A research team led by Harsa Mitra, Evelyn Nonamaker, Ria Corder, Luis Solorio and Arezoo Ardekani dove deep into this mystery by exploring the skin’s hidden layers. Their work, recently published in Annals of Biomedical Engineering, examines skin from three body regions—the breast, belly and neck—from distinct depths using shear rheology and lipid analysis.

The results tell a rich story. Under low strain the skin behaves predictably. But crank up the pressure and things change. At high strain amplitudes the tissue stiffens or thins depending on where it is sampled. Lipid content adds another layer of influence. Layers rich in lipids showed less stiffness under stress. And sex matters too; female human skin differs in its softness and shear-thinning behaviour when compared to male skin.

Then there’s the minipig comparison. Yucatan minipig skin from belly and neck closely mirrored male human tissue in both viscoelastic response and lipid content. But minipig breast tissue varied widely between layers making it less reliable as a model for subcutaneous delivery.

What does it all mean for injections? Quite a bit. The study shows that skin isn’t uniform. Depth, location, sex and even species create meaningful mechanical and biochemical differences. Preclinical models that ignore this variability risk missing important human tissue behaviour.

By measuring these details Purdue researchers are building a case for smarter injection design. Imagine injection devices or protocols tailored to a patient’s belly fat layer or to how stiff the skin is at different depths. It could mean more consistent drug delivery, less pain and better treatment outcomes.

This isn’t just lab work. It’s a step toward personalized medicine. As Professor Solorio explains, understanding how skin mechanics vary across the body gives us the starting point for designing injections that work the first time every time.