The Development and Characterization of a Novel Instrument for Measuring Vaginal Closing Force: EVE the Elevated-Surface Vaginal Elastometer

Abstract

In 2012 there were an estimated 35.3 million people living with HIV [1]. Microbicides address an important gap in HIV prevention for vulnerable groups unable to implement other prevention strategies [2]. Models are being developed in our lab to optimize microbicide delivery vehicle properties so that the microbicide will coat the entire vaginal epithelial surface, stay in place for the duration of possible exposure, and coat thick enough to deliver sufficient active ingredient to prevent infection. A complete model should incorporate vaginal closing force to understand how a delivery vehicle will be distributed and retained in the vagina. However, the physiological magnitudes of vaginal closing forces are not known. Several previous methods have been utilized to determine an appropriate magnitude of one or several components, but they all neglect important features to measure the forces relevant to microbicide delivery vehicle spreading. An ideal measurement device to measure all aspects of vaginal closing force should: be controllable, operate in a variety of modes, have a constant contact area, be able to measure at different places along the vaginal axis and in different directions, be modular, be convenient and easy to operate in a clinical environment, and be safe to operate. This dissertation describes the design and testing of a new instrument to measure vaginal closing force, its calibration process, and the software to control it. Throughout this document the identified obstacles and the strategies used to mitigate them are discussed. Validation testing was performed on tissue phantoms and by bench testing using the calibration instrument. Validation testing shows that the instrument has the ability to differentiate between phantoms. Future testing on more tissue phantoms will allow further quantification of the instrument and a better determination of the precision of the measurements. Two alternate approaches have also been developed for the possible refinement of the EVE instrument. Utilizing force sensing within the probe body would violate the initial design constraints, but might be a relatively simple way to address the issues which have disrupted the instrument’s development. Alternatively a new probe which completely eliminates the hydraulic system in favor of mechanical linkages, although more drastic of a change, might allow for data generation without compromising the initial design requirements. The EVE instrument is a successful step forward in properly measuring vaginal tissue closing force. Many of the initial design challenges have been overcome, and a majority of the programming necessary has been completed. Measurements of phantom tissue elasticity are now possible. EVE is nearly ready for in vivo testing.