Most of us sit down to eat without fear of choking—barring the occasional forkful of spaghetti that goes down the wrong way. But for those who struggle to swallow with ease—a condition known as dysphagia—mealtime can be a life-threatening event. Marco Ramaioli from the University of Surrey in the UK hopes to solve this problem by studying the physics of swallowing and using his findings to design special foods, drinks, and drugs. Ramaioli shared several of his group’s recent findings at the Physics in Food Manufacturing Conference held in early January in Gloucestershire, UK.
“Eating is an important social activity and a huge source of pleasure,” Ramaioli says. “But when people lose the ability to eat, food becomes a source of huge anxiety.” Dysphagia is most common among the elderly, affecting 22% of those above the age of 50, according to studies. The ailment can cause minor nuisances like heartburn, or more serious problems like aspiration pneumonia, a lung infection that develops after inhaling food. Those unable to safely consume normal foods may stop going out for dinner and withdraw from other food-related communal activities, Ramaioli says, which quickly leads to isolation. Sufferers may also limit their intake of nutrients, causing malnutrition and dehydration.
Clinicians commonly help patients with dysphagia by liquifying foods and thickening drinks, which makes them easier and safer to consume. But while this approach has proven effective, finding the right “recipes” for a patient can involve a lot of time-consuming trial and error. And patients may still feel isolated for fear that they won’t have the means to prepare their meals when going out. To make improvements, Ramaioli believes doctors and scientists need a more detailed understanding of why some fluids are easier to swallow than others.
Ramaioli studies the oral phase of swallowing—the part of swallowing that involves the mouth, tongue, and upper throat muscles. During chewing, food and saliva combine to form a ball. Meanwhile, the motion of the tongue triggers a series of reflexes that transport this ball from the front to the back of the mouth and down the throat. To study this process, Ramaioli’s group collaborated with researchers in the food industry to design and build a model of a human mouth and throat out of membranes and rollers. As described in a poster from the conference, this "mouth model" uses the turning of its rollers to simulate the wavelike contractions of the tongue and throat muscles that propel food to the esophagus. The membranes provide a soft surface that mimics the mouth’s palate and holds food in place.
With this setup, the team can add liquids to solid foods and monitor the effect of liquid properties, such as viscosity, on the speed and ease of swallowing. They can also determine whether any fluid is left in the throat after swallowing. This information is useful because a thin fluid “coating” can contribute to an enjoyable eating experience through the aromas it releases. But a thick coating can be dangerous if it’s aspirated into the lungs when the airway opens back up.
In one set of experiments—the first to study ingestion of solid objects in vitro—the team looked at how swallowing a plastic tablet of a fixed volume depends on its shape. Each tablet was suspended in either glycerol or orange juice, spooned into the mouth model, and then gulped down by the membranes and rollers. The researchers found that the mouth model swallowed elongated tablets (long cylinders with rounded ends) 20% faster than spherical or disk-shaped tablets. Ramaioli says that the elongated objects are easier to swallow than spheres or disks of the same volume because of their smaller cross-sectional area when they align with the flow of the surrounding fluid. He hopes that experiments such as these can persuade manufacturers to adopt more palatable pill shapes that could make patients more likely to take their medication.
Swallowing lots of tiny tablets, rather than one big one, can also ease ingestion. Ramaioli and colleagues studied this scenario by suspending micrometer-sized cellulose pellets in either water or a thickened fluid that becomes less viscous when it flows—a property known as shear-thinning that is found in, for instance, xanthan gum. As they expected, the researchers found that suspensions containing large pellets were harder for the mouth model to ingest than those containing small pellets. But they showed they could ease the flow of large pellets down the throat by tailoring the fluid to be thicker or more shear-thinning. In addition, experiments with humans indicated that thicker fluids mask the presence of the pellets, making the fluid easier to swallow. Thicker fluids did, however, have a downside: they were more likely to leave residue on the throat.
Optimizing drug delivery has been the main experimental focus for Ramaioli’s group so far. But he says that their results could also help in designing foods and drinks for dysphagia sufferers—the ultimate goal of his research. For example, he imagines inventing foods that disintegrate into small pieces when placed in the mouth, forming safe-to-swallow particle suspensions. “I want to really understand how to create foods and drinks that can be adapted to different consumers with different needs while preserving…the social ritual of eating,” he says.
Katherine Wright is a Senior Editor of Physics.