Tuning Cell and Tissue Development by Combining Multiple Mechanical Signals

Soft bioreactor systems: a necessary step toward engineered MSK soft tissue?

A key objective of tissue engineering (TE) is to produce in vitro funcional grafts that can replace damaged tissues or organs in patients. TE uses bioreactors, which are controlled environments, allowing the application of physical and biochemical cues to relevant cells growing in biomaterials. For soft musculoskeletal (MSK) tissues such as tendons, ligaments and cartilage, it is now well established that applied mechanical stresses can be incorporated into those bioreactor systems to support tissue growth and maturation via activation of mechanotransduction pathways. However, mechanical stresses applied in the laboratory are often oversimplified compared to those found physiologically and may be a factor in the slow progression of engineered MSK grafts towards the clinic. In recent years, an increasing number of studies have focused on the application of complex loading conditions, applying stresses of different types and direction on tissue constructs, in order to better mimic the cellular environment experienced in vivo. Such studies have highlighted the need to improve upon traditional rigid bioreactors, which are often limited to uniaxial loading, to apply physiologically relevant multiaxial stresses and elucidate their influence on tissue maturation. To address this need, soft bioreactors have emerged. They employ one or more soft components, such as flexible soft chambers that can twist and bend with actuation, soft compliant actuators that can bend with the construct, and soft sensors which record measurements in situ. This review examines types of traditional rigid bioreactors and their shortcomings, and highlights recent advances of soft bioreactors in MSK TE. Challenges and future applications of such systems are discussed, drawing attention to the exciting prospect of these platforms and their ability to aid development of functional soft tissue engineered grafts.

4D Printing of Personalized-Tunable Biomimetic Periosteum with Anisotropic Microstructure for Accelerated Vascularization and Bone Healing

An ideal biomimetic periosteum is expected to wrap various bone surfaces to orchestrate an optimal microenvironment for bone regeneration, including facilitating local vascularization, recruiting osteoblasts, and mineralizing the extracellular matrix (ECM). To mimic the role of the natural periosteum in promoting bone repair, a 4D printing technique to inlay aligned cell sheets on shape-shifting hydrogel is used, containing biophysical signals and spatially adjustable physical properties, for the first time. The outer hydrogel layer endows the biomimetic periosteum with the ability to digitally coordinate its 3D geometry to match the specific macroscopic bone shape to maintain a bone healing microenvironment. The inner aligned human mesenchymal stem cells (hMSCs) layer not only promotes the migration and angiogenesis of co-cultured cells but also exhibits excellent osteogenic differentiation properties. In vivo experiments show that apart from morphing preset shapes as physical barriers, the aligned biomimetic periosteum can actively facilitate local angiogenesis and early-stage osteogenesis. Altogether, this present work provides a novel route to construct a personalized biomimetic periosteum with anisotropic microstructure by introducing a tunable shape to maintain the bone reconstruction microenvironment and this strategy can be extended to repair sophisticated bone defects.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Review: Advanced Atomic Force Microscopy Modes for Biomedical Research

Visualization of biomedical samples in their native environments at the microscopic scale is crucial for studying fundamental principles and discovering biomedical systems with complex interaction. The study of dynamic biological processes requires a microscope system with multiple modalities, high spatial/temporal resolution, large imaging ranges, versatile imaging environments and ideally in-situ manipulation capabilities. Recent development of new Atomic Force Microscopy (AFM) capabilities has made it such a powerful tool for biological and biomedical research. This review introduces novel AFM functionalities including high-speed imaging for dynamic process visualization, mechanobiology with force spectroscopy, molecular species characterization, and AFM nano-manipulation. These capabilities enable many new possibilities for novel scientific research and allow scientists to observe and explore processes at the nanoscale like never before. Selected application examples from recent studies are provided to demonstrate the effectiveness of these AFM techniques.

Mechanical regulation of the early stages of angiogenesis

Favouring or thwarting the development of a vascular network is essential in fields as diverse as oncology, cardiovascular disease or tissue engineering. As a result, understanding and controlling angiogenesis has become a major scientific challenge. Mechanical factors play a fundamental role in angiogenesis and can potentially be exploited for optimizing the architecture of the resulting vascular network. Largely focusing on in vitro systems but also supported by some in vivo evidence, the aim of this Highlight Review is dual. First, we describe the current knowledge with particular focus on the effects of fluid and solid mechanical stimuli on the early stages of the angiogenic process, most notably the destabilization of existing vessels and the initiation and elongation of new vessels. Second, we explore inherent difficulties in the field and propose future perspectives on the use of in vitro and physics-based modelling to overcome these difficulties.

The lectin Discoidin I acts in the cytoplasm to help assemble the contractile machinery

Cellular functions, such as division and migration, require cells to undergo robust shape changes. Through their contractility machinery, cells also sense, respond, and adapt to their physical surroundings. In the cytoplasm, the contractility machinery organizes into higher order assemblies termed contractility kits (CKs). Using Dictyostelium discoideum, we previously identified Discoidin I (DscI), a classic secreted lectin, as a CK component through its physical interactions with the actin crosslinker Cortexillin I (CortI) and the scaffolding protein IQGAP2. Here, we find that DscI ensures robust cytokinesis through regulating intracellular components of the contractile machinery. Specifically, DscI is necessary for normal cytokinesis, cortical tension, membrane-cortex connections, and cortical distribution and mechanoresponsiveness of CortI. The dscI deletion mutants also have complex genetic epistatic relationships with CK components, acting as a genetic suppressor of cortI and iqgap1, but as an enhancer of iqgap2. This work underscores the fact that proteins like DiscI contribute in diverse ways to the activities necessary for optimal cell function.

Structurally Dynamic Hydrogels for Biomedical Applications: Pursuing a Fine Balance between Macroscopic Stability and Microscopic Dynamics

Owing to their unique chemical and physical properties, hydrogels are attracting increasing attention in both basic and translational biomedical studies. Although the classical hydrogels with static networks have been widely reported for decades, a growing number of recent studies have shown that structurally dynamic hydrogels can better mimic the dynamics and functions of natural extracellular matrix (ECM) in soft tissues. These synthetic materials with defined compositions can recapitulate key chemical and biophysical properties of living tissues, providing an important means to understanding the mechanisms by which cells sense and remodel their surrounding microenvironments. This review begins with the overall expectation and design principles of dynamic hydrogels. We then highlight recent progress in the fabrication strategies of dynamic hydrogels including both degradation-dependent and degradation-independent approaches, followed by their unique properties and use in biomedical applications such as regenerative medicine, drug delivery, and 3D culture. Finally, challenges and emerging trends in the development and application of dynamic hydrogels are discussed.

Mapping cellular-scale internal mechanics in 3D tissues with thermally responsive hydrogel probes

Local tissue mechanics play a critical role in cell function, but measuring these properties at cellular length scales in living 3D tissues can present considerable challenges. Here we present thermoresponsive, smart material microgels that can be dispersed or injected into tissues and optically assayed to measure residual tissue elasticity after creep over several weeks. We first develop and characterize the sensors, and demonstrate that internal mechanical profiles of live multicellular spheroids can be mapped at high resolutions to reveal broad ranges of rigidity within the tissues, which vary with subtle differences in spheroid aggregation method. We then show that small sites of unexpectedly high rigidity develop in invasive breast cancer spheroids, and in an in vivo mouse model of breast cancer progression. These focal sites of increased intratumoral rigidity suggest new possibilities for how early mechanical cues that drive cancer cells towards invasion might arise within the evolving tumor microenvironment.

MechanoBioTester: A Decoupled Multistimulus Cell Culture Device for Studying Complex Microenvironments In Vitro

Increasingly being recognized is the role of the complex microenvironment to regulate cell phenotype; however, the cell culture systems used to study these effects in vitro are lagging. The complex microenvironment is host to a combination of biological interactions, chemical factors, and mechanical stimuli. Many devices have been designed to probe the effects of one mechanical stimulus, but few are capable of systematically interrogating all combinations of mechanical stimuli with independent control. To address this gap, we have developed the MechanoBioTester platform, a decoupled, multi-stimulus cell culture model for studying the cellular response to complex microenvironments in vitro. The system uses an engineered elastomeric chamber with a specially defined region for incorporating different target materials to act as the cell culture substrate. We have tested the system with several target materials including: polydimethylsiloxane elastomer, polyacrylamide gel, poly(1,8-octanediol citrate) elastomer, and type I collagen gel for both 2D and 3D co-culture. Additionally, when the chamber is connected to a flow circuit and our stretching device, stimuli in the form of fluid flow, cyclic stretch, and hydrostatic pressure are able to be imparted with independent control. We validated the device using experimental and computational methods to define a range of capabilities relevant to physiological microenvironments. The MechanoBioTester platform promises to function as a model system for mechanobiology, biomaterial design, and drug discovery applications that focus on probing the impact of a complex microenvironment in an in vitro setting. The protocol described within provides the details characterizing the MechanoBioTester system, the steps for fabricating the MechanoBioTester chamber, and the procedure for operating the MechanoBioTester system to stimulate cells.

Multifunctional Biomaterials: Combining Material Modification Strategies for Engineering of Cell-Contacting Surfaces

In the human body, cells in a tissue are exposed to signals derived from their specific extracellular matrix (ECM), such as architectural structure, mechanical properties, and chemical composition (proteins, growth factors). Research on biomaterials in tissue engineering and regenerative medicine aims to recreate such stimuli using engineered materials to induce a specific response of cells at the interface. Although traditional biomaterials design has been mostly limited to varying individual signals, increasing interest has arisen on combining several features in recent years to improve the mimicry of extracellular matrix properties. Tremendous progress in combinatorial surface modification exploiting, for example, topographical features or variations in mechanics combined with biochemical cues has enabled the identification of their key regulatory characteristics on various cell fate decisions. Gradients especially facilitated such research by enabling the investigation of combined continuous changes of different signals. Despite unravelling important synergies for cellular responses, challenges arise in terms of fabrication and characterization of multifunctional engineered materials. This review summarizes recent work on combinatorial surface modifications that aim to control biological responses. Modification and characterization methods for enhanced control over multifunctional material properties are highlighted and discussed. Thereby, this review deepens the understanding and knowledge of biomimetic combinatorial material modification, their challenges but especially their potential.

Topographically Defined, Biodegradable Nanopatterned Patches to Regulate Cell Fate and Acceleration of Bone Regeneration

If only allowed to proceed naturally, the bone-healing process can take several weeks, months, or even years depending on the injury size. In terms of bone-healing speed, many studies have been conducted investigating the deliverance of various growth factors of implantable biomaterials to shorten the time for bone regeneration. However, there may be side effects such as nerve pain, infection, or ectopic bone formation. As an alternative method, we focused on biophysical guidance, which provided similar topographical cues to the cellular environment to recruit host cells for bone defect healing. In this study, we hypothesized that aligned nanotopographical features have enhanced osteoblast recruitment, migration, and differentiation without external stimuli. We designed and fabricated a biodegradable poly(lactic- co-glycolic acid) nanopatterned patch using simple solvent casting and capillary force lithography. We confirmed that a biodegradable nanopatterned patch (BNP) accelerated the migration of osteoblasts according to the orientation of the patterned direction. These highly aligned osteoblasts may contribute to in vitro osteogenic differentiation, such as alkaline phosphate activity, mineralization, and calcium deposition, compared to the biodegradable flat patch (BFP). To demonstrate bone defect healing by BNP guidance in vivo, we implanted either whole or bridge BNP on the critical size defect of mouse calvarial ( ø 4 mm) or tibia bone (3 × 7 mm²). Only the BNP-treated group showed faster new bone formation and compact bone regeneration at the calvarial or tibia bone defect area compared to BFP at 4 or 8 weeks. Bridge BNP guided, in particular, the regeneration of new bone formation along the parallel direction of nanopatterned substrates. Here, we show that a BNP with biophysical guidance should be suitable for use in bone tissue regeneration through accelerated migration of the intact host cell.

Vascular Endothelial Cell Behavior in Complex Mechanical Microenvironments

The vascular mechanical microenvironment consists of a mixture of spatially and temporally changing mechanical forces. This exposes vascular endothelial cells to both hemodynamic forces (fluid flow, cyclic stretching, lateral pressure) and vessel forces (basement membrane mechanical and topographical properties). The vascular mechanical microenvironment is "complex" because these forces are dynamic and interrelated. Endothelial cells sense these forces through mechanosensory structures and transduce them into functional responses via mechanotransduction pathways, culminating in behavior directly affecting vascular health. Recent in vitro studies have shown that endothelial cells respond in nuanced and unique ways to combinations of hemodynamic and vessel forces as compared to any single mechanical force. Understanding the interactive effects of the complex mechanical microenvironment on vascular endothelial behavior offers the opportunity to design future biomaterials and biomedical devices from the bottom-up by engineering for the cellular response. This review describes and defines (1) the blood vessel structure, (2) the complex mechanical microenvironment of the vascular endothelium, (3) the process in which vascular endothelial cells sense mechanical forces, and (4) the effect of mechanical forces on vascular endothelial cells with specific attention to recent works investigating the influence of combinations of mechanical forces. We conclude this review by providing our perspective on how the field can move forward to elucidate the effects of the complex mechanical microenvironment on vascular endothelial cell behavior.

Strategies for directing cells into building functional hearts and parts

This review presents the current state-of-the-art, emerging directions and future trends to direct cells for building functional heart parts.