Second, hypoxia is also a key factor to cause drug resistance.[19] According to the results, hypoxia‐inducible factor‐1α (HIF‐1α) was accompanied with a higher mRNA expression in PDA‐modified group compared with 2D group, while lower HIF‐1α level than that of nonmodified group (Figure 5A). The gene discussed is HIF1A; the disease is Patent ductus arteriosus.