Invasive human augmentation is so frowned upon that it stands to reason the only widely accepted implant would be an independent, self-evolving organ. The monad is an ancient concept, whose first iterations date back to the Low Age. Designed as artificial glands used to filter out toxins in decaying ecosystems and polluted zones, monads have evolved away and beyond their original purpose to serve as the universal symbiotic companion of interstellar humans.

The monad can be understood as a semi-artificial organ, about five centimetres in size, present at the base of the neck, wrapped around the thyroid. It is implanted during childhood, and develops from a transbiological seed into a full-fledged gland whose primary function is to synthesise a vast array of organic compounds, which are then liberated in the blood and lymphatic vessels. Historical monads were limited to antibody production and were thus employed as additional protection against toxins and poisons. Modern monads remain indefatigable immune sentinels, but have also evolved to cover a much wider variety of organic productions, ranging from hormones to drugs, stimulants and even specialised immune or blood cells. Monads thus play a crucial role in attuning their wearer to the conditions of a new planet or space station; their compounds are used to combat gravity-induced dizziness, regulate muscle growth, neuter allergies and accelerate the adaptation of the immune system to local pathogens. A monad's hormone production can be used to replace dysfunctional organs — such as the thyroid, which is often pre-emptively removed on spationauts — as well as perform hormone replacement therapy. More advanced functions involve control over the intestinal biome and partial nerve integration. Monads are also precious allies in combating tumours, as they can target cancerous cells with molecular precision. Being organs themselves, they are susceptible to ageing and their performance degrades over time, with a marked decrease in effectiveness noted ninety to a hundred years after implantation. Monads may also suffer from degenerative or cancer-like growths, especially after heavy exposure to radiation; such issues must be brought to the attention of specialised physicians and surgeon bots. Though the complete ablation of a dysfunctional monad is rarely practised, partial removal followed by a new seeding is common among older or high-risk populations. Due to the symbiotic nature of q-augs, it is a technology that evolves very slowly, and full replacement of an old monad with a more advanced one is a rare occurrence.

The initial monad surgery is performed around ages 5-6 (implantation at younger ages is possible, but not recommended due to immune and skeletal issues). It is a minor operation, which can be done under local anaesthesia. A small transbiological seed is inserted at the base of the neck, near the superficial superior lymph node, and attached to the local blood and lymphatic vessels. The initial growth takes between four and five years, and can cause very moderate amounts of pain. Rejection rates are extremely low, and often corrected through immune therapy. As the monad matures, it metamorphises from an implant into a symbiotic organ, integrating host cells and DNA into its definitive shape. By ages 12-15, all basic functionalities of the monad are in place, and upon reaching adulthood, the monad's integration to the nerve system is deep enough to allow for instinctive fine control without medication. As the wearer grows older, the monad becomes wiser and better attuned to their lifestyle — for instance, repeated exposure to high-g trauma will lead a monad to become better at synthesising anti-g drugs, while common exposure to pathogens will reinforce a monad's antibody assembly process. While this process has obvious limits — no amount of exposure will make a monad useful against Sequence bioweapons — a monad will always end up reflecting one's life and experiences. The final evolution of the monad, happening in the third to fourth decade of the wearer, is the development of a complex neural network, the monad weave, which retains elements of sensory inputs and ambient thoughts, acting as a complex library of memories and pathways, reflective of one's life. The weave is primarily used as the entry port for q-augs with nerve interface capabilities, be they designed for entertainment, performance enhancement or technological interaction. 

Though the uninformed public might conceive of weaves as a way to transfer consciousness to an external storage, they are too simplistic to allow for this kind of trick — which, to date, hasn't been achieved in any capacity, even for artificial intelligences. However, weaves can still allow one's memory to outlive their bodies; while post-mortem weave extraction is an extremely regulated activity, some extrasolar cultures have developed a tradition of using the weaves of fallen soldiers or spationauts to create custom muscle memories, passed on from generation to generation.

Character illustration from a stock archive by PO-Art and monad from Steven Sander's Symbiosis Creative Commons artbook, licensed under a Creative Commons Attribution Non-Commercial Share-alike 3.0 unported license.