Dr. John R. Callahan was a pioneer in the field of dentistry and particularly dental research in the late 19th and early 20th centuries. Born in Hillsboro, Ohio in 1853, Callahan received his dental degree from the Philadelphia Dental College in 1877. He practiced dentistry in San Francisco for two years before returning to Hillsboro where he practiced until 1890. He then moved to Cincinnati, Ohio joining the practice of Dr. C. R. Taft. In 1884 he was selected Secretary of the Ohio State Dental Society and served until 1890. In 1892 he was elected President of the Society and was a member of the Board of Directors from 1894-1918. He served as President of the Cincinnati Dental Society 1906-1907. His major research was in diseases of the dental pulp. He also contributed papers on materials and methods for filling root canals; notably, chloro-percha. His original contribution of the use of sulfuric acid for opening root canals was significant at the time as were his investigations on dental materials and patient management problems. Callahan was a member of the Institute on Dental Research of the National Dental Association. The Jarvie Fellowship Medal was awarded to him in 1917. 

Hillsboro