Compound adsorption by soft polymer biochips and the consequences on preclinical research
The use of the silicon polymer polydimethylsiloxane (PDMS) for organ chip models has become widespread, despite its high adsorption rates of hydrophobic chemicals. The result are models that are less suitable for proper in vitro adsorption, distribution, metabolism, and elimination (ADME) modeling, important for effective drug development.
In this article, we will discuss common issues observed when working with biochips made of PDMS, most commonly used alternatives, and desired properties for biochip materials. But most importantly, we will present alternative biochips showing low adsorptions rates but also unique biocompatibility for high grade drug screening.
Common materials used for biochips
The chosen biochip material depends on a number of factors, including the functionality of the final device, microfabrication strategy, readouts and biocompatibility (Leung 2022 et al.). Classical materials used in tissue culture such as glass and plastic are typically too hard and not elastic enough, leading to cellular stress impacting physiological tissue growth and behavior. Therefore, soft silicon polymers such as PDMS have become one of the most widely used materials for organ-on-chip platforms. Their low cost and ease of prototyping as well as fabrication have made them a popular choice for commercially available solutions. However, PDMS has significant disadvantages limiting its application for drug testing and in vitro pharmacokinetics/ pharmacodynamics, which led to the development of several alternatives (Campbell 2021 et al.).
Materials used for organ chips or microphysiological systems (MPS):
/ PDMS (polydimethylsiloxane)
/ Thermoplastic polymers
/ Inorganic materials
All the listed materials have their advantages and disadvantages. The decision as to which material is best will depend upon desired functionality, access to fabrication facilities and development stage of the product (Leung 2022 et al.).
In the following chapters, we will discuss evidence from different publications on the deficits of PDMS and properties that are desirable for organ chip materials. Should you want more information on the properties of the listed material, we recommend you visit table 1 in Campbell 2021 et al., which holds a very good comparison of available materials for biochips.
Issues observed when working with biochips made of PDMS
Biochips made of PDMS have distinct properties such as biocompatibility, transparency, gas permeability, porosity, and the ability to adsorb hydrophobic small molecules. Especially the latter two pose a challenge to drug discovery, proteomic analysis, and cell culture applications as it can drastically increase compound adsorption rates and cause a nutritional misbalance (Toepke 2006 et al.).
Another artifact observed in PDMS is that even after prolonged curing times uncrosslinked oligomers are left, that can freely diffuse and are therefore prone to leak into the solution and thus contaminate the tissue culture (Rodrigues 2022 et al.), a process also referred to as leaching. A study by Regehr 2009 et al. even showed that PDMS oligomers were found in the plasma membranes of NMuMG cells (epithelial-like cells from the mammary gland of a mouse).
More specifically, Grant 2021 et al. and Meer 2017 et al. showed that PDMS can significantly impact the adsorption of drugs in organ-on-chip devices. Grant et al. showed that FITC (fluorescein isothiocyanate) a compound similar to amodiaquine, an anti-malarial drug, diffused into the matrix and was adsorbed by the PDMS chip. Meer et al. studied the adsorption rates of cardiac drugs and found that adsorption varied greatly but was highest for bepridil followed by verapamil.
In a wider test, Auner 2019 et al. characterized the loss of compound using 19 chemicals in two different PDMS-surface-to-solution-volume ratios. Their results showed binding of chemicals by PDMS for 8 of the investigated substances, and concluded that binding was strongest for chemicals with high octanol-water partition coefficient (logP) and low H-bond donor number. They went further and measured the depletion and return of chemical from solution over a prolonged timeframe to determine the binding-kinetics. Notably, propiconazole and bisphenol A bound irreversibly and didn’t show any desorption when brought into fresh solvent.
Taken together, those findings highlight that we should critically evaluate the results obtained from organ-on-chip models cultured in biochips made of PDMS. They not only have the potential to alter dose-response curves, but also to strongly impact physiological tissue growth.
What are desired properties for materials used in organ chips?
Clearly, the answer to this question does lie in part in the eye of the observer. From a commercial point of view cost and ease of fabrication/prototyping are clearly important factors. Cost-efficient and simple fabrication procedures enable production on large scale and ultimately could benefit the customer by providing inexpensive consumables. PDMS harbors all those properties but as we have learned is a less than desired material.
Much more than commercial factors, the success of a material for biochips for a researcher is defined by its properties enabling high quality disease modeling and compound screening. Factors to be considered here are (Campbell 2021 et al.):
/ Potential for chemical modifications
/ Low adsorption rates
/ Absence of leaching
/ Potential for cell ingrowth
Oxygen permeability is a factor to consider, but will depend upon the aim of the experiment. Sometimes lack of permeability is desired, if the aim is to study a system under anaerobic conditions.
Additional factors to consider are the properties that enable in depth analysis of the models, such as:
/ Optical clarity
/ Tunable fluorescence
/ Accessibility of the biological samples
Especially, the biocompatibility and reduced adsorption levels are important when it comes to organ models used for clinical research. A biocompatible material poses non-toxic to the cells in the tissue and does not activate immune cells, something that commonly happens with plastic materials. Low adsorption rates enable effective drug testing and realistic dose prediction for the clinical phase of drug development. That said, also the ability to access the sample after cultivation on the biochip is critical to enable downstream analysis. If the organ tissue cultivated on the biochip is destroyed upon removal from the biochip most in depth analysis is hindered.
So where can I find biochips aligning with the aforementioned criteria, you might ask? Read on if you would like to find out.
Superior materials to enable highest in class drug testing
At Dynamic42 we provide biochips made of a unique medical-grade plastics ensuring biocompatibility and low drug adsorption even of very hydrophobic compounds.
We tested Dynamic42 biochip adsorption levels by measuring a variety of hydrophobic compounds and therapeutic antibodies. At first, we measured adsorption levels of propiconazole by our chip material. Propiconazole is a very hydrophobic compound with an octanol-water partition coefficient of 3.72. We perfused the whole system with the compound and collected the remaining solution after 4 and 24h to measure propiconazole levels remaining in the medium. On average, more than 85% of the compound were left in the medium, indicating minimal adsorption of the compound by the chip material (Fig. 1). Strikingly, Auner 2019 et al. showed, that propiconazole was among the compounds that bound irreversibly to PDMS and didn’t show any desorption when brought into fresh solvent.
Further, we also measured adsorption levels for troglitazone, a very hydrophobic compound with a logP of 5.1. After perfusion of the chips for 24h and 48h, there was still 63.29% and 42.88% of trogliazone left in the medium, respectively.
In the following experiments, we took a closer look at the individual components of our perfusion system, testing the chip, two different tubing materials, the reservoir and combined setups. We determined the adsorption levels for acetaminophen (APAP) and tolcapone, with an octanol-water partition coefficients of 2 and 3.3, respectively, after 24 h and 48 h.
While the chip and the reservoir did not show adsorption values higher than 15 % for acetaminophen, larger amounts of the substance were adsorbed by the tubing material (remaining substance after 48h: tubing A 76% vs tubing B 54%). This showed that the tubing material is a decisive component (should you want to find out the exact material used, please do get in touch). Tubing material A proved to be more suitable compared to material B, which is often used in pharmaceutical and medical applications. Particularly for the more hydrophobic substance tolcapone, the amount of adsorbed compound was drastically increased when tubing material B was used (remaining substance after 48h: tubing A 56% vs tubing B 27%).
Lastly, we went on to measure adsorption levels of a variety of therapeutic antibodies by analyzing their binding to our chip material. We perfused the whole system and collected the remaining antibody solution after 24 hours and determined the IgG concentration via IgG ELISA. Compared to the stock solution, we had a remaining antibody concentration of over 70 % to 80 % for IgG isotype controls and 100 % of our therapeutic antibodies (Fig.4). This confirms that therapeutic antibodies do not tend to stick to our biochip material and will remain in the system.
That said, low adsorption level of hydrophobic compounds is just one of the attributes Dynamic42 biochips offer. Additionally, they come in a microscopic slide format and are therefore adapted to work with standard cell culture methods. Each biochip comprises two individual cavities (2 chips in one) which are subdivided by a porous polymer membrane into a top and bottom channel. The membrane is tightly sealed into the cavities and customizable in terms of polymer material, pore size and pore density. The Dynamic42 biochip connects via standard luer ports to perfusion tubing and pump equipment. There are different channel and cavity dimensions available, supporting volumes from 55 µl to 140 µl. Furthermore, the design of the channels makes removal of air bubbles easy, rendering the system ideal for beginners. Additionally, the two individual cavities on the chip can be connected, to create multi-organ-chips, to study crosstalk between organs enabling a more realistic dose prediction for clinical trials.
In summary, the results of our measurements and aforementioned attributes render our biochip a very user-friendly but foremost ideal system for disease modelling and substance screening.
The widespread use of PDMS for commercial biochips poses a large challenge to preclinical research. PDMS has been shown in numerous publications to have high adsorption rates for compounds with high octanol-water partition coefficient and low H-bond donor number. The result are models that aren’t equipped for proper in vitro ADME modeling, important for effective drug development. While PDMS is greatly suitable for rapid prototyping, it is less favorable as a reliable and easy system for preclinical testing. Organ Chips that enable effective and physiological preclinical research use materials that are biocompatible, have low adsorption rates and don’t show leaching. At Dynamic42 we provide biochips made of a unique medical-grade material ensuring biocompatibility and low drug adsorption even of very hydrophobic compounds such as trogliazone and propiconazole.
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