## 6 Discussion  
### Open-Source Hardware  
Open-source hardware has started to revolutionise wet labs in recent years. Numerous devices have been proposed in order to reduce the costs and make tools accessible [8; 9; 10; 11; 25; 26; 27]. Most of these works are based on a combination of 3D printing and open-source electronics, which reduces the complexity involved in building the devices. While this is a general trend, it creates new advantages and challenges that should be taken into account. A key advantage of open-source hardware is that users can customize their robot to fulfil specific requirements. Previous open-source liquid handling robots were designed to automate only one kind of experiment. These can still be modified by adding sensors or other devices, but this requires significant modifications [9], which in most cases will reduce their liquid handling abilities. In contrast, modifications to EvoBot are made through new modules. EvoBot has a simple software and mechatronic interface. Therefore, it does not require deep knowledge of the robot to interface a new module to it. As an example, one of our computer science students was able to develop a pH meter module just by modifying the design of the dispensing module; see Figure 10. The part that holds the different nozzles has been redesigned to hold a pH meter in a chamber with two connectors. At the bottom connector, a luer tip adapter has been installed to be able to plug a needle to aspirate and dispense liquids. The second connector allows us to connect the chamber to a peristaltic pump. Therefore, the pH can be measured by aspirating the sample through the needle until reaches the chamber with the pH meter. Once the sampled is measured, the robot can discard the sample and rinse the chamber with an electrode filling solution to keep the pH electrode moist. Another interesting advantage of EvoBot's design is that it makes it possible to customize the size of the robot simply by cutting the aluminium profiles to the required lengths; all the remaining partsdo not change. Thus, some users built a small robot to fit inside a fume hood, while others built a robot with a large workspace. One challenge for open source hardware is that it requires a workshop to be able to build the device. We have put effort into making it as easy as possible for end-users to produce and assemble EvoBot. We have avoided complex machining techniques, and the parts can be bought off-the-shelf, or they can be produced with simple rapid prototyping tools such as 3D printers, laser cutters, milling machines, and saws. Most university laboratories and fablabs have them, but the production could also be easily out-sourced if desired. In house production of the custom electronics of EvoBot can be more challenging, but most of the electronics are reused from the 3D printing community. Thus, only two kinds of PCBs should be fabricated. The bare PCBs can be ordered from a manufacturer, and the components can be mounted with a hot air rework station or using an open source reflow oven, built by modifying a toaster oven, as it was in our case. In addition to the workshop, it is also necessary that the person that builds the robot has some skills. All seven EvoBots built so far were assembled at the IT University of Copenhagen by the authors and two research assistants. However, the fact that you need to build your own robot is a challenge for most people without mechatronic skills. In order to tackle this issue, we have provided good documentation in our repositories which includes, among others, a bill of materials and design files, a well-documented building manual, and a wiki with detailed tutorial and information about the robot. Additionally, we have also provided a page on Thingiverse that gives information on how to avoid the most difficult parts by externalizing them ([[https://www.thingiverse.com/thing:2776125](https://www.thingiverse.com/thing:2776125)]([https://www.thingiverse.com/thing:2776125](https://www.thingiverse.com/thing:2776125))).