Biomedical Engineering Reference
In-Depth Information
20.1 INTRODUCTION
Biomaterials have been used for many years in the treatment of gastrointestinal conditions. Their
applications range from biodegradable sutures and meshes to assist in the healing of laparotomy
procedures, to the use of alginate-based raft-forming formulations, for the symptomatic treatment
of heartburn and esophagitis. In addition to the continued use and development of traditional thera-
pies, there is a growing interest in the design and fabrication of new biomaterials for a variety of
other therapeutic applications associated with the gastrointestinal tract, such as nanotechnology-
based drug delivery systems and tissue engineering. Before new therapies involving biomaterials
can be successfully implemented, consideration needs to be given to the complexity of the gas-
trointestinal tract and the effect of this complexity on the delivery, longevity, and effi cacy of the
biomaterial.
The gastrointestinal tract in humans (also called the alimentary canal or gut) is a complex system
of organs that carries out vital functions to maintain health. Its primary purpose is to extract energy,
nutrients, and water from ingested food, which is achieved through a process called digestion before
the remaining waste is expelled. In healthy adult humans, the gastrointestinal tract is approximately
7.5-9 m in length, extending from the mouth down to the anus. The gastrointestinal tract has vari-
ous specialized regions within it that aid the different stages of the digestion process and help to
maintain homeostasis. The upper gastrointestinal tract consists of the mouth, pharynx, esophagus,
and stomach. The lower gastrointestinal tract consists of the small intestine (which is subcatego-
rized into duodenum, jejunum, and ileum), the large intestine (which is subcategorized into cecum,
colon, and rectum), and the anus. In addition to these regions, there are a number of related organs
whose functions are integrated with those of the gastrointestinal tract. These organs include the
liver, which secretes bile into the small intestine, and the pancreas, which secretes enzymes into the
small intestine. Thus, both organs assist in the process of digestion. The wall of the gastrointestinal
tract is made up of four concentric layers of tissue. The mucosa is the innermost layer facing the
lumen of the gastrointestinal tract and is composed of the epithelium, lamina propria, and muscu-
laris propria. Villi extending from the mucosal surface signifi cantly increase the absorptive surface
area of the intestine. The mucosa carries out highly specialized functions for each organ of the
gastrointestinal tract, including the processes of absorption and secretion associated with digestion.
It also maintains a barrier function against potentially harmful microorganisms that colonize the
gut. The submucosa is composed of connective tissue and contains blood vessels, lymphatics, and
nerves that extend into the mucosa and musclaris layers. The muscularis externa controls peristalsis
and is composed of a circular inner muscle layer that contracts and prevents backwash of food in
the lumen and a longitudinal outer muscle layer that contracts and shortens the tract. The outermost
layer of the gastrointestinal tract is called the adventitia (or serosa) and is composed of several layers
of connective tissue.
In addition to assisting nutrient absorption, the gastrointestinal tract is also a prominent part
of the immune system, harboring abundant lymphoid tissues and immune cells. This system is
particularly important since the gastrointestinal tract also plays host to a population of microfl ora,
consisting predominantly of bacteria, whose density increases from the upper (almost sterile in the
stomach) to the lower part (10
12
bacteria per gram of colonic content) of the tract [1]. In healthy indi-
viduals, the immune system is programmed to distinguish resident commensal (“good”) bacteria
that colonize and coexist with the gut from harmful pathogenic bacteria that can cause illness. The
immune system provides just one example of the potential problems faced when developing new
devices for the gastrointestinal tract. Biomaterials need to avoid eliciting an immune response or
become affected by other innate components associated with the gastrointestinal immune system,
such as mucus and low pH (1-4) of the stomach, which might also interfere with biomaterials pass-
ing through the tract.
The recognition of biocompatibility as a fundamental requirement for the success of a biomate-
rial has led to the development of a variety of synthetic and modifi ed natural polymers that offer not
