# Water Jacket Systems for Temperature Control of Petri Dish Cell Culture Chambers
## 1 Introduction  
Temperature is a major environmental parameter that affects cellular processes, ranging from enzyme activities and trafficking of signaling receptors across the membrane to gene expression, survival, signaling, proliferation, and differentiation of cells [1; 2; 3; 4]. Temperature can also affect several physical properties of cells, including the fluidity and permeability of the plasma membrane, organization of the cytoskeleton, and the morphology and stiffness of cells [5; 6]. Petri dish cell culture chambers are routinely used in biomedical laboratories for various in vitro studies. In particular, Petri dish chambers are useful for monitoring the response of cells using various microscopic techniques. Petri dish chambers are placed on the microscope stage, and are exposed to the surrounding environment, which generally has a temperature of 20-25 \({}^{\circ}\)C. This demonstrates the need for temperature control systems to set the temperature of Petri dish chambers to ensure the cultured cells are maintained at their physiological temperature, i.e., 37 \({}^{\circ}\)C for human cells. Microscopes with integrated incubation chambers have been used to control the temperature of the sample within the Petri dish [7; 8; 9]. Temperature control is facilitated by circulating warm air through a relatively large chamber that encloses the microscope stage. Despite their widespread application, the chamber occupies a large volume, which also limits access to the sample. Temperature control is generally limited to heating, and is associated with long temperature transition periods due to the large volume of the air enclosed within the chamber. These limitations have inspired researchers to introduce alternative methods to control the temperature of samples. These include objective heaters, in which a flexible heating element is wrapped around the microscope objective to heat the air surrounding the Petri dish [10; 11]. Using a similar principle, objective coolers have been introduced, in which water tubes are wrapped around the microscope objective [12; 13; 14]. Despite their simplicity, the distance between the objective and the Petri dish (which depends on the objective magnification) limits the accuracy and results in quick changes in temperature. Infrared heaters, enabled by emitting laser beams into the sample, have also been used to heat the sample [15]. Heating stages are also used for heating the sample [16; 17; 18]. These heaters consist of circular metallic rings with a large aperture in the middle to accommodate the Petri dish and are heated by the Joule heating effect after applying a small voltage. The metallic heating ring can be reconfigured to facilitate heating of multi-well plates. Implementing supplementary equipment to adjust carbon dioxide and humidity levels facilitates miniaturized cell incubation chambers [19; 20; 21]. In comparison, cooling stages, also known as cooling chambers or water jackets, are used for both heating and cooling samples. These heat exchangers consist of circular or square metallic chambers that recirculate temperature-controlled water with an aperture in the middle to insert the Petri dish. The temperature of the recirculating water is controlled using heating elements or Peltier heat sinks. Despite their simplicity and high degree of controllability, the heat transfer across a stationary sample inside a Petri dish is dominated by conduction (diffusion), which leads to relatively large temperature gradients across the sample and long temperature transition periods, as will be comprehensively discussed here. An alternative solution is to provide a constant flow of liquid through the Petri dish to reduce the dominance of diffusion. This can be achieved by inserting a channel-shaped silicon structure (also known as Petri dish perfusion inserts) inside the Petri dish to drive the cell culture medium between the inlet/outlet ports of the channel [22; 23]. In this case, the Petri dish is effectively converted into a perfusion chamber.