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What is Which of the following material form hard tissue barrier? and explain in detail?
Two composite resin-based materials, Retroplast and Geristore have been advocated for use as root-end filling materials.
There is evidence that Retroplast promotes hard-tissue formation at the root apex, and some have suggested that this is a form of cementum. 
In a limited number of case reports, Retroplast root-end fillings have demonstrated regeneration of the periodontium with a cementum layer over the root-end restoration.
Retroplast is a dentin-bonding composite resin system developed in 1984 specifically for use as a root-end filling material.
The formulation was changed in 1990, when the silver was replaced with ytterbium trifluoride and ferric oxide. Retroplast is a two-paste system that forms a dual-cure composite resin when mixed.