Fundamentals of the gut for capsule engineers

The gastrointestinal (GI) tract is a complex environment comprised of the mouth, esophagus, stomach, small and large intestines, rectum and anus, which all cooperate to form the complete working GI system. Access to the GI using endoscopy has been augmented over the past several decades by swallowable diagnostic electromechanical devices, such as pill cameras. Research continues today and into the foreseeable future on new and more capable miniature devices for the purposes of systemic drug delivery, therapy, tissue biopsy, microbiome sampling, and a host of other novel ground-breaking applications. The purpose of this review is to provide engineers in this field a comprehensive reference manual of the GI environment and its complex physical, biological, and chemical characteristics so they can more quickly understand the constraints and challenges associated with developing devices for the GI space. To accomplish this, the work reviews and summarizes a broad spectrum of literature covering the main anatomical and physiological properties of the GI tract that are pertinent to successful development and operation of an electromechanical device. Each organ in the GI is discussed in this context, including the main mechanisms of digestion, chemical and mechanical processes that could impact devices, and GI motor behavior and resultant forces that may be experienced by objects as they move through the environment of the gut.