In this thesis organs-on-chips were studied which contain micrometer-sized fluid-filled channels. ‘Herein various human cells can be cultured,’ says Marinke van der Helm. ‘Thus, a controlled environment is created that resembles the microenvironment of a certain organ. I focused on two of them: on blood-brain barrier (BBB) and on gut tissue.’
In both systems, drug delivery strategies face (often) one cell thick barriers hampering successful drug uptake on the spot. ‘Especially, in the brain, for treating Parkinson’s and Alzheimer’s disease, this problem is current and relevant,’ Marinke shares.
The main focus was on developing and understanding electrical measurements of barrier function. Using Matlab software as a numerical computing environment, Marinke was able to build and run simulations. In this way, valuable insights were gained into these electrical measurements currently used on cell-cultured chips. ‘One of the more surprising findings was, that we could relate experimental and simulation results back towards the morphology of the living cells under study,’ she says.
‘During the project I felt no real stress. Within the BIOS Lab on a Chip Group my supervisors and colleagues excellently helped me: discussing and interpreting the results, identifying the progress made, and coming up with ideas on how to proceed. During the PhD project, I grew as an independent researcher, able to make a successful research strategy by myself.
At first, in this PhD work, several general challenges for barrier tissues-on-chips were identified. Transendothelial / transepithelial electrical resistance (TEER) measurements, always play a crucial role in investigating barrier-forming tissues within organ-on-a-chip devices, Marinke explains.
‘However, the exact principles underlying the measurement procedures are still poorly studied. Standardization and higher throughput strategies are requisite, in order to achieve a widely accepted research tool. By tuning mechanical, biochemical and geometrical aspects, organs-on-chips hold great promise, to refine current in vitro methods and, partly, to replace animal testing. ’
Marinke described new research strategies for organs-on-chips, such as several ways to arrive at predictive TEER measurements. ‘We proposed a simple and universally applicable method, to directly determine the TEER in various microfluidic organs-on-chips,’ she says.
Electrodes were used, integrated into both the apical and basal channels outside of the culture area, without the need for a cleanroom environment. Using four electrodes and six different measurement configurations, the TEER was directly derived, independent of channel resistance properties. ‘Though its applicability is shown in a BBB-on-chip, this simple and robust method is also applicable to any other organ-on-chip device with two channels separated by a porous membrane,’ Marinke says.
Further, simulations were presented to correct for the effects of device geometry on the measurements. Here, TEER measurements were simulated in a gut-on-a-chip with long and shallow channels, in which two electrodes were inserted. ‘The model was validated by comparing the corrected TEER values obtained in a microfluidic gut-on-a-chip to the current standard, a so-called Transwell culture system,’ Marinke says.
Following an extensive research strategy of computer simulations and experiments, finally it was shown that impedance spectroscopy correctly predicted villi differentiation of gut tissue. ‘This strategy resulted into a tool to monitor villi differentiation without the need for microscopy,’ Marinke says. ‘The route towards this result gradually developed. We focussed more and more on this topic, betting on extra horses so to say. This result is the most rewarding for me.’
Lastly, in Chapter 6, first results were presented of a multiplexed organ-on-chip. This chip comprised eight parallel channels that could be addressed at once via a common access port, for cell seeding and parallelized culture under identical environmental conditions without the need for extra pipetting steps.
‘This extra line within my PhD work is highly relevant for increased throughput research strategies in the future,’ Marinke says. ‘I enjoyed working on this topic with the help of master students, who did an excellent job on creatively designing, fabricating and performing first experiments. Working with them always provides fresh views and new ideas. For example within the standard fabrication protocols I used, they came up with clever tricks for improvement, thus contributing directly to the progress of research.’
After her PhD Defence, Marinke will be working at Groene Hart Hospital in Gouda and Erasmus Medical Centre in Rotterdam as a clinical chemist in training.
‘Following a training path, I intend to develop myself as a medical specialist,’ Marinke says. ‘Using my expertise with organs-on-chips research and my technical background, I hope to be of extra value in the clinical chemistry laboratory and in patient care.‘
‘Perhaps one day new diagnosis or treatment strategies might result from this research. During the PhD project I have collaborated with other groups that were interested in my research approach: such as the Paulusse Research Group (in delivering nano-encapsulated medicine into brain environments) and the Nanobiophysics Group (to develop new medicine strategies in treating Alzheimer’s disease).’