  We study deformations of graded braided bialgebras using cohomological
methods. In particular, we show that many examples of Nichols algebras,
including the finite-dimensional ones arising in the Andruskiewitsch-Schneider
program of classification of pointed Hopf algebras, are rigid. This result can
be regarded as nonexistence of "braided Lie algebras" with nontrivial bracket.
