Synthetic Replacement Cell Architecture (SRCA): A Contract-Based Framework for Progressive Cellular Substrate Replacement with Functional Continuity

Synthetic Replacement Cell Architecture (SRCA) is a theoretical, contract-based framework for engineering artificial or hybrid cellular systems specifically as progressive functional replacements for natural cells within an existing multicellular organism. Instead of requiring molecular identity with the original cell, SRCA defines replacement through Cell Contracts and Tissue Contracts specifying required inputs, outputs, physical interactions, homeostatic functions, lifecycle behavior, integration, handover, and safe exit. The framework introduces the Replacement Continuity Requirement (Cᵣ), dual Host Signaling and Synthetic Signaling planes, a Synthetic Control Layer, a functional Synthetic Protein Instruction Set abstraction, and two initial validation benchmarks: the Synthetic Replacement Erythrocyte (SRC-E) and the Synthetic Replacement Microvascular Endothelial Cell (SRC-MEC). Neural replacement is positioned as an extreme high-continuity case in which persistent network information must also be preserved. SRCA reframes long-term rejuvenation as a maintenance problem in which persistent biological organization may be supported by controlled replacement of finite-lived cellular substrates rather than by indefinite preservation of each original component. The work is theoretical and does not provide a therapeutic protocol, laboratory procedure, genetic sequence, or clinical implementation recipe.