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Pathologies affecting the small intestine contribute significantly to the disease burden of both the developing and the developed world, which has motivated investigation into the disease mechanisms through in vitro models. Although existing in vitro models recapitulate selected features of the intestine, various important aspects have often been isolated or omitted due to the anatomical and physiological complexity. The small intestine's intricate microanatomy, heterogeneous cell populations, steep oxygen gradients, microbiota, and intestinal wall contractions are often not included in in vitro experimental models of the small intestine, despite their importance in both intestinal biology and pathology. Known and unknown interdependencies between various physiological aspects necessitate more complex in vitro models. Microfluidic technology has made it possible to mimic the dynamic mechanical environment, signaling gradients, and other important aspects of small intestinal biology. This review presents an overview of the complexity of small intestinal anatomy and bioengineered models that recapitulate some of these physiological aspects. Keywords: gut-on-a-chip, microfluidic models of intestine, small intestine model, tissue engineered intestine Impact Statement There is a need for improved cell culture models of the small intestine. To develop such models, it is necessary to give engineers a better understanding of intestinal biology, and conversely, to give biologists an idea of what is possible with microfluidic devices and biomaterial platforms. This work provides a detailed overview of aspects of intestinal biology that are important for tissue engineers to understand when designing models of the small intestine and provides a summary of current approaches. This review highlights both unique and common features of various strategies for modeling the intestinal cell environment, not only using microfluidic chip-based systems but also several elegant designs that use a materials-based static culture approach.

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Modelling organ-blood barriers through the inclusion of microvessel networks within in vitro tissue models could lead to more physiologically accurate results, especially since organ-blood barriers are crucial to the normal function, drug transport, and disease states of vascularized organs. Microvessel networks are difficult to form, since they push the practical limit of most fabrication methods, and it is difficult to coax vascular cells to self-assemble into structures larger than capillaries. Here we present a method for rapidly forming networks of microvessel-like structures using sacrificial, alginate structures. Specifically, we encapsulated endothelial cells within short alginate threads, then embedded them in collagen gel. Following enzymatic degradation of the alginate, the collagen gel contained a network of hollow channels seeded with cells, all surrounding a perfusable central channel. This method uses a 3D printed coaxial extruder and syringe pumps to generate short threads in a way that is repeatable and easily transferrable to other labs. The cell-laden, sacrificial alginate threads can be frozen after fabrication and thawed before embedding without significant loss of cell viability. The ability to freeze the threads enables future scale up and ease of use. Within microfluidic devices that restrict access to media, the threads enhance cell survival under static conditions. These results indicate the potential for use of this method in a range of tissue engineering applications.