Organ-on-a-chip
By acting as a more sophisticated in vitro approximation of complex tissues than standard cell culture, they provide the potential as an alternative to animal models for drug development and toxin testing.
Organs that have been simulated by microfluidic devices include brain, lung, heart, kidney, liver, prostate, vessel (artery), skin, bone, cartilage and more.
Many aspects of subsequent microphysiometry aim to address these constraints by modeling more sophisticated physiological responses under accurately simulated conditions via microfabrication, microelectronics and microfluidics.
Nevertheless, even the best 3D culture models fail to mimic an organ's cellular properties in many aspects,[6] including tissue-to-tissue interfaces (e.g., epithelium and vascular endothelium), spatiotemporal gradients of chemicals, and the mechanically active microenvironments (e.g. arteries' vasoconstriction and vasodilator responses to temperature differentials).
[8][9] Brain-on-a-chip devices can span multiple levels of complexity in terms of cell culture methodology and can include brain parenchyma and/or blood-brain barrier tissues.
Organotypic brain slices are an in vitro model that replicates in vivo physiology with additional throughput and optical benefits,[8] thus pairing well with microfluidic devices.
[11] Slice-based systems also provide experimental access with precise control of extracellular environments, making it a suitable platform for correlating disease with neuropathological outcomes.
The standard procedure for culturing organotypic brain slices (around 300 microns in thickness) uses semi-porous membranes to create an air-medium interface,[14] but this technique results in diffusion limitations of nutrients and dissolved gases.
[22] The human gut-on-a-chip contains two microchannels that are separated by the flexible porous Extracellular Matrix (ECM)-coated membrane lined by the gut epithelial cells: Caco-2, which has been used extensively as the intestinal barrier.
[28] By inducing suction in the vacuum chambers along both sides of the main cell channel bilayer, cyclic mechanical strain of stretching and relaxing are developed to mimic the gut behaviors.
Dongeun Huh from Wyss Institute for Biologically Inspired Engineering at Harvard describes their fabrication of a system containing two closely apposed microchannels separated by a thin (10 μm) porous flexible membrane made of PDMS.
[42] Another lab-on-a-chip similarly combined a microfluidic network in PDMS with planar microelectrodes, this time to measure extracellular potentials from single adult murine cardiomyocytes.
[43] A reported design of a heart-on-a-chip claims to have built "an efficient means of measuring structure-function relationships in constructs that replicate the hierarchical tissue architectures of laminar cardiac muscle.
The design and fabrication process of this particular microfluidic device entails first covering the edges of a glass surface with tape (or any protective film) such as to contour the substrate's desired shape.
After the cutting of the thin films into two rows with rectangular teeth, and subsequent placement of the whole device in a bath, electrodes stimulate the contraction of the myocytes via a field-stimulation – thus curving the strips/teeth in the MTF.
[50] The neonatal rat micro-engineered cardiac tissues (μECTs) stimulated by this design show improved synchronous beating, proliferation, maturation, and viability compared to the unstimulated control.
[50] The contraction rate of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) is observed to accelerate with 100-fold less isoprenaline, a heart block treatment, when having electrical pacing signal (+ES) compared to that without ES.
For instance, cardiac hypertrophy and fibrosis are studied via the respective biomarker level of the mechanically stimulated μECTs, such as atrial natriuretic peptide (ANP) for the former[53] and transforming growth factor-β (TGF-β) for the latter.
Rapid dissemination and availability of low cost, high resolution 3D printing technology is revolutionizing this space and opening new possibilities for building patient specific heart and cardiovascular systems.
In the device, this section is merely a straight channel, but blood particles going to the filtrate have to cross the previously mentioned membrane and a layer of renal proximal tubule cells.
[72] A study by Emulate researchers assessed advantages of using liver-chips predicting drug-induced liver injury which could reduce the high costs and time needed in drug development workflows/pipelines, sometimes described as the pharmaceutical industry's "productivity crisis".
Future goals for liver-on-a-chip devices focus on recapitulating a more realistic hepatic environment, including reagents in fluids, cell types, extending survival time, etc.
[77][6] PDMS developments have enabled the creation of microfluidic systems that offer the benefit of adjustable topography, gas and liquid exchange, as well as an ease of observation via conventional microscopy.
Axel Gunther from the University of Toronto argues that such MEMS-based devices could potentially help in the assessment of a patient's microvascular status in a clinical setting (personalized medicine).
Finally, the last pair of microchannels is used to provide superfusion flow rates, in order to maintain the physiological and metabolic activity of the organ by delivering a constant sustaining medium over the abluminal wall.
[94] Issues such as detachment of the collagen scaffolding from microchannels,[94] incomplete cellular differentiation,[95] and predominant use of poly(dimethysiloxane) (PDMS) for device fabrication, which has been shown to leach chemicals into biological samples and cannot be mass-produced[96] stymie standardization of a platform.
One study attempted to address this problem by comparing the qualities of collagen scaffolding from three different animal sources: pig skin, rat tail, and duck feet.
[99] Dynamic perfusion may also improve cell viability, demonstrated by placing a commercial skin equivalent in a microfluidic platform that extended the expected lifespan by several weeks.
[103] "The development of the μCCA laid the foundation for a realistic in vitro pharmacokinetic model and provided an integrated biomimetic system for culturing multiple cell types with high fidelity to in vivo situations", claim C. Zhang et al.
The development of MEMS-based biochips that reproduce complex organ-level pathological responses could revolutionize many fields, including toxicology and the developmental process of pharmaceuticals and cosmetics that rely on animal testing and clinical trials.
