Exploring the role of rheology in bolus flow using an in-vitro approach
Doctoral thesis, 2019

Swallowing disorders, termed ‘dysphagia’, are more common in the elderly but can also affect younger persons. Approximately, 8% of the world’s population suffers from dysphagia. A Texture Modified Diet (TMD), which increases bolus viscosity to adjust for the sluggish bolus handling mechanism, is the most common intervention. Other rheological properties, such as bolus elasticity, shear rate, and yield stress, are often ignored. TMDs, which often comprise gum- and starch-based thickeners, were characterised for their rheological properties. The gum-based thickeners were considerably more elastic and exhibited a mild yield stress, showing a fine-stranded network structure on the nanoscale length when visualised by electron microscopy, as compared to the starch-based thickeners. Among the rheological properties, elasticity is cited as the most important for safe swallowing. Therefore, an in vivo study was performed in which fluids that have elastic properties were assessed for efficacy of safe swallowing in patients with dysphagia. These fluids showed easy swallowability, in terms of the sensory response and transit times during the oral and pharyngeal stages. While clinical examination is the standard and most appropriate way to diagnose dysphagia, difficulties arise in relation to the use of contrast media, ethics, and patient discomfort. To overcome these difficulties and to reduce the frequency of clinical analysis, an in vitro approach was adopted. And an in vitro swallowing model was developed that can be used to perform experimental bolus visualisation and manometry, mimicking the in vivo counterpart of video fluoroscopy and manometry. To study bolus transport, Pulsed Ultrasound Doppler Velocimetry was used. Pressure sensors were embedded in the model pharynx body to measure the bolus pressure during transit. The device delivers the bolus, at an appropriate speed and volume, and can handle boluses with different consistencies. In the device, bolus velocities ranging from 0.04 m/s to 0.48 m/s were measured, while bolus consistency was varied from nectar-thick to pudding-thick following National Dysphagia Diet scale. These velocities are within the range that is often reported for in vivo experiments. The acquired velocities yielded shear rates in the range of 13–229 s-1, which is both lower and mostly higher shear rates than 50s-1 commonly referred to for deglutition. Similarly, when gum and starch-thickened boluses were injected into the model pharynx, the starch-thickened bolus often disintegrated, leaving residues in the model pharynx. When thickened boluses were analysed using manometry, by varying the bolus composition, shape, and volume, the pressure values at different locations in the model pharynx were in the range often noticed in clinical assessments. The device can simulate abnormal swallowing conditions, such as delayed epiglottis and Upper Esophageal Sphincter (UES) closure. Simulations of abnormal UES conditions, i.e. reduced UES area, yielded different pressure values in the lower pharynx. Therefore, the device can be used as a pre-clinical study tool to elucidate the relationship between bolus rheology and deglutition.

extensional rheology

ultrasound viscometery

Bolus rheology

in vivo analysis

in vitro manometry

In-vitro simulations


microstructure analysis

thickened fluids


Hörsalsvagen 7
Opponent: Professor Peter Fishcher, ETH Zurich, Switzerland


Muhammad Waqas

Chalmers, Industrial and Materials Science, Engineering Materials

Eating and drinking are one of the most enjoyable acts we perform in our daily life. During the process, the senses experience different flavors and aromas of the food and drinks. At the same time, we acquire nutrients such as proteins, carbohydrates, fats, minerals and vitamins from the complex structure of the food material which enable us to perform our daily tasks. In certain situations, people cannot eat and swallow food in a normal way. E.g. in the older age or in the case of neurological impairments such as Parkinson’s disease, dementia or stroke. This condition is called dysphagia and is estimated to influence 8% of the Swedish and world population.

People suffering from dysphagia cannot swallow low viscosity fluids. Commonly the mechanism that closes the airways during food/drinks transport is slower, which might result in liquids of low consistency leaking into the lungs causing pulmonary pneumonia. Therefore, the liquids need to be thickened with thickening powders that are commercially available to increase their viscosity and therefore make them travel slower during swallowing. This enables the slow responding swallowing mechanism of the patient to adapt and direct food towards the stomach. Having knowledge of the consistency and flow properties of these thickening powders is vital to ensure people suffering from dysphagia are treated properly. Traditionally patients suffering from severe dysphagia are diagnosed in clinical studies using x-rays or inserting a tube with pressure sensors inside the patients swallowing tract. These practices are cumbersome and possess ethical issues.

In this project, the properties of thickened foods used in dysphagia management were studied using an in-vitro approach. A simulator has been built that performs swallowing analysis. The simulator performs the in-vivo equivalent of bolus visualization using ultrasonics and manometry assessments using pressure sensors. The simulator can form boluses of different volumes and handles viscosities of different levels. The simulator can deliver the bolus into the model swallowing tract with the fixed speed similar to how the human tongues pushes to bolus into the swallowing tract. 

The results from the in-vitro simulator showed fluids with different consistencies flows with different speeds and recorded different pressures in different locations in the simulator and it was noted that some consistencies such as liquids with springy properties are safer to swallow. The recorded bolus velocities and pressure values were in the range that is often reported in clinical examinations. The simulator is therefore ideal for studying flow properties of food and drinks that are made for the patients suffering from dysphagia. This project will hopefully reduce the frequency of clinical examinations which are expensive and which leads to patients discomfort.

Subject Categories

Applied Mechanics

Other Medical Engineering

Areas of Advance

Life Science Engineering (2010-2018)

Materials Science



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4528



Hörsalsvagen 7

Opponent: Professor Peter Fishcher, ETH Zurich, Switzerland

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