#### Vol. 6, No. 1-4, 2011

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Deep penetration and liquid injection into adipose tissue

### Kerstyn Comley and Norman Fleck

Vol. 6 (2011), No. 1-4, 127–140
##### Abstract

The subcutaneous injection of porcine adipose tissue by a hypodermic needle involves two stages: tissue penetration followed by the delivery of liquid into the tissue. The force required to penetrate adipose tissue by a series of conically tipped and flat-bottomed circular punches has been measured. Scanning electron microscopy and light microscopy are used to observe the mechanism of crack formation during penetration. The experiments reveal that penetration by either a flat bottomed or $4{5}^{\circ }$ conically tipped punch involves the formation of a mode II ring crack. The predicted penetration pressure according to the Shergold–Fleck model (Proc. R. Soc. Lond. A 460 (2004), 3037–3058) is in good agreement with the measured pressure on the punch. The subsequent delivery of liquid into adipose tissue by the hypodermic needle has also been examined: the injection pressure for phosphate buffered saline has been measured for a range of flow rates. X-ray images of the injected liquid suggest that micro-cracks are formed by the fluid pressure within the tissue and this leads to an increase in permeability. A seepage model is developed, based on the Darcy flow law, to relate the volumetric flow rate to the injection delivery pressure. Finally, a model of hydraulic fracture is used to assess the toughness associated with the formation of the micro-cracks during injection.

##### Keywords
adipose tissue, soft solids, deep penetration, injection, fracture mechanics, toughness