Plunger Sub-Assembly with Radially Flexing Prongs for Auto-Injector
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Solution Overview
Problem
Automatic injectors for retractable syringes often fail to provide adequate safety features, such as automatic needle retraction and clear end-of-dose indication, leading to potential needlestick injuries and misuse by self-administering patients.
Innovation Solution
The development of a plunger sub-assembly with a low retraction activation force mechanism, incorporating a compression spring and engagement prongs that radially flex to disengage from the plunger inner, allowing for automatic needle retraction and true end-of-dose indication, integrated into an automatic injector design with a housing, activation mechanism, and syringe cartridge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a compression spring is used to provide retraction force, then the retraction force is sufficient to retract the needle, but the activation force required to initiate retraction is too high
Solution Approach 1:
The engagement prongs are designed to transition from a rigid engaged state during drug delivery to a flexible disengaged state during retraction. The prongs flex radially outward when the plunger outer reaches a specific position, dynamically changing the mechanical engagement to reduce activation force for needle retraction.
Solution Approach 2:
The engagement prongs change their mechanical properties by flexing radially, altering the engagement parameter from locked to disengaged. This parameter change allows the compression spring to release and retract the needle with low activation force, while maintaining high retraction force once initiated.
2Reliability
If the engagement prongs are rigid to maintain structural integrity, then the device is reliable, but the activation force to disengage them is too high
Solution Approach 1:
The engagement prongs transition from a rigid structural element during drug delivery to a flexible component during retraction. This dynamic behavior maintains structural integrity when needed while enabling easy activation when the retraction phase is reached.
3Device complexity
If the plunger outer and plunger inner remain engaged throughout the stroke, then the device is simple, but the needle cannot be automatically retracted
Solution Approach 1:
The engagement between plunger outer and plunger inner is dynamic rather than static. The engagement prongs automatically disengage when the plunger outer reaches the retraction position, enabling automatic needle retraction without complex additional mechanisms.
Solution Approach 2:
The plunger sub-assembly automatically initiates needle retraction through the flexing engagement prongs and compression spring mechanism without requiring external intervention. The system serves itself by using the existing mechanical components to achieve automatic retraction.
4Ease of operation
If the engagement prongs flex radially to disengage, then the activation force is reduced, but the manufacturing precision required is higher
Solution Approach 1:
The engagement prongs are designed with specific geometric parameters that allow controlled radial flexion. By optimizing the prong dimensions and material properties, the design achieves the desired flexibility while maintaining manufacturability and functional reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides enhanced safety by automatically retracting the needle, preventing injuries, and ensuring users are aware when the dose is complete, reducing the force required for retraction and simplifying the device's operation and manufacturing.
Implementation Method 1
a plunger biasing member, which may be a spring such as a compression spring, is retained in a first energized state between the plunger outer and plunger inner
Implementation Method 2
The one or more engagement prongs are capable of flexing substantially radially to release from engagement with the shoulder of the plunger inner to permit the plunger spring to expand
Data Source
AI summary
A low retraction activation force plunger sub-assembly for an automatic injector includes: a plunger outer having one or more engagement prongs, a plunger inner having a shoulder, and a plunger biasing member retained in a first energized state between said plunger outer and plunger inner when the engagement prongs of the plunger outer are releasably engaged with the shoulder of the plunger inner. An automatic injector includes a housing, an activation mechanism, an actuation mechanism, and a syringe cartridge having the plunger sub-assembly and a needle assembly, wherein the actuation mechanism includes comprises an actuation biasing member residing in an initial energized state substantially within an upper portion of an actuation pill. A method of assembling the automatic injector includes the steps of: assembling the plunger sub-assembly and inserting the plunger sub-assembly into the housing such that a proximal end of the plunger sub-assembly contacts the actuation pill.