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Application of thermosensitive polymers as an embolic material for intravascular neurosurgery was investigated. We intended to use thermosensitive polymers to occlude vessels by precipitation in response to body temperature. Copolymers of N-isopropylacrylamide (NIPAM) and N-n-propylacrylamide (NPAM) were selected as thermosensitive polymers. To determine the optimal lower critical soluble temperature (LCST) for the embolic material, we developed an in vitro flow model. In this study the copolymers with an LCST of 24-26 degrees C showed appropriate precipitation. To prove the occlusion of vessels in vivo, we injected the copolymers into a rabbit kidney through a microcatheter. The extent of embolization was judged by angiography and histological examination. An acute toxicity test of the copolymer of NIPAM and NPAM was performed in comparison with that of the NIPAM monomer. The copolymer used in this paper showed no acute toxicity in mice. Water solubility, non-adhesiveness, and non-toxicity are the advantages of the use of thermosensitive polymers as an embolic material. By changing the LCST, various embolic materials can be designed. Based on our results, we believe that the application of thermosensitive polymers as a new embolic material is very promising.

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Light scattering measurement was performed on three monodisperse polystyrene samples (M = 10 X 103, 43.6 X 103, and 180 x 103) in cyclohexane in the temperature range from the theta point to 1 °C above the cloud point. From the data obtained the Flory-Huggins interaction parameter was expressed as a function of temperature T, polymer volume fraction , and molecular weight M. This expression for was extended to T below the cloud point to calculate critical points, cloud point curves, and spinodals for a series of M values. Good agreement, certainly better than obtained by previous formulations, was found between the calculated results and some typical experimental data. The success is due primarily to the formulation of the experimental finding that at high concentration depends significantly on M, in contrast to the usual concept on thermodynamic behavior of concentrated polymer solutions.

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