This paper deals with the design of metamaterial (MTM) substrates to be used in electromagnetic devices. In particular, the approach has been considered for different investigations having the scope the realization of antennas on flexible substrates. The importance of the topic resides in the potential of conforming the antenna to/on desirable shapes. Flexibility is well exploitable either in advanced communication systems or in biomedical applications, just to mention some. The proposed MTM is made of metallic spherical inclusions of AISI52100, which are embedded in a polymer host. The paper aims to assess the feasibility of increasing the performance of a microstrip patch antenna, and to decrease its size by using the MTM substrate, which is able to locally control the permittivity of the substrate and to create electromagnetic band-gap regions outside of the patch.