Animal Experiments to Assess Toxicity Following Subcutaneous Administration of N-Isopropylacrylamide Gels Having Biochemo-Mechanical Function

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Hydrogels consisting of N-Isopropylacrylamide (NIPAAm) undergo a thermosensitive and discontinuous volume-phase transition (I.e., volume collapse) at a temperature close to 33.5 ℃, which is of interest in the development of stimulus-sensitive biomaterials such as biochemo-mechanical and drug-delivery devices. However, NIPAAm, the neurotoxicity of which may not be much stronger than acrylamide, and acrylamide have been known to produce peripheral neuropathy in experimental animals. Nevertheless, this is a large barrier for discussing potential uses of NIPAAm gels in the aforementioned biomaterials. Thus, we performed animal experiments to assess toxicity following subcutaneous administration of NIPAAm gels.Significant differences (with a "risk" less than 1% or 5%, according to a statistical analysis by Student's t-test) between the monomer-injected and control groups were observed in the food and water intakes, body weight, and locomotor activity. Also, the monomer administration produced a significant increase in kidney weight and a decrease in spleen weight. No significant differences were seen in blood examinations, but the monomer tended to cause an increase in GOT and GPT levels. In contrast, there were no significant differences in these toxicity indicators between the gel administered and control animals. Histological observations showed an inflammation of the tissue around the gel injected until 3 or 4 days, although a complete cure was attained after 6 day.The results obtained from the monomer-treated groups are consistent with those of previous studies, each of which indicated the neurotoxicity of NIPAAm monomer but were different in the methodology from our study. However, NIPAAm gels produce no differences in toxicity indicators under comparison with control experiments. From these results, we speculate that the gel has little toxicity compared with the monomer. However, further careful experiments are required in order to reach a conclusion.