In 1994 he successfully completed his MD thesis - tutored by M. Reim - on Examinations on cornea and conjunctiva after alkali burns by means of scanning electron microscopy and energy-dispersive of x-ray analysis.
During this period he also was associated as an honorary scientist to the Institute of Biomedical Optics (former Medical Laser Center) of the University of Lübeck.
From 1997 until 2000 (funded by the DFG and Special Trustees) and in 2003 he worked at Moorfields Eye Hospital and the associated Institute of Ophthalmology, University College London receiving subspecialist training from John KG Dart, J Richard O Collin and Geoffrey E Rose in corneal and oculoplastic surgery.
[3] Geerling's clinical and scientific interests and activities are focused on natural tear substitutes, tissue constructs for ocular surface reconstruction and regeneration as well as new transplantation techniques for the treatment of corneal and ocular surface disorders, in particular dry eye, Corneal diasease and keratectasias.
He was one of the first to introduce modern lamellar corneal graft techniques in Germany and intraoperative optical coherence tomography in Ophthalmology.