Reading 02 · Mycelium
Members' library

Where fabrication becomes biology

How to audit the biological consequences of every step in a living-material process.

Raphael Kim · 1 Aug 2026 · Content v13 · 24 min read · 38 min listen
Mycelium · Process · Method
Photograph · Printed panel, raking light, gaps between lines visible · 1110 x 340
Fig. 1  A printed panel before growth. Every line has air on two or three sides, and that is the fabrication decision doing biological work.
In short

Grinding, mixing, printing, moulding and assembling are not neutral operations carried out on biological matter. Each one changes the conditions the organism is living in.

In this Reading
24 min · the audit is at the end
01
The division that is not real
02
The project
03
Five decisions
04
What the dish and the panel show
05
Why these decisions escape scrutiny
06
The five analyses
07
Where the effect lands
08
The whole-process biological audit
09
Honest edges, and the sources

The division that is not real

Anyone running a living-material process sorts decisions into two piles. One is biological: species, feedstock, water, temperature, time. The other is fabrication: grind size, nozzle width, layer height, mould material, fill depth, orientation, assembly.

The division organises a week of work. It is also biologically false.

Every fabrication decision is a biological decision.
Particle size, water content, pH, binders, nozzle width, line thickness, toolpath, mould geometry, orientation and assembly do not only determine how a living material is manufactured. They alter the conditions in which the organism grows, feeds and builds the material you will use.

Three terms, since the rest depends on them.

Terms used here
Mycelium
The body of a fungus, a mass of fine threads growing through whatever it eats. Those threads hold a grown material together.
Enzymes
The tools it releases into its food to break it down, having no mouth and absorbing what it dissolves.
Oxygen
A fungus needs it, and in wet solid material it is hard to move around. Most of what follows returns to that.

The project

Céline Oberholzer, Jennifer Marie Yabut and Ehsan Baharlou built a material from two things in their own household waste: cat litter made from corn and potato, and used coffee grounds.

They screened five recipes in dishes over nineteen days, scoring sideways spread, growth above the surface and growth down into the material separately. The best recipe was printed as one continuous folded line with roughly 3 mm gaps between lines, so threads could bridge across. Panels about 23 cm square and seven layers thick grew for three weeks, were killed, dried on a schedule borrowed from pottery, and mounted into a wooden screen.

Their stated limitation: three weeks was not enough for threads to grow through 2.1 cm.

Why this project and not another

This Reading is possible because of how that paper is written. The authors published their full recipe, recorded the iterations that failed, explained their choices, and stated their limitation without softening it. Papers doing all four are rare, partly because it exposes the work to exactly this kind of reading.

Five decisions

No.
Decision
Fabrication problem solved
Biological condition changed
01
Grind the coffee finer
Extrusion and layer adhesion
Air gaps between particles
02
Water at 59% by weight
Extrudability
Hydration, and how oxygen moves
03
Widen the printed line by about 150%
Cracking, bowing, layer separation
Distance from any surface to the centre of a line
04
Adjust the mix to pH 5.8 to 6.2
Chemical stability and printability
Enzyme activity and growth rate
05
Guar gum at 0.8%
Holding a 9 mm line across seven layers
Available food
Tab. 1  The third column is the one the field does not routinely fill in.

Filling that third column took about twenty minutes a row, working backwards. The material sits in mycology, enzyme chemistry and a century of commercial mushroom growing, almost none of it in the biofabrication literature anyone building a printed composite reads first.

The column is not a scorecard. It describes an instrument the field does not have yet.

What the dish and the panel show

The same finely ground recipe was tested twice, in two shapes.

Photograph · 9 cm dish at day 19 · 539 x 230
In a dish
Spread sideways
38 mm
Growth above the surface
None
Growth down into the material
None
Fig. 2a  Oberholzer et al., 2026, Table 3 and Figure 5.
Photograph · Printed panel at week 3 · 539 x 230
Printed into panels
Kinds of growth recorded
All four
Panels showing them
10 of 10
Growth down into the material
Present
Fig. 2b  Same paper, p. [LOC?]

Same recipe, same fungus, same three weeks. The obvious difference is shape. In a dish, wet fine paste is a dense bed with air only at the top. In a printed panel, almost every line has air on two or three sides.

This is not a controlled experiment

Sample size, wall thickness, evaporation and handling changed alongside shape, so it does not establish that geometry caused the difference.

The biological effect of a fabrication parameter cannot be read off the parameter alone.
What it does establish is narrower and more useful than causation. A grind size does not carry a fixed biological meaning from one shape to another, which is why the second half of this Reading is a method rather than a list of better settings.

Why fabrication decisions escape biological scrutiny

The paste answers first

Fabrication problems announce themselves. The line sags, the nozzle blocks, the panel cracks in the dryer. You get an answer that afternoon and act on it while the file is open.

Biological consequences do not. Incomplete growth, a slowed enzyme, oxygen running out inside a printed line: none is visible from outside. Three weeks later somebody cuts a panel open, and the decision responsible is five steps back, indistinguishable from four others made the same week.

So work gets optimised against the feedback available, which is what anyone does when one channel reports in hours and the other reports in weeks, if at all.

The field has the same shape.

How well the field understands its own settings
46 studies
Fixed before the fungus is alive
Well understood
Happening during growth
Moderate to poor
Applied after it is dead
Well understood
Chart 1  Ratings as published in Biront, Van Dijck and Wurm, 2026, Figure 2 on p. 34. Not re-scored here.
Members from here
The five analyses, the geometry diagnostic and the audit.
Roughly 4,000 words, 24 sources, and a twenty minute method you can run on your own recipe this week. Membership opens the whole Library, seven shelves, with a new Reading every two weeks.
Still to come in this Reading
01  Particle size
02  Water content
03  Printed line width
04  pH
05  Guar gum
+  The whole-process audit

Reading the five analyses

Each decision is set out the same way, with evidence kept at three separate levels.

Observed here
What the paper reports.
Established elsewhere
Measured in published work outside it, cited.
Working hypothesis
Untested reasoning, mine unless attributed.

The labels are there because plausible reasoning is not a result.

01
Decision

Particle size

Fabrication problem
Coarse grounds extruded badly and layers separated. Grinding to fine espresso fixed both.
Condition changed
Air gaps between particles, and how far oxygen reaches into the material.
Observed here
No growth downward in a dish; growth downward once printed.
Established elsewhere
Grinding too fine "may reduce porosity, limit aeration, and impede oxygen transfer", with a useful window of 1 to 5 mm for the two species most used here (Liadi et al., 2026, p. [LOC?]); finely ground composite leaves "minimal amounts of air between particles" (Rigobello and Ayres, 2022, p. [LOC?]). Oxygen moves roughly ten thousand times faster through air than water (Somerville and Proctor, 2013, p. [LOC?]), so a water-filled gap is close to a wall. In a damp solid bed with a fungus growing on it, oxygen was undetectable one tenth of a millimetre down (Oostra et al., 2001, p. [LOC?]).
Working hypothesis
Screening in flat dishes undersells recipes destined for open printed shapes, because the dish removes air the printed geometry restores. If so, the screening stage is quietly rejecting recipes that would have worked.
Cheapest test
Screen each recipe twice, in a dish and as a short printed piece at production line width and spacing, scoring downward growth in both.
02
Decision

Water content

Fabrication problem
The paste has to extrude and hold a bead. Water was set at 59% by weight, the largest single component.
Condition changed
Hydration, and how much pore space holds water rather than air.
Observed here
At 59% water the paste extruded cleanly and held its shape through printing; wetter mixes slumped, drier mixes tore.
Established elsewhere
Water-filled pores block gas exchange far more than air-filled ones do at the same porosity (Oostra et al., 2001, p. [LOC?]), so raising water content to fix extrusion can work against the aeration that grinding was meant to protect.
Working hypothesis
Particle size and water content were set separately but interact: fine particles need more water to extrude, and more water fills the pores that fine grinding already narrowed. The 59% figure may be a compromise forced by particle size rather than an independent optimum.
Cheapest test
Vary water content at two particle sizes in a small grid, scoring both extrusion quality and downward growth, to see whether the two variables trade off against each other.
03
Decision

Printed line width

Fabrication problem
Cracking, bowing and separating layers. The nozzle went from 4 mm to 6 mm and the line to about 9 mm, roughly 150% (Oberholzer et al., 2026, p. [LOC?]).
Condition changed
The distance from any surface of a line to its centre, which is how far oxygen must travel through wet material.
Observed here
The change and its fabrication rationale. No biological examination.
Established elsewhere
The tenth-of-a-millimetre measurement above.
Working hypothesis
The centre of a 9 mm line sits about 4.5 mm from the nearest surface, far beyond the depth at which oxygen remained measurable in that study. If it is colonised at all, it is colonised from the open ends and through channels the threads open as they go. I have found no measurement of oxygen inside a printed line of living paste, which makes this the most testable claim here.
Diagram · Section through a 9 mm line
surfaces, centre, 4.5 mm · 539 x 250
Fig. 3  The distance that matters is not the line width, it is half of it.
Distance oxygen must cross
Millimetres
Measured limit
0.1
Old line, half width
2.0
New line, half width
4.5
Chart 2  The measured limit is from a different material in a different study. It is a scale marker, not a threshold for this paste.
Cheapest test
Print at three line widths, grow for the same period, then section the lines to see how far in the threads reached.

This is for paid subscribers

The full breakdown of all five decisions, with sourced evidence and testable hypotheses, is available to paid subscribers of Biodesign Academy.

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Where the effect lands

A fabrication parameter has no fixed biological direction. It has a direction relative to where the material must work and where it is likely to fail.

Two archetypes are useful, diagnostic rather than exhaustive, with most real objects somewhere between them.

Archetype 01
Interior-limited
Looks like
A solid block, a thick wall, a cast part, a thick printed line
The limit
Whether the middle receives oxygen
The currency
Connected air space, and anything that packs the material tighter spends it
Archetype 02
Interface-limited
Looks like
A stack, a joined assembly, an open lattice, a thin shell
The limit
Whether two surfaces knit together
The currency
Contact area, and packing tighter can help, because more surface touches more surface

The dish and the panel are the same recipe landing in different places on that axis. In the dish the interior is the whole problem and a fine grind removes what little air there is. In the panel every line has air on more than one side, so geometry relieves the interior limit.

A published number therefore carries its geometry with it, and moving it into a different shape can change what it does.

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