The article examines donor-hair selection for capsule extensions with the goal of reproducing native hair optics and biomechanics. Relevance follows from the industry’s shift away from bulky bonds toward micro- and nano-capsules that lower mechanical load and visual detectability, while patient expectations trend toward medical-grade planning. Novelty lies in translating quantitative thresholds from surgical trichology (shaft diameter classes, density ceilings, hair–skin contrast) into an extension-specific selection protocol and embedding these rules in the author’s InvisiCaps Method. The work synthesizes anatomical, physiological, and therapeutic literature; evaluates non-invasive diagnostic tools for pre-installation profiling; and considers materials science evidence for keratin systems. Special attention is given to caliber matching, shade/contrast control, capsule geometry zoning, and maintenance timing under concurrent therapies. The study goal is to define reproducible criteria for natural outcomes. Methods include comparative analysis, critical appraisal, and integrative synthesis across peer-reviewed sources and technical reports. Sources cover transplantation references, guidelines, therapy updates, nutraceutical trials, mechanistic reviews, non-invasive diagnostics, computational modeling, and keratin microcapsule fabrication. The conclusion operationalizes selection rules into a four-step workflow and decision thresholds that practitioners can adopt in premium extension services.

donor hair selection, capsule extensions, micro-capsules, keratin bonds, shaft diameter calibration, density planning, trichoscopy, InvisiCaps Method, hair–skin contrast, maintenance strategy.

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