A Frobenius category within the monomorphism category

Given a selfinjective artin algebra Λ, we consider the category Sinj(Λ) of all embeddings of a left Λ-module in a finitely generated injective left Λ-module. We show that Sinj(Λ) is a Frobenius category with Auslander-Reiten sequences such that the categories Λ-mod and Sinj(Λ) are stably equivalent and Sinj(Λ) has twice as many indecomposable injective objects as Λ-mod.