Most modern embedded systems are now required to satisfy seemingly divergent critical properties like safety and security. It is therefore becoming increasingly important that any systems development methodology employed should support modeling of system requirements in a manner that it facilitates validation and verification of such critical properties. In the paper we present the result of applying the Genetic Software Engineering (GSE) method to design an ambulatory infusion pump (AIP) which must satisfy a number of safety and security properties. The safety and security requirements are integrated with the rest of the systems requirements in the form of integrated behavior tree (IBT), which is systematically refined into a design behavior tree (DBT). The integrated behavioral view of the requirements provides a platform for requirements conflict resolution, defect detection and requirements validation. The formal semantics of the behavior tree (BT) notation, used to specify the requirements, makes formal verification of critical properties in the final design possible.