Design of a Biomedical Device Through Non Linear Analysis
Authors: Baglioni Stefano , Braccesi Claudio , Cianetti Filippo , Ficola Antonio , Anile Carmelo 
Published: March 2016

The most widely accepted hypothesis to explain normal pressure hydrocephalus (NPH) points at the increase of cerebrospinal fluid (CSF) outflow resistance as the fundamental cause. Some clinical and experimental studies do not agree with this hypothesis and suggest that NPH is related to an alteration of the CSF pulse pressure waveform, while intracranial pressure (ICP) mean value has negligible effects.

The current treatment of hydrocephalus is based on the first hypothesis and consists in the implantation of CSF shunts. An improved treatment can be obtained by damping the ICP pressure peaks and keeping unchanged the mean value.

The target of this work is to design a special ICP regulator valve, that will be implanted in a human body and that must be characterized by a purely mechanical working principle avoiding any electrical equipment (sensors, actuators...).

This device is currently patented [1] and in virtue of that the paper will focus only on the general device working principle and design methodology rather than specific data.

Since the device must be implanted inside the patient head, the system must satisfy very restrictive requirements: low weight and dimensions in order to avoid possible patient discomfort or obstacles to the normal life activities, in addition, being the valve application place close to a delicate organ such the brain is, the mechanism must be very simple and must reach very high reliability standards (almost zero maintenance and possible failures).

The idea is to realize a device in which the hydraulic flow is governed by a spring with variable stiffness with respect to the applied loads (intracranial pressure: characterized by both a mean constant component and by random oscillatory phenomenon).

To maximize the valve effect about pressure peaks reduction, the spring will be designed with a strongly non-linear behavior characterized by bistable working principle.

The systems that show this properties are innumerable, but according to the author hypothesis to realize a mechanism as simpler as possible the choice done falls into the thin curved plate (shell) category.

In particular, the goal is to obtain a plate behavior called “Buckling Behavior”: under determined load conditions the plate geometric configuration must suddenly switch from an equilibrium position to another. The two target parameters which describe this phenomenon are the buckling critical load that is the applied load value for which the plate change the geometric configuration (valve activation point) and the load application point displacement (evacuation pipe opening).

The adopted design method is the non-linear analysis developed in a finite element analysis (F.E.A.) environment, by which it is possible to analyze a component behavior also in case of large displacements.

To identify the optimal component geometry the load application point displacement versus the acting load was evaluated as function of the main parameters describing the plate profile: plate semi-length, curvature radius and semi-length of the plate plane portion.

This work represents only a preliminary study oriented to demonstrate the feasibility in realizing a biomedical valve for fluids pressure control, adopting a thin curved plate with “Buckling Behavior”. Moreover it provides useful information for the designer who wants to realize curved plate with buckling behavior showing the influence of the main geometric parameters on this phenomenon.

Further in depth studies oriented to: the spring stiffness regulation for different patients, best material choice and productive process must be accomplished before the device realization.