Muscle on a chip: in vitro contractility assays for smooth and striated muscle

Microengineered vascular systems for drug development

Recent advances in microfabrication technologies and advanced biomaterials have allowed for the development of in vitro platforms that recapitulate more physiologically relevant cellular components and function. Microengineered vascular systems are of particular importance for the efficient assessment of drug candidates to physiological barriers lining microvessels. This review highlights advances in the development of microengineered vascular structures with an emphasis on the potential impact on drug delivery studies. Specifically, this article examines the development of models for the study of drug delivery to the central nervous system and cardiovascular system. We also discuss current challenges and future prospects of the development of microengineered vascular systems.

Microfluidic organs-on-chips

An organ-on-a-chip is a microfluidic cell culture device created with microchip manufacturing methods that contains continuously perfused chambers inhabited by living cells arranged to simulate tissue- and organ-level physiology. By recapitulating the multicellular architectures, tissue-tissue interfaces, physicochemical microenvironments and vascular perfusion of the body, these devices produce levels of tissue and organ functionality not possible with conventional 2D or 3D culture systems. They also enable high-resolution, real-time imaging and in vitro analysis of biochemical, genetic and metabolic activities of living cells in a functional tissue and organ context. This technology has great potential to advance the study of tissue development, organ physiology and disease etiology. In the context of drug discovery and development, it should be especially valuable for the study of molecular mechanisms of action, prioritization of lead candidates, toxicity testing and biomarker identification.

Assessment of cardiomyocyte contraction in human-induced pluripotent stem cell-derived cardiomyocytes

Functional changes to cardiomyocytes are a common cause of attrition in preclinical and clinical drug development. Current approaches to assess cardiomyocyte contractility in vitro are limited to low-throughput methods not amenable to early drug discovery. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) were used to assess their suitability to detect drug-induced changes in cardiomyocyte contraction. Application of field stimulation and measurement of cardiac contraction (IonOptix edge detection) and Ca(2+) transients confirmed hiPS-CMs to be a suitable model to investigate drug-induced changes in cardiomyocyte contractility. Using a live cell, fast kinetic fluorescent assay with a Ca(2+) sensitive dye to test 31 inotropic and 20 non-inotropic compounds in vivo, we report that hiPS-CMs provide a high-throughput experimental model to detect changes in cardiomyocyte contraction that is applicable to early drug discovery with a sensitivity and specificity of 87% and 70%, respectively. Moreover, our data provide evidence of the detection of this liability at therapeutically relevant concentrations with throughput amenable to influencing chemical design in drug discovery. Measurement of multiple parameters of the Ca(2+) transient in addition to the number of Ca(2+) transients offered no insight into the mechanism of cardiomyocyte contraction.

Biomimetic cardiac microsystems for pathophysiological studies and drug screens

Microfabricated organs-on-chips consist of tissue-engineered 3D in vitro models, which rely on engineering design and provide the physiological context of human organs. Recently, significant effort has been devoted to the creation of a biomimetic cardiac system by using microfabrication techniques. By applying allometric scaling laws, microengineered cardiac systems simulating arterial flow, pulse properties, and architectural environments have been implemented, allowing high-throughput pathophysiological experiments and drug screens. In this review, we illustrate the recent trends in cardiac microsystems with emphasis on cardiac pumping and valving functions. We report problems and solutions brought to light by existing organs-on-chip models and discuss future directions of the field. We also describe the needs and desired design features that will enable the control of mechanical, electrical, and chemical environments to generate functional in vitro cardiac disease models.

Microscale screening systems for 3D cellular microenvironments: platforms, advances, and challenges

The increasing interest in studying cells using more in vivo-like three-dimensional (3D) microenvironments has created a need for advanced 3D screening platforms with enhanced functionalities and increased throughput. 3D screening platforms that better mimic in vivo microenvironments with enhanced throughput would provide more in-depth understanding of the complexity and heterogeneity of microenvironments. The platforms would also better predict the toxicity and efficacy of potential drugs in physiologically relevant conditions. Traditional 3D culture models (e.g., spinner flasks, gyratory rotation devices, non-adhesive surfaces, polymers) were developed to create 3D multicellular structures. However, these traditional systems require large volumes of reagents and cells, and are not compatible with high-throughput screening (HTS) systems. Microscale technology offers the miniaturization of 3D cultures and allows efficient screening of various conditions. This review will discuss the development, most influential works, and current advantages and challenges of microscale culture systems for screening cells in 3D microenvironments.

The role of tissue engineering and biomaterials in cardiac regenerative medicine

In recent years, the development of 3-dimensional engineered heart tissue (EHT) has made large strides forward because of advances in stem cell biology, materials science, prevascularization strategies, and nanotechnology. As a result, the role of tissue engineering in cardiac regenerative medicine has become multifaceted as new applications become feasible. Cardiac tissue engineering has long been established to have the potential to partially or fully restore cardiac function after cardiac injury. However, EHTs may also serve as surrogate human cardiac tissue for drug-related toxicity screening. Cardiotoxicity remains a major cause of drug withdrawal in the pharmaceutical industry. Unsafe drugs reach the market because preclinical evaluation is insufficient to weed out cardiotoxic drugs in all their forms. Bioengineering methods could provide functional and mature human myocardial tissues, ie, physiologically relevant platforms, for screening the cardiotoxic effects of pharmaceutical agents and facilitate the discovery of new therapeutic agents. Finally, advances in induced pluripotent stem cells have made patient-specific EHTs possible, which opens up the possibility of personalized medicine. Herein, we give an overview of the present state of the art in cardiac tissue engineering, the challenges to the field, and future perspectives.

Microphysiological modeling of the reproductive tract: a fertile endeavor

Preclinical toxicity testing in animal models is a cornerstone of the drug development process, yet it is often unable to predict adverse effects and tolerability issues in human subjects. Species-specific responses to investigational drugs have led researchers to utilize human tissues and cells to better estimate human toxicity. Unfortunately, human cell-derived models are imperfect because toxicity is assessed in isolation, removed from the normal physiologic microenvironment. Microphysiological modeling often referred to as 'organ-on-a-chip' or 'human-on-a-chip' places human tissue into a microfluidic system that mimics the complexity of human in vivo physiology, thereby allowing for toxicity testing on several cell types, tissues, and organs within a more biologically relevant environment. Here we describe important concepts when developing a repro-on-a-chip model. The development of female and male reproductive microfluidic systems is critical to sex-based in vitro toxicity and drug testing. This review addresses the biological and physiological aspects of the male and female reproductive systems in vivo and what should be considered when designing a microphysiological human-on-a-chip model. Additionally, interactions between the reproductive tract and other systems are explored, focusing on the impact of factors and hormones produced by the reproductive tract and disease pathophysiology.

An in vitro culture system that supports robust expansion and maintenance of in vivo engraftment capabilities for myogenic progenitor cells from adult mice

Muscle cell therapy and tissue engineering require large numbers of functional muscle precursor/progenitor cells (MPCs), making the in vitro expansion of MPCs a critical step for these applications. The cells must maintain their myogenic properties upon robust expansion, especially for cellular therapy applications, in order to achieve efficacious treatment. A major obstacle associated with MPCs expansion is the loss of "stemness," or regenerative capacity, of freshly isolated cells, presumably due to the absence of the native cellular niches. In the current study, we developed an in vitro system that allowed for long-term culture and massive expansion of murine MPCs (mMPCs) with the preservation of myogenic regeneration capabilities. Long term in vitro expanded mMPC expressed the myogenic stem cell markers Pax3 and Pax7 and formed spontaneously contracting myotubes. Furthermore, expanded mMPC injected into the tibialis anterior muscle of nude mice engrafted and formed myofibers. Collectively, the method developed in this study can be potentially adapted for the expansion of human MPCs to high enough numbers for treatment of muscle injuries in human patients.

Organs-on-a-chip: a new tool for drug discovery

INTRODUCTION

The development of emerging in vitro tissue culture platforms can be useful for predicting human response to new compounds, which has been traditionally challenging in the field of drug discovery. Recently, several in vitro tissue-like microsystems, also known as 'organs-on-a-chip', have emerged to provide new tools for better evaluating the effects of various chemicals on human tissue.

AREAS COVERED

The aim of this article is to provide an overview of the organs-on-a-chip systems that have been recently developed. First, the authors introduce single-organ platforms, focusing on the most studied organs such as liver, heart, blood vessels and lung. Later, the authors briefly describe tumor-on-a-chip platforms and highlight their application for testing anti-cancer drugs. Finally, the article reports a few examples of other organs integrated in microfluidic chips along with preliminary multiple-organs-on-a-chip examples. The article also highlights key fabrication points as well as the main application areas of these devices.

EXPERT OPINION

This field is still at an early stage and major challenges need to be addressed prior to the embracement of these technologies by the pharmaceutical industry. To produce predictive drug screening platforms, several organs have to be integrated into a single microfluidic system representative of a humanoid. The routine production of metabolic biomarkers of the organ constructs, as well as their physical environment, have to be monitored prior to and during the delivery of compounds of interest to be able to translate the findings into useful discoveries.

Integrating in vitro organ-specific function with the microcirculation

There is significant interest within the tissue engineering and pharmaceutical industries to create 3D microphysiological systems of human organ function. The interest stems from a growing concern that animal models and simple 2D culture systems cannot replicate essential features of human physiology that are critical to predict drug response, or simply to develop new therapeutic strategies to repair or replace damaged organs. Central to human organ function is a microcirculation that not only enhances the rate of nutrient and waste transport by convection, but also provides essential additional physiological functions that can be specific to each organ. This review highlights progress in the creation of in vitro functional microvessel networks, and emphasizes organ-specific functional and structural characteristics that should be considered in the future mimicry of four organ systems that are of primary interest: lung, brain, liver, and muscle (skeletal and cardiac).

Shrink-induced biomimetic wrinkled substrates for functional cardiac cell alignment and culture

The anisotropic alignment of cardiomyocytes in native myocardium tissue is a functional feature that is absent in traditional in vitro cardiac cell culture. Microenvironmental factors cue structural organization of the myocardium, which promotes the mechanical contractile properties and electrophysiological patterns seen in mature cardiomyocytes. Current nano- and microfabrication techniques, such as photolithography, generate simplified cell culture topographies that are not truly representative of the multifaceted and multi-scale fibrils of the cardiac extracellular matrix. In addition, such technologies are costly and require a clean room for fabrication. This chapter offers an easy, fast, robust, and inexpensive fabrication of biomimetic multi-scale wrinkled surfaces through the process of plasma treating and shrinking prestressed thermoplastic. Additionally, this chapter includes techniques for culturing stem cells and their cardiac derivatives on these substrates. Importantly, this wrinkled cell culture platform is compatible with both fluorescence and bright-field imaging; real-time physiological monitoring of CM action potential propagation and contraction properties can elucidate cardiotoxicity drug effects.

Engineered skeletal muscle tissue for soft robotics: fabrication strategies, current applications, and future challenges

Skeletal muscle is a scalable actuator system used throughout nature from the millimeter to meter length scales and over a wide range of frequencies and force regimes. This adaptability has spurred interest in using engineered skeletal muscle to power soft robotics devices and in biotechnology and medical applications. However, the challenges to doing this are similar to those facing the tissue engineering and regenerative medicine fields; specifically, how do we translate our understanding of myogenesis in vivo to the engineering of muscle constructs in vitro to achieve functional integration with devices. To do this researchers are developing a number of ways to engineer the cellular microenvironment to guide skeletal muscle tissue formation. This includes understanding the role of substrate stiffness and the mechanical environment, engineering the spatial organization of biochemical and physical cues to guide muscle alignment, and developing bioreactors for mechanical and electrical conditioning. Examples of engineered skeletal muscle that can potentially be used in soft robotics include 2D cantilever-based skeletal muscle actuators and 3D skeletal muscle tissues engineered using scaffolds or directed self-organization. Integration into devices has led to basic muscle-powered devices such as grippers and pumps as well as more sophisticated muscle-powered soft robots that walk and swim. Looking forward, current, and future challenges include identifying the best source of muscle precursor cells to expand and differentiate into myotubes, replacing cardiomyocytes with skeletal muscle tissue as the bio-actuator of choice for soft robots, and vascularization and innervation to enable control and nourishment of larger muscle tissue constructs.

Physiologically relevant organs on chips

Recent advances in integrating microengineering and tissue engineering have generated promising microengineered physiological models for experimental medicine and pharmaceutical research. Here we review the recent development of microengineered physiological systems, or also known as "ogans-on-chips", that reconstitute the physiologically critical features of specific human tissues and organs and their interactions. This technology uses microengineering approaches to construct organ-specific microenvironments, reconstituting tissue structures, tissue-tissue interactions and interfaces, and dynamic mechanical and biochemical stimuli found in specific organs, to direct cells to assemble into functional tissues. We first discuss microengineering approaches to reproduce the key elements of physiologically important, dynamic mechanical microenvironments, biochemical microenvironments, and microarchitectures of specific tissues and organs in microfluidic cell culture systems. This is followed by examples of microengineered individual organ models that incorporate the key elements of physiological microenvironments into single microfluidic cell culture systems to reproduce organ-level functions. Finally, microengineered multiple organ systems that simulate multiple organ interactions to better represent human physiology, including human responses to drugs, is covered in this review. This emerging organs-on-chips technology has the potential to become an alternative to 2D and 3D cell culture and animal models for experimental medicine, human disease modeling, drug development, and toxicology.

Microfluidic heart on a chip for higher throughput pharmacological studies

We present the design of a higher throughput "heart on a chip" which utilizes a semi-automated fabrication technique to process sub millimeter sized thin film cantilevers of soft elastomers. Anisotropic cardiac microtissues which recapitulate the laminar architecture of the heart ventricle are engineered on these cantilevers. Deflection of these cantilevers, termed Muscular Thin Films (MTFs), during muscle contraction allows calculation of diastolic and systolic stresses generated by the engineered tissues. We also present the design of a reusable one channel fluidic microdevice completely built out of autoclavable materials which incorporates various features required for an optical cardiac contractility assay: metallic base which fits on a heating element for temperature control, transparent top for recording cantilever deformation and embedded electrodes for electrical field stimulation of the tissue. We employ the microdevice to test the positive inotropic effect of isoproterenol on cardiac contractility at dosages ranging from 1 nM to 100 μM. The higher throughput fluidic heart on a chip has applications in testing of cardiac tissues built from rare or expensive cell sources and for integration with other organ mimics. These advances will help alleviate translational barriers for commercial adoption of these technologies by improving the throughput and reproducibility of readout, standardization of the platform and scalability of manufacture.

Micropatterning Alginate Substrates for in vitro Cardiovascular Muscle on a Chip

Soft hydrogels such as alginate are ideal substrates for building muscle in vitro because they have structural and mechanical properties close to the in vivo extracellular matrix (ECM) network. However, hydrogels are generally not amenable to protein adhesion and patterning. Moreover, muscle structures and their underlying ECM are highly anisotropic, and it is imperative that in vitro models recapitulate the structural anisotropy in reconstructed tissues for in vivo relevance due to the tight coupling between sturcture and function in these systems. We present two techniques to create chemical and structural heterogeneities within soft alginate substrates and employ them to engineer anisotropic muscle monolayers: (i) microcontact printing lines of extracellular matrix proteins on flat alginate substrates to guide cellular processes with chemical cues, and (ii) micromolding of alginate surface into grooves and ridges to guide cellular processes with topographical cues. Neonatal rat ventricular myocytes as well as human umbilical artery vascular smooth muscle cells successfully attach to both these micropatterned substrates leading to subsequent formation of anisotropic striated and smooth muscle tissues. Muscular thin film cantilevers cut from these constructs are then employed for functional characterization of engineered muscular tissues. Thus, micropatterned alginate is an ideal substrate for in vitro models of muscle tissue because it facilitates recapitulation of the anisotropic architecture of muscle, mimics the mechanical properties of the ECM microenvironment, and is amenable to evaluation of functional contractile properties.

Microfabricated mammalian organ systems and their integration into models of whole animals and humans

While in vitro cell based systems have been an invaluable tool in biology, they often suffer from a lack of physiological relevance. The discrepancy between the in vitro and in vivo systems has been a bottleneck in drug development process and biological sciences. The recent progress in microtechnology has enabled manipulation of cellular environment at a physiologically relevant length scale, which has led to the development of novel in vitro organ systems, often termed 'organ-on-a-chip' systems. By mimicking the cellular environment of in vivo tissues, various organ-on-a-chip systems have been reported to reproduce target organ functions better than conventional in vitro model systems. Ultimately, these organ-on-a-chip systems will converge into multi-organ 'body-on-a-chip' systems composed of functional tissues that reproduce the dynamics of the whole-body response. Such microscale in vitro systems will open up new possibilities in medical science and in the pharmaceutical industry.