Hemofiltration removes water and small molecules from the blood via nanoporous filtering membranes. This paper discusses a pump‐free hemofiltration device driven by the pressure dif-ference between the artery and the vein. In the design of the filtering device, oncotic pressure needs to be taken into consideration. Transmembrane pressure (TMP) determines the amount and direc-tion of hemofiltration, which is calculated by subtracting the oncotic pressure from the blood pres-sure. Blood pressure decreases as the channels progress from the inlet to the outlet, while oncotic pressure increases slightly since no protein is removed from the blood to the filtrate in hemofiltra-tion. When TMP is negative, the filtrate returns to the blood, i.e., backfiltration takes place. A small region of the device with negative TMP would thus result in a small amount of or even zero filtrates. First, we investigated this phenomenon using in vitro experiments. We then designed a hemofiltra-tion system taking backfiltration into consideration. We divided the device into two parts. In the first part, the device has channels for the blood and filtrate with a nanoporous membrane. In the second part, the device does not have channels for filtration. This design ensures TMP is always positive in the first part and prevents backfiltration. The concept was verified using in vitro experiments and ex vivo experiments in beagle dogs. Given the simplicity of the device without pumps or electrical components, the proposed pump‐free hemofiltration device may prove useful for either implantable or wearable hemofiltration.
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