Human Gastric Simulator (Riddet Model)
Maria J. Ferrua and R. Paul Singh
Abstract An in vitro 'dynamic' model for food digestion diagnosis, the Human Gastric Simulator (HGS), has been designed to reproduce the fluid mechanical conditions driving the disintegration and mixing of gastric contents during digestion. The HGS simulates the stomach as a flexible compartment, and mimics its contractive motility by a series of rollers that continuously impinge and compress the compartment wall with increasing amplitude. Operated at 37 °C, the HGS facilitates a precise control of the mechanical forces to which foods are exposed during the process, as well as of the rate of simulated gastric secretions and emptying patterns.
Applications of the HGS have illustrated the need to better understand, and mimic, the fluid mechanic conditions that develop during digestion to improve the performance and reliability of novel in vitro models. To date, the HGS has been used to analyse the digestion behaviour of different foods, and the role of their materials properties on the physicochemical changes that they experience during the process. While the ability of the HGS to reproduce the gastric forces that develop in vivo has been proved, further studies are needed to achieve a thorough validation of its digestive capabilities.
Keywords Human gastric simulator • In vitro model • Digestion • Gastric motility
• Digesta fluid mechanics
Origins of the HGS
Central to the delivery of optimal nutrition, the stomach is, after the mouth, the main site for food disintegration during digestion (Wickham et al. 2012). Once in the stomach, products are stored, digested and progressively emptied into the duodenum by a synergy of physicochemical processes triggered and regulated by the motor and secretory activities of the gastric wall (Barrett and Raybould 2010a; Mayer 1994).
From a functional point of view, the stomach is divided into two main regions. Within the proximal region (upper half), changes in the compliance and secretory activity of the gastric wall allow the stomach to accommodate the ingested meal and provide the biochemical environment needed for its conditioning (Schwizer et al. 2002; Wickham et al. 2012). The distal region, on the other hand, is expected to play a major role in the structural disintegration of the meal. It is within this region where a series of peristaltic antral contraction waves (ACWs) continuously mix, compress and shear gastric contents during the process (Schwizer et al. 2006; Schulze 2006). As a result, food is converted into a semi-liquid mass of partially digested food, whose emptying from the stomach is feedback-regulated by a series at physicochemical receptors within the intestine (Barrett and Raybould 2010b).
Despite the complexities of gastric processes, increasing evidence indicates that the hydrodynamic conditions that develop during digestion have a central role on the material response and subsequent bioavailability of nutrients and bioactive compounds (Dikeman et al. 2006; Lentle and Janssen 2010). In particular, the poor in vitro–in vivo performance of many of the in vitro models currently used for digestion diagnosis has been largely attributed to their inability to reproduce the in vivo mechanics of the gastrointestinal (GI) tract (Yoo and Chen 2006).
Significant efforts have been made during the last decade to better understand the overall functioning of the human stomach and to develop a new generation of in vitro models of enhanced biochemical and mechanical relevance (Boulby et al. 1999; Faas et al. 2002; Kunz et al. 2005; Goetze et al. 2007, 2009; Kwiatek et al. 2006; Marciani et al. 2001a, 2007, 2012; Marciani 2011; Schwizer et al. 2002, 2006; Steingoetter et al. 2005; Treier et al. 2006; Mackie et al. 2013). More notably among those models are the TNO and DGM systems discussed in the previous sections. However, it is noteworthy that there is still no consensus agreement on the way in which these models reproduce the hydrodynamic conditions that develop in vivo, with none of them being able to replicate the actual motility of the gastric wall during digestion.
The Human Gastric Simulator (HGS) was specifically designed and developed by Kong and Singh (2010) to mimic the peristaltic activity of ACWs as reported in vivo (Kwiatek et al. 2006; Schwizer et al. 2006). Aimed at reproducing one of the main features driving the dynamics of gastric contents, this model is expected to better simulate the fluid mechanical forces driving food disintegration during digestion. Since its development, the HGS has been used to investigate not only the physicochemical changes experienced by different food products during digestion, but also the role of gastric motility on the outcomes of the process.