Did you know that variable loading conditions at valve closure and opening during AWT studies inadvertently increase development cost through premature failure of valve prosthesis test samples?

Older generation AWT systems generate variable loading conditions at valve closure and opening which inadvertently increase development cost through premature failure of test samples, system management inefficiency and/or increase the duration of fatigue studies.

WHAT IS THE EFFECT ON STROKE VOLUME ADJUSTMENT AND BYPASS VALVE ON CLOSING MOMENTUM (CM)?

The closing speed of the valve leaflets with a full cycle depends on the degree of opening of the bypass and the amplitude of the piston displacement. Assuming that for a given valve, at a given frequency, the peak Differential Pressure, DP, can be kept constant.

If Peak DP = P1(deg, amp) and if V = V1 (deg, amp),

The definition of closing momentum associated with Water-hammer effects, mechanical wear, hemolysis and cavitation potential can be expressed as

Closing momentum = CM1 = P1. V1

Then for constant differential pressure,

P1= α then CM1 =P1.V1 =V1.α

In this case, the closing momentum is linear with leaflet closing velocity.

When the valve is fully opened and the flow starts to reverse in a backflow direction, leaflets slowly start to move from open to close position. As leaflets move to a close position, leaflets continue to accelerate and the leaflet velocity increases, to reach a peak velocity at closure.  Therefore, for a given valve at a given frequency and within a full cycle (open to close position), leaflet velocity is variable, regardless of the mode of control. Thus, for each cycle the leaflet accelerates to reach maximum velocity at closure and the closing momentum, CM, increases proportionally with increasing closing leaflet velocity.

This closing momentum is exacerbated when Peak DP is gradually increased with manual bypass valve adjustments and single pressure control mechanism running in automatic pressure regulation modes. The variable leaflet motion/kinematics generated during AWT studies is a test artifact generated by the design limitations of the pressure control mechanism of older AWT systems.

BASIC DIFFERENTIAL PRESSURE CONTROL TECHNOLOGY GENERATES VARIABLE LOADING CONDITIONS AT VALVE CLOSURE AND OPENING

• Single pressure control mechanism with automatic pressure regulation mode

In older generation systems, AWT units compensate the progressive increase of valve leakage due to wear by automatically increasing the system stroke volume to maintain target pressure difference across the closed valve.  And overtime, during the entire duration of the AWT study, the gradual stroke volume augmentation increases the valve opening position beyond physiological conditions. Larger opened valve positions generated by greater stroke volumes results in greater leaflet trajectories. With longer leaflet trajectories to travel and greater back flow velocities generated by larger stroke volumes, leaflet velocities and stresses at closure are increased.

The gradual strain and stress augmentation resulting from the steady augmentation of system stroke volume throughout AWT studies increases the risk of premature failure and false negatives results of perfectly sound designs.

• Manual bypass valve adjustment

In older generation systems, the bypass valve is manually adjusted by the operator to regulate the differential pressure across the valve and at the same time to mitigate the impact of valve over-opening.  But between manual adjustments, the system automatic pressure control mode continues to regulate the stroke volume which severely affects valve kinematics by gradually increasing valve opening.

Thus, between adjustments, and during the opening phase of the valve, the leaflets are subjected to increasingly higher strains and upon closure leaflets are subjected to higher stress.  The combined effect of gradual strain and stress augmentation throughout AWT studies increases the risk of premature failure and false negatives results of perfectly sound designs.

• Single pressure control mechanism – Deactivation of automatic pressure regulation mode

Many customers choose to deactivate the automatic pressure control mechanism on the stroke volume to avoid overloading valve samples and run AWT systems in manual mode for the entire duration of the study.   Not only this deactivation is done at the expense of operator’s time and efficiency but more importantly it also reduces the number of passing cycles.  In the end, the duration of AWT studies becomes unpredictable, forcing operators to continue the study beyond the initial target number of cycles until obtaining 100% of passing total target cycles.

ADVANCED DIFFERENTIAL PRESSURE TECHNOLOGY GENERATES CONSTANT LOADING CONDITIONS AT VALVE CLOSURE AND OPENING

The ViVitro AWT-V8 System with DCT^TM is a fully software-controlled instrument with a proprietary user-friendly interface to control and maintain AWT test conditions specified in ISO 5840. Unlike older generation AWT systems on the market, the ViVitro Labs AWT-V8 System uses two separate position feedback sensors and algorithms to automatically control the piston displacement and the bypass valve independently for up to 6 independent test modules with pure standalone architecture.

• Differential Pressure Technology with Dual Control

The advanced differential pressure technology of the ViVitro Labs AWT -V8 system with DCT^TM provide a unique opportunity for device manufacturers to mitigate the impact of costly test artifacts generated by older generation AWT systems.

The Dual Control Technology (DCT^TM) of the ViVitro Labs AWT-V8 system protects valve samples from being subjected to variable opening positions during AWT studies by producing repeatable valve kinematics and generating constant loading conditions at valve closure and opening regardless of wear on the leaflet. The advanced differential pressure technology of the ViVitro Labs AWT-V8 system regulates the differential pressure across the valve sample independently of the stroke volume using two separate control mechanisms.

By keeping the stroke volume constant, the loading conditions at valve closure and opening can be kept constant and stable throughout the duration of the AWT study.  Simultaneously, to mitigate the effect of the pressure variation due to valve wear, the bypass valve automatically regulates the differential pressure across the valve with minimal impact on leaflets kinematics.   By controlling stroke volume throughout the duration of the study, the ViVitro Labs AWT-V8 system with DCT^TM locks the driving parameter that affects valve motion and strains experienced by leaflet materials the most. The independent control of the bypass valve optimizes the differential pressure across the test sample with minimal impact on valve motion while optimizing passing cycles. At the end of the AWT study, customers have 100% confidence that cycles have achieved ISO 5840 requirements.