As noted in this project’s research paper, “Mutual capacitance-based multi-touch sensing is now a ubiquitous and high-fidelity input technology. However, due to the complexity of electrical and signal processing requirements, it remains very challenging to create interface prototypes with custom-designed multi-touch input surfaces.”
In other words, while you might use a single capacitive sensor to replace a button in your next Arduino Project, creating a complex matrix such as the one used to interface with a smartphone is generally out of reach for those without extensive electrical engineering knowledge.
For the rest of us, researchers across several universities have been working on the amply named: "Multi-Touch Kit." This technique allows makers to use an Arduino boards or other similar hardware to take in multi-touch capacitive signals on sensing grids made with a variety of materials, and of different sizes and even shapes.
An Arduino firmware library, as well as a Processing library and an API form the “tools” used, along with the OpenCV library to determine finger positions. These techniques have been tested in various setups — a conductive 16x16 electrode matrix, measuring in at 177x177mm; on conductive yarn; and even on electrodes arranged on a 3D-printed rabbit. Hardware used includes an Arduino Uno, Mega, and LillyPad.