Piston Centering Elements for Drug Delivery Dosing Accuracy
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Solution Overview
Problem
Drug delivery devices, particularly pen-type injectors, face challenges in ensuring accurate and centralized force transmission to the piston, leading to potential dosing inaccuracies due to manufacturing and assembly tolerances, which can result in piston tilting and increased displacement force requirements.
Innovation Solution
The design incorporates a plunger with a circular or annular receptacle and a piston featuring centering elements arranged in a circular structure, ensuring mutual alignment and even thrust distribution across the piston's thrust receiving surface, compensating for radial offsets and reducing point stresses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional plunger-piston interface is used without centering elements, then the device structure remains simple, but manufacturing and assembly tolerances cause radial offsets leading to non-central force transmission, piston tilting, and dosing inaccuracies
Solution Approach 1:
The patent employs distance elements (protrusions) on the piston's thrust receiving surface that are designed to be compressed during operation. These elements temporarily deform to accommodate radial offsets between the plunger and piston, then recover to maintain proper spacing. This allows the system to tolerate manufacturing tolerances while maintaining dosing accuracy, effectively discarding the need for tight tolerances and recovering the functional relationship through elastic deformation.
Solution Approach 2:
The centering elements (grooves in the plunger, protrusions on the piston) act as intermediary features that mediate the interface between plunger and piston. These elements guide the relative positioning and ensure centralized force transmission by providing geometric constraints that compensate for radial offsets, thereby improving dosing accuracy without requiring high-precision manufacturing throughout the entire assembly.
2Reliability
If distance elements protrude from the thrust receiving surface to prevent piston adhesion, then mutual adhering is prevented, but thrust application becomes concentrated on these protrusions causing point stresses and reduced dosing accuracy
Solution Approach 1:
The distance elements are designed as compressible protrusions that temporarily deform under thrust load. They discard their rigid shape during force application to distribute stress, then recover their original shape when the load is removed. This elastic behavior allows them to prevent adhesion while avoiding permanent deformation and maintaining dosing accuracy over repeated use cycles.
Solution Approach 2:
The patent changes the mechanical parameter of the distance elements from rigid to elastic/compressible. By making these elements capable of deformation, the system transforms the stress distribution from concentrated point loads to distributed forces across the thrust receiving surface, thereby maintaining both adhesion prevention and dosing accuracy.
3Ease of manufacture
If radial offsets between plunger and piston occur due to tolerances, then assembly is easier with standard tolerances, but the displacement force required increases substantially and dosing accuracy decreases
Solution Approach 1:
The compressible distance elements serve as a mechanical compliance feature that absorbs radial misalignment through elastic deformation. Instead of requiring precise alignment (which would increase manufacturing difficulty), the system allows offsets and recovers proper force transmission geometry through the compression and rebound of these elements, maintaining acceptable displacement forces despite manufacturing variations.
Solution Approach 2:
The distance elements pre-compress when radial offsets occur, providing a cushioning effect that prevents hard impacts and excessive force requirements. This beforehand cushioning through elastic elements protects the system from force spikes that would otherwise result from direct rigid contact between misaligned plunger and piston surfaces.
4Adaptability or versatility
If the piston is made flexibly deformable to accommodate misalignment, then some tolerance compensation is possible, but excessive flexibility causes piston tilting and reduces dosing accuracy
Solution Approach 1:
The distance elements provide controlled, localized flexibility through elastic compression rather than overall piston flexibility. They discard rigid geometry temporarily to accommodate misalignment, then recover to maintain proper piston orientation. This localized compliance allows tolerance compensation without causing the piston body to tilt or deform excessively, preserving dosing accuracy.
Solution Approach 2:
The patent applies flexibility locally at the distance elements rather than making the entire piston flexible. This localized quality change allows the specific regions needing compliance (the distance elements) to deform while the piston body maintains sufficient rigidity to prevent tilting and ensure accurate dosing.
Data Source
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AI summary
The present invention relates to a piston for a cartridge (24) for a drug delivery device comprising: at least one annular sealing surface (35) to radially abut against an inner side wall (24) of a cartridge (33), a distal surface (42) to confine a drug receiving volume of the cartridge (33), a thrust receiving surface (40) adapted to receive a thrust exerting plunger (10, 32) of a drug delivery device for displacing the piston in a proximal direction relative to the side wall (24), - wherein the thrust receiving surface (40) comprises numerous centering elements (34) protruding from the thrust receiving surface (40) towards the plunger (10, 32) and wherein the centering elements (34) comprise an outer shape and geometry that matches with a corresponding receptacle (38) of the plunger (10, 32), and wherein the centering elements (34) are arranged and aligned on the circumference of an imaginary annular or circular structure.