A Six Degree of Freedom Micromanipulator for Opthalmic Surgery

Needs which have arisen in ophthalmic research have motivated the development of a six degree of freedom parallel micromanipulator. The first application of the tool will be in the treatment of retinal vemus occlusion, for which micron-scale spherical movement of a glass micropipette tip inside the eye is required. The initial speration mode will be open loop while future operation will be in a force-reflecting bilateral (macro-master / micro-slave) arrangement. Presented here are some of the design criteria, the mathematical tools used in evaluating various parallel manipulator designs, and the final kinematic configuration. 1. Background and Motivation An active area of research is the treatment of retinal venous occlusion by delivery of anticoagulant drugs directly to blood clots in retinal vessels [1,2,41. Shortcomings of micro manipulators currently being used in such research have motivated the development of a new tool. In order to better explain the design constraints, some steps which are included in a typical procedure are outlined here. A hypodermic needle is inserted through the wall of the eye to gain access to the interior surface. A rigid glass micropipette is inserted through the hypodermic needle far enough that its tip protrudes beyond the end of the needle. The micropipette is guided to various injection sites on the retina by pivoting the hypodermic needle about its puncture point, that is, the point of intersection between the needle and the wall of the eye. By keeping the needle nearly stationary at the point where it passes into the eye's interior, damage to the wall of the eye is minimized. A third allowable degree of freedom is movement along the axis of the hypodermic needle since it does not deform the wall of the eye at the puncture point significantly. Most commercially available micromanipulators are not useful for guiding the hypodermic needle in the way described, since they exhibit Cartesian movements. The spherical movement needed can be found in some commercially available micromanipulators, and some have been constructed for this type of research [9], but they are circular track serial devices, (figure I), which are mechanically constrained to move on a spherical surface. :igure 1: Schematic of a traditional sphericalmovement micromanipulator. 1050-4729/93 $3.00