The invention provides compositions comprising a polyamidoamine (PAA) polymer comprising a pendant disulphide, sulphydryl, or activated sulphydryl moiety, and methods for their manufacture. The invention extends to the use of such polyamidoamine polymers to form cross-linked compositions, and hydrogels comprising the same, and the use of such compositions in various biological and non-biological applications, such as the delivery of biomolecules to target sites, and for tracking fluid flows. The invention also provides carrier particles, which may be used to deliver biomolecules, and to methods of treatment. The invention also provides a fluid tracking system for monitoring fluid flow.