Direct chemical implementations of CRNs have been theorised and realised, but involve complicated reaction mechanisms (Shin 2011). The most common substrate for chemical kinetics is DNA strand displacement (DSD), which involves the displacement of DNA strands in solution. These strands are labelled with the chemical species and, once the reaction has taken place, an outputting strand represents an output from the CRN that the strand displacement system is trying to emulate. DNA strand displacement has been shown to be a universal substrate for chemical kinetics, specifically for bi-molecular reactions used here (Soloveichik et al. 2010). Most importantly, the AM circuit seen in Fig. 8b has been implemented as a strand displacement device (Lakin et al. 2012). However, a potential difficulty with this approach is scalability: as the number of components increases, the number of chemical species representing them also increases. Large numbers of chemical species result in large numbers of DSD complexes in solution, and consequently crosstalk needs to be accounted for. Fortunately, a recent experiment with the implementation of a square-root circuit in solution provided a new experimental ceiling on the number of species that can be used (Qian and Winfree 2011).