Provided is the use of a capsule holding a catalyst, such as an enzyme. The capsule has a shell of material that is a supramolecular cross-linked network. The network is formed from a host-guest complexation of a host, such as cucurbituril, and one or more building blocks comprising suitable guest functionality. The complex non-covalently crosslinks the building block and/or non-covalently links the building block to another building block thereby forming the network. The shell of the capsule encapsulates the catalyst. The capsules holding the catalyst are suitable for use as microreactors, and the catalyst can be used as such whilst it is held within the capsule.