Question 12
List four properties of polylactic acid (PLA) that make it a useful material for medical implants.
[4 marks]
A PLA screw holds a broken bone while it heals, then the body absorbs it
Each stage of the implant’s life calls on one property. Watch QCAA’s four sample properties light up below as the screw needs them.
In the body PLA breaks down slowly, over months to years, by which time the bone carries the load again. That is why biodegradable is the property that makes PLA an implant material.
Pick four properties and see which ones earn a mark
Choose up to four cards. Some are properties QCAA accepted, some repeat a property you already have, and some are not properties of PLA at all. Switch to PA6 to try the same question with the 2026 implant plastic.
Work through the solution one mark at a time
Try each step yourself before you reveal it. The guide runs a ladder: each correct property moves you up one rung.
QCAA’s sample answer: “1. Biodegradable”
Start with the property that makes PLA the implant plastic. The body absorbs it once the bone has healed, so the patient needs no second operation. List it as a word or a short phrase. The verb is list, so no explanation is needed.
Marker: provides 1 correctly identified PLA property [1 mark]
QCAA’s sample answer: “2. High tensile strength”
A screw, pin or plate has to hold bone under load until the bone can carry it again. Strength is the second thing the job needs.
Marker: provides 2 correctly identified PLA properties [2 marks]
QCAA’s sample answer: “3. Non-toxic in solid form”
Anything left inside the body must be safe against living tissue. The notes column also accepts plain “non-toxic”.
Marker: provides 3 correctly identified PLA properties [3 marks]
QCAA’s sample answer: “4. Resistant to chemicals”
The implant sits in body fluid for months, so it must survive that chemical environment while it does its job. The notes accept other properties for any of the four lines: “non-toxic”, “good barrier properties (moisture, heat etc.)”, “smooth appearance” and “or other suitable response consistent with a reasonable understanding”.
2. High tensile strength
3. Non-toxic in solid form
4. Resistant to chemicals
Marker: provides 4 correctly identified PLA properties [4 marks]
Each phrase of the question gave you something
This is pure recall with no stimulus, and the band comes from QCAA. The 2020 subject report says six of the seven Section 2 questions were simple familiar and the one complex familiar question was worth 6 marks, so this 4-mark list is simple familiar. For 2026, polylactic acid is not in the syllabus at all, and medical implants now sit on the PA6 list. The four-mark recall form, last seen in 2023 Question 11, has not appeared since. What transfers is the recall ladder, one mark per correct item, and the habit of choosing properties that the job needs. If a 2026 question asked for implant properties, it would be about PA6, where biodegradable earns nothing.
If I had to guess where you might lose marks, it would be listing biodegradable and then “breaks down in the body” as two separate lines. That is one property said twice, and it costs you a rung on the ladder. The other leak is writing an application, such as screws or 3D printing, where the question asks for a property.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording, any figures and marking-guide steps are from the 2020 QCAA Engineering external assessment, © State of Queensland (QCAA) 2020, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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