  Oocyte retrieval forms a crucial part of in vitro fertilisation treatment and
its ultimate outcome. Standard double-lumen needles, which include a sequence
of aspiration and flushing steps, are characterised by a similar success rate
to single-lumen needles, despite their increased cost. A novel
hydrodynamics-based needle called the OxIVF needle is proposed here, which is
geared towards the generation of an internal flow field within the full
follicular volume via laterally, rather than frontally, oriented flushing,
leading to successful retrievals with no additional stress on the oocyte. A
two-dimensional digital twin of the follicular environment is created and
tested via multi-phase flow direct numerical simulation. Oocyte initial
location within the follicle is varied, while quantities of interest such as
velocity magnitude and vorticity are measured with a high level of precision.
This provides insight into the overall fluid motion, as well as the trajectory
and stresses experienced by the oocyte. A comparative benchmark set of tests
indicated a higher success rate of the OxIVF needle of up to 100%, marking a
significant improvement over the traditional double-lumen design whose success
rate of no more than 75% was also highly dependent on the location of the
needle tip inside the follicle. All forces measured during these tests showcase
how the oocyte experiences stresses which are no larger than at the aspiration
point, with the flow field providing a gentle steering effect towards the
extraction region. Finally, the flow generation strategy maximises oocyte
yield, unlocking new capabilities in both human and veterinary contexts.
