Organ-on-chip (OOC) or organ-on-a-chip (OOAC) is a much-hyped field of research. However, it is in fact, still in it’s infancy. In the last 10 years, the development of different organ-on-chip models has exploded with numerous examples of lung-on-a-chip [1,2], liver-on-a-chip , kidney-on-a-chip [4,5], heart-on-a-chip  and so on. But researchers doing the work know that the experiments are not so easy to set up.
Like any field of research, there are still some challenges to overcome and in this field, they are broadly divided into two areas: Biological and Technical. Biological challenges include sourcing induced pluripotent stem cells (iPSCs), appropriate organ scaling, vascularisation of tissues and inclusion of immune components . Here, we will focus on the technical challenges and what Cellix can do to help resolve some of the issues you may experience with organ-on-chip set-ups.
Cellix's Top Tips:
1. Poor tubing connections - a cause for contamination: Despite advances in microfluidics, joining the macro world to the micro-world is still not easy. This is mainly because most organ-on-chip systems are connected to external pumps via tubing and connections with multiple connection points which can be cumbersome to connect and are often a source of contamination due to disconnecting and reconnecting.
Cellix's top tips:
Always wear gloves when assembling and connecting tubing and connectors.
Avoid multiple connection points - this will reduce potential contamination points.
Use autoclaveable tubing and connectors.
Cellix's Kima Tubing Kit is a reusable, autoclaveable 8-way connection from your microfulidic pump to your chip and from your chip to waste. The spacing between the pins is 4.5mm (SBS standard distance between wells on a 384 well-plate) so it is easily integrated with SBS standards including robotics. The 8-way outlet tubing with connectors enables easy collection of culture media thereby reducing manual pipetting and handling - a source of potential contamination.
2. Avoiding Bubbles: avoiding bubbles is still a problem. If they are large and move (travelling through the tubing into your organ-on-chip), they can potentially ruin an entire experiment. They can push out cells that have seeded and are culturing in your chip, washing away your entire experiment.
Cellix's top tip:
Prime (or 'wet') the tubing and inlet pin/connection before connecting to your chip:
Bubbles are usually introduced into the chip at the start of the experiment and can potentially ruin an entire experiment. To avoid bubbles, using your microfluidic pump, pump your liquid (e.g culture media) through the tubing from the sample reservoir (e.g. cell culture bottle) until you see a droplet on the outlet pin of your tubing. This ensures that all the air is pushed out of your tubing and your fluidic path to your chip is "primed". You should also visually check the tubing to ensure there are no bubbles collected inside.
3. Flow rate differences between platforms: Flow control is a crucial element of organ-on-chip experimental set-ups and most organ-on-chips connect to external microfluidic pumps. Pressure pumps, such as Cellix's 4U or UniGo pumps, are a popular choice for organ-on-chip systems as they can facilitate long-term cell culture in microfluidic chips via connection to a large cell culture bottle and they have the flexibility of pumping both liquid and air samples. However, pressure pumps alone will only read the pressure within the organ-on-chip system. So if a microchannel in your organ-on-chip becomes blocked, the resistance changes, and the flow rate will be reduced (or increased if a seal is broken and there is a leak). This means the flow rate and sample volume will be incorrect!
Cellix's top tip:
Use a flow sensor! To ensure precise flow control, connect your pressure pump to a flow sensor and keep the distance between the flow sensor and your organ-on-chip as short as possible. This ensures that the active feedback measured by the flow sensor is measuring exactly what's happening at the chip level - always delivering the correct flow rate and sample volume. Cellix's Flow sensors are calibrated by liquid type (e.g. aqueous flow sensor for culture media) and flow range.
4. Different flow rates required by different organ systems: Like the organs they represent, different organ-on-chip systems have different flow rate requirements. This becomes trickier for models that integrate different organ-on-chip systems, e.g. integration of a lung-on-a-chip system with a heart-on-a-chip system. To model these systems, researchers require pumps that have independent channel control.
Cellix's top tips:
Use a multi-channel system capable of independent channel control.
If you're using a pressure pump, make sure you connect it to a flow sensor!
Cellix's 4U pump can simultaneously and independently control 4-channels giving the user great flexibility.
5. Drug adsorption and binding to PDMS microfluidic chips: PDMS is still the material of choice for many researchers to explore their different models. However, the surface of PDMS is highly hydrophobic, binding many of the drugs and compounds that are introduced into your organ-on-chip system. This can lead to incorrect results for example, for drug concentration curves for your organ-on-chip.
Cellix's top tips:
Coat with BSA: this will block all non-specific binding sites.
Plasma-treat your chip: usually only lasts a short time but should be long enough for your experiment.
Coat with inert polymer.
Dongeun Huh et al. Reconstituting Organ-Level Lung Functions on a Chip. Sience 25 Jun 2010: Vol 328, Issue 5986, pp. 1662-1668. DOI: 10.1126/science.1188302.
Dongeun Huh et al. A Human Disease Model of Drug Toxicity-Induced Pulmonary Edema in a Lung-on-a-Chip Microdevice. Sience Translational Medicine 07 Nov 2012: Vol 4, Issue 159, pp. 159ra147. DOI: 10.1126/scitranslmed.3004249.
Yosuke Nakao et al. Bile canaliculi formation by aligning rat primary hepatocytes in a microfluidic device. Biomicrofluidics 5, 022212 (2011). DOI: 10.1063/1.3580753.
H. C. Huang et al. Enhancement of renal epithelial cell functions through microfluidic-based coculture with adipose-derived stem cells. Tissue Eng Part A, 19 (17-18) (2013), pp. 2024-2034. DOI: 10.1089/ten.tea.2012.0605.
M. Zhou et al. Induction of epithelial-to-mesenchymal transition in proximal tubular epithelial cells on microfluidic devices. Biomaterials, 35(5) (2014), pp. 1390-1401. DOI: 10.1016/j.biomaterials.2013.10.070.
Haitao Liu et al. Heart-on-a-Chip Model with Integrated Extra- and Intracellular Bioelectronics for Monitoring Cardiac Electrophysiology under Acute Hypoxia. Nano Lett. 2020, 20, 4, 2585-2593. DOI: 10.1021/acs.nanolett.0c00076.
Juan Eduardo Sosa-Hernandez et al. Organs-on-a-Chip Module: A Review from the Development and Applications Perspective. Micromachines (Basel). 2018 Oct; 9(10): 536. DOI: 10.3390/mi9100536.
If you're struggling with your organ-on-chip set-up and would like some advice, click here to learn more about Cellix's Microfluidic Solutions for Organ-on-Chip or get in touch with Cellix now- we'd be happy to help!