A disposable molecular valve, consisting of a membrane coated with alginate gel, which is partially cross-linked with Ca(2+), was developed based on a volume change and/or dissolution of the gel in response to the cation concentration. The valve automatically opened upon an increase in the concentration of cations such as H(+), Na(+), Ca(2+), Mg(2+), Cu(2+), or Al(3+). The concentration where the valve opens was 1 x 10(-4) to 2 x 10(-4) M for Ca(2+), Cu(2+), and Al(3+), but the solution volume passed vs. concentration increased in the order of Ca(2+), Cu(2+), and Al(3+). For Na(+) and Mg(2+) the valve opening concentration was ca. 1 x 10(-2) and 1 x 10(-3) M, respectively. Two mechanisms were proposed to account for the opening of the valve. One is due to a shrinking of the gel caused by cations; the other is due to dissolution of the gel. The former corresponds for the cases of Ca(2+), Cu(2+), and Al(3+), and the latter does for Na(+) and Mg(2+). The valve closed over the pH range from 3 to 12, whereas it opened below and above this range. The effects of anions and pressure were also studied.