Welcome to chapter thirteen, the first chapter of Part Two, putting an object into a clinical world. A part coming off the machine, out of the printer, off the lathe, out of the mould, off the sewing machine or out of the annealing oven, is the beginning of the object's life, not the end of its design. This chapter is about what happens next: who handles it, how it is cleaned, and what you can and cannot claim about the materials it is made from.
Your object will be handled, carried, set down, cleaned by somebody who is not you with a process you did not choose, stored in a cupboard, used by the next person, dropped, repaired and thrown away. Every one of those stages is a design input, and most are decided at the design stage, by whether there is a cavity you cannot reach, a seam that traps fluid, or a surface the approved wipe cannot wet for long enough. This is also where makers most often make a claim they cannot support. It's PLA, that's food safe. It's surgical stainless. It's medical-grade silicone. Those sentences sound like evidence, and they are not. Choosing the fabrication method itself is the subject of chapter fourteen.
First, just enough microbiology.
Bacteria multiply on surfaces given moisture and nutrients, and some form biofilm. Some bacteria make spores, dormant and highly resistant forms that survive many disinfectants; sterilisation is defined partly by its ability to kill them. Viruses replicate only inside cells; those with an outer envelope, like influenza, are generally easier to inactivate than tough non-enveloped ones like norovirus. Fungi grow in damp, porous and absorbent materials. And prions, misfolded proteins, are extraordinarily resistant, a specialist domain where standard sterilisation is not assumed sufficient.
Biofilm deserves its own slide. Bacteria attached to a surface can secrete a protective matrix, forming a community dramatically more resistant to disinfectants and to mechanical removal than the same organisms floating free. It forms in exactly the places a maker creates without thinking: crevices, threads, lumens, scratches, porous surfaces, and the gaps between press-fitted parts. That is the microbiological reason behind what otherwise sounds like fussiness about surface finish. A scratched or porous surface is not merely harder to clean. It provides protected habitat.
Infection passes along a chain: the organism, its reservoir, a way out, transmission, a way in, and a susceptible host. Infection control works by breaking the cheapest link, and hand hygiene, the most effective single measure, breaks transmission. Your object sits at that link too. It is a fomite, an inanimate vehicle for indirect contact. The shared teaching model is handled by eight learners, set down, boxed, carried and handled again next week. No patient touched it, but it is still a surface in a healthcare environment. So the question is never does it touch a patient. It is where does it go, who touches it, and what do they touch next.
Next, three words that are not interchangeable.
Treating these as synonyms is the most common error in this area. Cleaning is physical removal of soil and contaminants. It need not kill anything, and it is the prerequisite for everything else, because residual soil shields organisms and can inactivate the chemical agent. Disinfection inactivates many disease-causing organisms, depending on agent, concentration, contact time, temperature and organism, and it is explicitly not sterilisation; many disinfectants do not reliably kill spores. Sterilisation is a validated process intended to eliminate all microbial life, defined to a sterility assurance level of one in a million. And cleaning always comes first.
The Spaulding classification decides what level of processing an item needs, based on how it is used. Critical items enter sterile tissue or the vascular system and must be sterilised: instruments, implants, needles. Semi-critical items contact mucous membranes or broken skin and need at least high-level disinfection: endoscopes, respiratory equipment. Non-critical items touch intact skin only: blood pressure cuffs, bed rails. The category is set by the use, not the object; the same piece of plastic is non-critical on a desk and critical inside a body. Your tabletop model is non-critical at most, and you should state that explicitly in your brief.
Autoclaving is the default where material permits. A typical gravity cycle holds about a hundred and twenty-one degrees Celsius for fifteen to thirty minutes; a pre-vacuum cycle about a hundred and thirty-two to a hundred and thirty-five for three to four. Actual parameters are validated by the institution. Autoclaving destroys much of what a general workshop produces. PLA softens around fifty five to sixty-five degrees and deforms in a dishwasher. Acrylic, many glued joints, elastic and hook-and-loop fabric, and almost all electronics are casualties. The survivors, 316 stainless, borosilicate glass, PEEK and many silicones, survive because they were chosen for it. Discover that constraint at the beginning, not the end.
Now the central idea of the chapter.
Food safe, medical grade, biocompatible, used in medical devices: these are claims about a material as supplied, usually under specified conditions. None of them is a claim about your finished object in your intended use. Between the raw material and your part sit the process and everything it touches. The regulator's framing, written for printed devices but true of every process, is that materials are evaluated in the context of a finished device and its intended use, not approved in the abstract.
Every method has its own list. Printing adds thermal history, additives and colourants, residue from the nozzle, layer voids, and for resins incomplete cure. Machining adds coolants, burrs and embedded swarf. Injection moulding adds release agents, regrind, parting lines and moulded-in stress that later cracks under disinfectant. Casting adds release agents, cure inhibition and bubbles. Welding adds heat tint, flux residue and free iron on stainless that needs passivation. Glass adds residual stress if annealing was inadequate. Textiles add finishes, shrinkage and stitch holes. Electronics add flux, coatings and ingress paths around buttons. A part made by any process is not the material on the datasheet.
Every process varies from run to run. In printing, orientation sets where the weak layer boundaries fall; infill creates sealed voids that become uncleanable once the shell is breached; and the same file on the same printer a month later is not guaranteed identical. Uncured resin is an irritant and sensitiser. In machining, tool wear and coolant matter, and a blind threaded hole is as hard to clean as any printed cavity. In moulding, a change of resin lot or moulder is a change of product. Under-cured castings are tacky and leach. And every wash changes a textile's dimensions and strength. Each method needs its own traceability record, so you can reproduce a part and connect a test result to it.
These rules cost nothing at the design stage and a lot later. Make every surface reachable; if a wipe on a gloved finger cannot reach it, it is not being cleaned. A press-fit seam is a capillary gap, so seal it or make it a deliberate tool-free disassembly. Radius internal corners and avoid exposed threads. Make metal joints continuous, ground flush, and passivate stainless after fabrication. For soft goods, decide the cleaning route at the start: laundered, wiped if coated and sealed, or single-patient use. Seal electronics with gaskets or membrane switches. Orient so liquid drains, design for inspection, and use labels that survive the cleaning, because a label that washes off takes your traceability with it.
Wood and fabric behave very differently in a clinical setting. Wood is porous and absorbent and usually inappropriate for shared objects that must be cleaned. Sealed wood raises further questions: which sealant, is it intact, what happens when scratched. Use wood where cleaning requirements permit, a workshop demonstration or a fixture that stays in your space, and record why. Fabric, by contrast, is everywhere in healthcare, because it has a recognised cleaning route of its own: laundering with thermal disinfection, for example holding sixty-five degrees for at least ten minutes in UK guidance; or a wipeable coated fabric with sealed seams; or single-patient use. Fabric, thread, elastic, fastenings and labels must all survive the route and still fit.
Now the engineering bridge: how to compare materials honestly.
Here is the kind of comparison you should produce. PLA, printed, softens at fifty-five to sixty-five degrees and carries unknown additives and porosity. Polypropylene, moulded, is often autoclavable depending on grade. Acrylic crazes and cracks with alcohol-based wipes, especially at laser-cut edges. Acetal machines beautifully but repeated steam degrades it. PEEK survives repeated sterilisation at a cost. 316L stainless is excellent once weld tint is removed and it is passivated. Borosilicate glass survives anything but breaks into sharps. Coated fabric wipes clean until its coating cracks. Notice how much of the honest answer is in the unresolved column, and never give any material a blanket hospital safe rating.
Infection prevention is a systems discipline. Its equipment includes autoclaves, where removing air is the critical step; washer-disinfectors; low-temperature sterilisers for heat-sensitive items; and endoscope reprocessors, one of the hardest problems in medicine, linked to real outbreaks. UV-C works only by line of sight and is a serious eye and skin hazard. In software, instrument tracking answers a recall question: if a cycle fails, which patients were affected? The training side is unusually accessible. Fluorescent markers under ultraviolet light are the best single teaching tool, and a set of matched cleanability coupons, printed, machined, cast, welded and sewn, soiled and wiped, shows this chapter's argument at a glance.
The best first build in this chapter is a UV hand hygiene box: a light-tight enclosure, a UV-A source, a viewing aperture and a hand opening, used with commercial fluorescent lotion. It is a genuinely mixed build: folded and powder-coated sheet metal or sealed laminate for the enclosure, UV-A LEDs with a driver, an interlock and timer in the electronics, and a wipeable gasket. It is cheap, it makes an invisible problem visible, and infection prevention teams use them constantly. And it draws on the whole book: the enclosure must be cleanable, the UV source must be shielded from the eyes, a hazard users cannot perceive, and the learning objective must be written before you build.
Your elbow model's pivot could have a closed one-piece shell or two removable halves. The shell has no fasteners but cannot be inspected or cleaned inside; the halves can, but add capillary gaps and parts to lose. The question that resolves it is not in the comparison: what is the cover for, and is it needed at all? Only then choose the method. A printed shell iterates fast but carries layer porosity. Machined acetal is smooth and tough. Folded stainless survives anything. A cast silicone boot has no seams. Laser-cut acrylic shows the mechanism but will craze with alcohol wipes. And if a fastener loosens at cycle thirty-two in your bench test, that is real evidence about durability and none about cleanliness.
For the exercises: draw a contact map of every surface touched during one full use and reset of your model, including the forgotten steps like carrying and boxing, with who touches what next. Compare three candidate materials made by different methods, marking what evidence is missing, without ranking them overall. Find three hard-to-clean features and redesign them. Write a traceability record that someone else could reproduce your part from. And make matched cleanability coupons by four methods, soil them with fluorescent lotion, wipe each once, and photograph them under UV.
The limits. Disinfectants and sterilisation cycles are institutional decisions, validated against specific loads and equipment. Safe is a claim about a finished object in a use, with evidence behind it, so you cannot declare any material safe. Validating a cleaning process involves defined soil challenges, recovery methods and acceptance criteria; wiping something and looking at it is not that. Keep bench durability evidence strictly separate from any claim about contamination. And biocompatibility testing has its own standards and specialists. Next, making it: choosing the technology and the method.