Guided cell migration on a graded micropillar substrate

Cargo Size Limits and Forces of Cell-Driven Microtransport

The integration of motile cells into biohybrid microrobots offers unique properties such as sensitive responses to external stimuli, resilience, and intrinsic energy supply. Here, biohybrid cell-cargo systems that are driven by amoeboid Dictyostelium discoideum cells are studied and how the cargo speed and the resulting viscous drag force scales with increasing radius of the spherical cargo particle are explored. Using a simplified geometrical model of the cell-cargo interaction, the findings toward larger cargo sizes, which are not accessible with the experimental setup, are extrapolated and a maximal cargo size is predicted beyond which active cell-driven movements will stall. The active forces exerted by the cells to move a cargo show mechanoresponsive adaptation and increase dramatically when challenged by an external pulling force, a mechanism that may become relevant when navigating cargo through complex heterogeneous environments.

Enhanced Cell Viability and Migration of Primary Bovine Annular Fibrosus Fibroblast-like Cells Induced by Microsecond Pulsed Electric Field Exposure

This study is the first to report the enhancement of cell migration and proliferation induced by in vitro microsecond pulsed electric field (μsPEF) exposure of primary bovine annulus fibrosus (AF) fibroblast-like cells. AF primary cells isolated from fresh bovine intervertebral disks (IVDs) are exposed to 10 and 100 μsPEFs with different numbers of pulses and applied electric field strengths. The results indicate that 10 μs-duration pulses induce reversible electroporation, while 100 μs pulses induce irreversible electroporation of the cells. Additionally, μsPEF exposure increased AF cell proliferation up to 150% while increasing the average migration speed by 0.08 μm/min over 24 h. The findings suggest that the effects of PEF exposure on cells are multifactorial-depending on the duration, intensity, and number of pulses used in the stimulation. This highlights the importance of optimizing the μsPEF parameters for specific cell types and applications. For instance, if the goal is to induce cell death for cancer treatment, then high numbers of pulses can be used to maximize the lethal effects. On the other hand, if the goal is to enhance cell proliferation, a combination of the number of pulses and the applied electric field strength can be tuned to achieve the desired outcome. The information gleaned from this study can be applied in the future to in vitro cell culture expansion and tissue regeneration.

Mechanobiology of cancer cell responsiveness to chemotherapy and immunotherapy: Mechanistic insights and biomaterial platforms

Mechanical forces are central to how cancer treatments such as chemotherapeutics and immunotherapies interact with cells and tissues. At the simplest level, electrostatic forces underlie the binding events that are critical to therapeutic function. However, a growing body of literature points to mechanical factors that also affect whether a drug or an immune cell can reach a target, and to interactions between a cell and its environment affecting therapeutic efficacy. These factors affect cell processes ranging from cytoskeletal and extracellular matrix remodeling to transduction of signals by the nucleus to metastasis of cells. This review presents and critiques the state of the art of our understanding of how mechanobiology impacts drug and immunotherapy resistance and responsiveness, and of the in vitro systems that have been of value in the discovery of these effects.

Rational Design of Soft-Hard Interfaces through Bioinspired Engineering

Soft-hard tissue interfaces in nature present a diversity of hierarchical transitions in composition and structure to address the challenge of stress concentrations that would otherwise arise at their interface. The translation of these into engineered materials holds promise for improved function of biomedical interfaces. Here, soft-hard tissue interfaces found in the body in health and disease, and the application of the diverse, functionally graded, and hierarchical structures that they present to bioinspired engineering materials are reviewed. A range of such bioinspired engineering materials and associated manufacturing technologies that are on the horizon in interfacial tissue engineering, hydrogel bioadhesion at the interfaces, and healthcare and medical devices are described.

A review on cell damage, viability, and functionality during 3D bioprinting

Three-dimensional (3D) bioprinting fabricates 3D functional tissues/organs by accurately depositing the bioink composed of the biological materials and living cells. Even though 3D bioprinting techniques have experienced significant advancement over the past decades, it remains challenging for 3D bioprinting to artificially fabricate functional tissues/organs with high post-printing cell viability and functionality since cells endure various types of stress during the bioprinting process. Generally, cell viability which is affected by several factors including the stress and the environmental factors, such as pH and temperature, is mainly determined by the magnitude and duration of the stress imposed on the cells with poorer cell viability under a higher stress and a longer duration condition. The maintenance of high cell viability especially for those vulnerable cells, such as stem cells which are more sensitive to multiple stresses, is a key initial step to ensure the functionality of the artificial tissues/organs. In addition, maintaining the pluripotency of the cells such as proliferation and differentiation abilities is also essential for the 3D-bioprinted tissues/organs to be similar to native tissues/organs. This review discusses various pathways triggering cell damage and the major factors affecting cell viability during different bioprinting processes, summarizes the studies on cell viabilities and functionalities in different bioprinting processes, and presents several potential approaches to protect cells from injuries to ensure high cell viability and functionality.

Recent advances in micro-/nanostructure array integrated microfluidic devices for efficient separation of circulating tumor cells

Circulating tumor cells (CTCs) released from the primary tumor to peripheral blood are promising targets for liquid biopsies. Their biological information is vital for early cancer detection, efficacy assessment, and prognostic monitoring. Despite the tremendous clinical applications of CTCs, development of effective separation techniques are still demanding. Traditional separation methods usually use batch processing for enrichment, which inevitably destroy cell integrity and affect the complete information acquisition. Considering the rarity and heterogeneity of CTCs, it is urgent to develop effective separation methods. Microfluidic chips with precise fluid control at the micron level are promising devices for CTC separation. Their further combination with micro-/nanostructure arrays adds more biomolecule binding sites and exhibit unique fluid barrier effect, which significantly improve the CTC capture efficiency, purity, and sensitivity. This review summarized the recent advances in micro-/nanostructure array integrated microfluidic devices for CTC separation, including microrods, nanowires, and 3D micro-/nanostructures. The mechanisms by which these structures contribute to improved capture efficiency are discussed. Two major categories of separation methods, based on the physical and biological properties of CTCs, are discussed separately. Physical separation includes the design and preparation of micro-/nanostructure arrays, while chemical separation additionally involves the selection and modification of specific capture probes. These emerging technologies are expected to become powerful tools for disease diagnosis in the future.

The role of cellular traction forces in deciphering nuclear mechanics

Cellular forces exerted on the extracellular matrix (ECM) during adhesion and migration under physiological and pathological conditions regulate not only the overall cell morphology but also nuclear deformation. Nuclear deformation can alter gene expression, integrity of the nuclear envelope, nucleus-cytoskeletal connection, chromatin architecture, and, in some cases, DNA damage responses. Although nuclear deformation is caused by the transfer of forces from the ECM to the nucleus, the role of intracellular organelles in force transfer remains unclear and a challenging area of study. To elucidate nuclear mechanics, various factors such as appropriate biomaterial properties, processing route, cellular force measurement technique, and micromanipulation of nuclear forces must be understood. In the initial phase of this review, we focused on various engineered biomaterials (natural and synthetic extracellular matrices) and their manufacturing routes along with the properties required to mimic the tumor microenvironment. Furthermore, we discussed the principle of tools used to measure the cellular traction force generated during cell adhesion and migration, followed by recently developed techniques to gauge nuclear mechanics. In the last phase of this review, we outlined the principle of traction force microscopy (TFM), challenges in the remodeling of traction forces, microbead displacement tracking algorithm, data transformation from bead movement, and extension of 2-dimensional TFM to multiscale TFM.

The Bumpy Road to Stem Cell Therapies: Rational Design of Surface Topographies to Dictate Stem Cell Mechanotransduction and Fate

Cells sense and respond to a variety of physical cues from their surrounding microenvironment, and these are interpreted through mechanotransductive processes to inform their behavior. These mechanisms have particular relevance to stem cells, where control of stem cell proliferation, potency, and differentiation is key to their successful application in regenerative medicine. It is increasingly recognized that surface micro- and nanotopographies influence stem cell behavior and may represent a powerful tool with which to direct the morphology and fate of stem cells. Current progress toward this goal has been driven by combined advances in fabrication technologies and cell biology. Here, the capacity to generate precisely defined micro- and nanoscale topographies has facilitated the studies that provide knowledge of the mechanotransducive processes that govern the cellular response as well as knowledge of the specific features that can drive cells toward a defined differentiation outcome. However, the path forward is not fully defined, and the "bumpy road" that lays ahead must be crossed before the full potential of these approaches can be fully exploited. This review focuses on the challenges and opportunities in applying micro- and nanotopographies to dictate stem cell fate for regenerative medicine. Here, key techniques used to produce topographic features are reviewed, such as photolithography, block copolymer lithography, electron beam lithography, nanoimprint lithography, soft lithography, scanning probe lithography, colloidal lithography, electrospinning, and surface roughening, alongside their advantages and disadvantages. The biological impacts of surface topographies are then discussed, including the current understanding of the mechanotransductive mechanisms by which these cues are interpreted by the cells, as well as the specific effects of surface topographies on cell differentiation and fate. Finally, considerations in translating these technologies and their future prospects are evaluated.

Effect of Electrical and Electromechanical Stimulation on PC12 Cell Proliferation and Axon Outgrowth

Peripheral nerve injuries have become a common clinical disease with poor prognosis and complicated treatments. The development of tissue engineering pointed a promising direction to produce nerve conduits for nerve regeneration. Electrical and mechanical stimulations have been incorporated with tissue engineering, since such external stimulations could promote nerve cell proliferation, migration and differentiation. However, the combination of electrical and mechanical stimulations (electromechanical stimulation) and its effects on neuron proliferation and axon outgrowth have been rarely investigated. Herein, silver nanowires (AgNWs) embedded polydimethylsiloxane (PDMS) electrodes were developed to study the effects of electromechanical stimulation on rat pheochromocytoma cells (PC12 cells) behaviors. AgNWs/PDMS electrodes demonstrated good biocompatibility and established a stable electric field during mechanical stretching. PC12 cells showed enhanced proliferation rate and axon outgrowth under electrical stimulation alone, and the cell number significantly increased with higher electrical stimulation intensity. The involvement of mechanical stretching in electrical stimulation reduced the cell proliferation rate and axon outgrowth, compared with the case of electrical stimulation alone. Interestingly, the cellular axons outgrowth was found to depend on the stretching direction, where the axons prefer to align perpendicularly to the stretch direction. These results suggested that AgNWs/PDMS electrodes provide an in vitro platform to investigate the effects of electromechanical stimulation on nerve cell behaviors and can be potentially used for nerve regeneration in the future.