A subcutaneous capsule for long-term studies

In assessing interstitial fluid dynamics, perforated capsules have a limited existence because of the ingrowth of connective tissue. A permanently patent capsule (1 yr) can be obtained by implanting 1.9-cm lengths of Tygon tubing (OD 7.9 mm, ID 4.8 mm). After implant, the capsule is invested in connective tissue, and a thin strand of connective tissue invades both ends of the capsule. In 2-3 wk, the connective tissue joins in the capsule and there is no further ingrowth of connective tissue. This tissue bridge through the center of the capsule is richly vascularized, eventually containing small central arteries and veins. Protein content of capsule fluid was 50-60% of plasma, with albumin proportionately higher in the capsule. Intracapsule pressures averaged -2.08 mmHg. Communication between capsule contents and vascular volume appeared to be size-dependent in that the [35S]sulfate and [3H]inulin count could be measured in tail blood 5 min after intracapsule injection and it increased over 30 min. This did not occur with [14C]dextran (mol wt 70,000) and [14C]globulins (mol wt 150,000). This capsule appears to lend itself to long-term studies related to interstitial fluid dynamics and/or capillary exchange.