architectures. We focus on methods that can be readily translated from engineering and materials science laboratories to the life-science community with the overall goal of helping to increase the physiological relevance of cell culture assays.