  We investigate the effect of 2-dimensional (in-plane) strain on the critical
transition temperature TH from the photoexcited hidden state in 1T-TaS$_2$ thin
films on different substrates. We also measure the effect of in-plane strain on
the transition temperature $T_{c2}$ between the nearly commensurate
charge-density wave state and the commensurate state near 200 K. In each case,
the strain is caused by the differential contraction of the sample and the
substrate, and ranges from 0.5 % compressive strain (CaF$_2$) to 2 % tensile
strain (sapphire). Strain appears to have an opposite effect on the H state and
the NC-C state transitions. TH shows a large and negative strain coefficient of
dT$_H$/de = - 8900+/-500 K, while $T_{c2}$ is not strongly affected by tensile
strain and shows a positive coefficient for compressive strain, which is
opposite to the effect observed for hydrostatic pressure.
