Hollow Core Microstructured Optical Fibers (HCMOFs) enable single mode guidance with large mode field diameters. HCMOFs can be applied for chemical sensing by filling the hollow core with appropriate analytes. We demonstrate preparation approaches for square shaped and hexagonal HCMOFs with a core diameter up to 30 µm. The prepared HCMOFs show a minimum loss of 3 dB/m and effective single mode propagation in the wavelength range 270 nm – 1500 nm. The HCMOFs are manufactured with very thin web bridges, typically 300 nm – 340 nm. We report on a preparation technique without supporting tubes to manufacture extremely thin bridges. The key is an intermediate cane drawing step. We show that the composition of the gas inside the preform cavities influences strongly the composition profile of the glass bridges by diffusion effects.