PEN RETENTION APPARATUS

1 . A pen retention device for releasably securing a pen to a tether, the device comprising:

the pen comprising an end and an annular interior surface defining a bore;

a cap comprising an expandable retention interface, the expandable retention interface disposed within the bore proximate the end and comprising a radially outwardly directed surface in contact with the annular interior surface; and

an anvil securedly attached to the tether;

wherein the expanding retention interface further comprises a radially inwardly directed surface in operable communication with the anvil;

wherein the expanding retention interface is responsive to an application of force less than a release force by the tether via the anvil to the radially inwardly directed surface to expand and secure cap to the pen, and

wherein the expanding retention interface is responsive to an application of force greater than the release force by the tether via the anvil to the radially inwardly directed surface to release the cap from the pen.

2 . The pen retention device of claim 1 , wherein the expanding retention interface comprises two or more discrete axially extending members.

3 . The pen retention device of claim 1 , wherein:

the interior surface of the pen further defines one or more radially outwardly directed recesses;

the expanding retention interface further comprises one or more radially outwardly directed protrusions having geometry complementary to geometry of the recesses; and

the protrusions are disposed upon the expanding retention interface at an axial location corresponding to an axial location of the recesses in response to disposal of the retention interface within the bore.

4 . The pen retention device of claim 1 , wherein:

the radially inwardly directed surface of the retention interface is disposed at an angle greater than zero degrees and less than ninety degrees relative to an axial centerline of the retention interface.

5 . (canceled)