An implantable needle-type fiber-optic microprobe was constructed to monitor in vivo fluorescent substances in anesthetized rats. This fiber-optic microprobe was composed of coaxial optical fibers that were catheterized using a thin-wall tube of stainless steel (o.d. similar to400 mum; i.d. similar to300 mum). When the fiber-optic microprobe was placed in solutions containing various concentrations of fluorescent nanospheres (20 nm), either in the presence or in the absence of 10% Lipofundin acting as an optical phantom, we observed nanosphere concentration-dependent responses of the fluorescence intensity. The microprobe was then implanted into the livers and brains of anesthetized rats to monitor the in situ extravasation of preadministered fluorescent nanospheres from vasculature following the hepatic and cerebral ischemic insults. Both types of ischemic insults showed immediate increases in fluorescent intensities when 20-nm fluorescent nanosphere were administered, but neither ischemic insult induces such an increase when we administered 1000-nm fluorescent nanospheres. Additional experiments can be performed to further narrow the size range of the increase in blood vessel permeability following ischemic insult; such "size" information may be valuable when formulating drugs for optimal local delivery. Although a wide variety of fluorescent substances are used intensively for in vitro biological studies, the in vivo and in situ monitoring of these substances is studied much less often, probably because of difficulties in the efficient assembly of miniaturized fiber optics to detect the relatively weak fluorescence signal arising within such a turbid medium as tissue. To our knowledge, the use of our implantable fiber-optic microprobe is the first minimally invasive technique capable of investigating the "size window" of vascular permeability for the in vivo delivery of nanospheres in both ischemic livers and brains.
