  Strain engineering has arisen as a powerful technique to tune the electronic
and optical properties of two-dimensional semiconductors like molybdenum
disulfide (MoS$_2$). Although several theoretical works predicted that biaxial
strain would be more effective than uniaxial strain to tune the band structure
of MoS$_2$, a direct experimental verification is still missing in the
literature. Here we implemented a simple experimental setup that allows to
apply biaxial strain through the bending of a cruciform polymer substrate. We
used the setup to study the effect of biaxial strain on the differential
reflectance spectra of 12 single-layer MoS$_2$ flakes finding a redshift of the
excitonic features at a rate of ~ -75 +- 35 meV/% of biaxial tension. We also
directly compare the effect of biaxial and uniaxial strain on the same
single-layer MoS$_2$ finding that the biaxial strain gauge factor is 2.3 times
larger than the uniaxial strain one.
