1.Place wakame in small bowl, cover with cold water; stand about 10 minutes or until softened. Drain then squeeze out excess water. Chop coarsely, removing any hard ribs or stems.
2.Combine ginger, chilli, garlic, lime leaves, stock, the water and chicken in large saucepan; bring to a boil. Reduce heat, simmer, uncovered, about 10 minutes or until cooked through. Cool chicken in broth 10 minutes; remove from pan. Strain broth through muslin-lined sieve over large bowl; discard solids, return broth to pan. Slice chicken thinly.
3.Meanwhile, cook soba in large saucepan of boiling water, uncovered, until just tender; drain. Divide soba, wakame and sprouts among serving bowls.
4.Bring broth to a boil; reduce heat and stir in sauce, juice and buk choy. Serve broth topped with chicken. Serve with lime wedges, if desired.