Again, primary lung fibroblasts from IPF patients and controls embedded in free-floating 3D collagen I and PAA hydrogels mimicking the stiffness of the ‘fibrotic’ and ‘normal’ lung presented with increased COL1A1 and reduced matrix metalloproteinase (MMP)-1 expression in response to TGF-β1 stimulation in stiff ‘fibrotic’ compared to the normal gels [108]. Here, TGFB1 is linked to idiopathic pulmonary fibrosis.