For all user-operated blood typing devices in today s market, including those designed by us in our previous research, a buffer-activation or buffer-washing step is required. The buffer-activation step, as is employed in some commercial blood typing devices, involves dissolving the antibodies deposited in the assaying zones of the device before the introduction of a blood sample for an assay. The buffer-washing step involves washing the blood sample in the assay zone in the end of the assay for result reporting. While all these devices work well, the activation or washing step does reduce the adaptability of those devices to resource-poor areas and under emergent circumstances. In this study, we designed a new device to perform forward and reverse blood typing assays without the buffer-activation or buffer-washing. Low-cost plastic slides were patterned to form channels containing dried grouping antibodies. Blood typing assays can be performed by simply placing a few microlitres of a blood sample into the channels and then tilting the slide. The sample flows along the channel under gravity, dissolving dried antibody and then spreading into a film, unveiling the reaction of red blood cells (RBCs) and antibodies. This device enables easy visual identification of the agglutinated and non-agglutinated RBCs in typically 1 minute. Both forward and reverse blood typing assays can be performed using this device. To optimize the device design, the antibody dissolution profile, assay sensitivity, and device longevity were investigated in this work.