In this paper, we propose in-vessel molecular multiple-input multiple-out (MIMO) communications to overcome the slow transmission rate of conventional molecular communications. Although the flow-assist channel model for MIMO systems has not been well investigated, we show the feasibility of MIMO systems and verify the system through a prototype. In the platform, an acid and a base are utilized to send information by changing the pH value. The testbed shows a higher data rate than single-input single-output (SISO) systems.