What monoliths are made of — and what the backbone actually decides

Three material families dominate preparative monolithic chromatography: polymethacrylate, styrenic or methacrylic polyHIPE foams, and cellulose. The backbone decides two different things, and they are worth keeping apart. It sets the surface your molecule touches — hydrophilic from the start, or hydrophilic only after modification. And through the manufacturing route it sets the topology: how much void the structure can carry before it collapses, and how much pressure it wastes. MonoCore™ is a modified cellulose monolith with a 15 µm channel size and a porosity around 80 %, supplied in 1.8 mL and 5.4 mL capsules with UNF 10-32 connectors.

The cauliflower: where the standard morphology comes from

The established polymethacrylate monolith is made by precipitation polymerisation. A mixture of monomer (usually glycidyl methacrylate), crosslinker (ethylene dimethacrylate), initiator and a porogenic solvent is polymerised; the growing polymer becomes insoluble in the porogen and precipitates. Nuclei grow into microglobules, microglobules stick together into clusters. What we call pores are simply the irregular gaps left over: macropores between clusters, mesopores between globules, and micropores inside them.

The porogen controls when the phase separation happens, and therefore how large the globules and the gaps become. In poly(GMA-co-EDMA) monoliths, raising the dodecanol content from 0 to 15 % shifts the mean pore size from 0.2 µm to 2.6 µm (J. Appl. Polym. Sci. 2012, DOI 10.1002/app.37514).

That structure has two consequences that rarely appear in product literature:

It cannot be made arbitrarily open. Porosity and mechanical strength move against each other, and the published correlation for this composition — extrapolated from measurements between 0.15 and 0.55 porosity — puts the mechanically viable ceiling at roughly 0.61.

It is hydraulically wasteful. The dimensionless form factor φ = d²ε/K measures how much pressure a structure squanders at a given pore size. An empty tube would be 32. Silica monoliths sit at 22 to 64, polyHIPE foams at 66 to 100, and methacrylate monoliths at about 130. In the same published comparison, the measured permeability of a commercial methacrylate monolith disc is 5.74·10⁻¹⁵ m² — the lowest of the media compared, below that of a packed Protein A column (Ind. Eng. Chem. Res., DOI 10.1021/acs.iecr.5c01179).

That last number is worth pausing on, because it corrects a widespread belief. The advantage of a polymethacrylate monolith is the transport mechanism — convection instead of diffusion, and resolution that holds as the flow rises. It is not permeability.

Topology is a consequence of the process, not of the monomer

This is the part where an honest article has to contradict the convenient story. The cauliflower is the signature of the precipitation route, not of the methacrylate. Change the route and the same chemistry gives a different structure.

The clearest current demonstration is in the recent literature. In a 2026 paper, an inverted-flow morphology is described in which hollow spheres are packed into an interconnected network, reaching a void volume of up to 80 % and roughly one third of the pressure drop of conventional thermally polymerised methacrylate discs (Wheelwright et al., Sep. Purif. Technol. 394, 137559, 2026, DOI 10.1016/j.seppur.2026.137559). The material remains methacrylic. What changed is where the solid sits.

So the field has two axes, and they move independently:

• Topology — how the void is arranged, how much of it the structure carries, how much pressure it costs.
• Surface — what the molecule touches when it arrives.

Addressing one does not address the other. A monolith with an excellent void structure still has the surface chemistry of its backbone.

One family is missing from the list above on purpose, and it is worth naming: the same permeability comparison reports a cellular ceramic monolith an order of magnitude above both the methacrylate disc and a packed Protein A bed. Ceramic monoliths are a real and hydrophilic fourth option for preparative protein work — they are simply not a commercial format for bioprocess affinity capture yet.

PolyHIPE: the best topology, with its own bill

Foams made from a high internal phase emulsion — more than 74 % internal phase — give the most favourable mechanics-to-porosity combination of the three families, with open cells connected by windows, porosities of 72 to 83 %, and hypercrosslinked variants above 95 %.

The bill comes in two parts, both documented. The emulsion requires 5 to 50 % surfactant relative to the external phase, and complete removal at scale is described in the literature as difficult, which matters for applications with demanding purity requirements. And the materials tend towards brittleness, a property directly attributed to the surfactant load.

Cellulose: hydrophilic from the start

The bill for cellulose comes in three parts, and none of them is in the literature the way the methacrylate numbers are. Mechanically it is the least characterised of the families: the modulus-porosity relationship published for cauliflower and foam morphologies has no cellulose counterpart, and a polysaccharide skeleton at high void fraction has to be engineered against compression rather than assumed rigid — which is one reason a MonoCore™ capsule is specified to 0.8 MPa rather than to the pressures a rigid methacrylate disc tolerates. Cellulose also responds to its solvent: pH, ionic strength and organic content change the hydration of the skeleton, where a crosslinked methacrylate does not. And caustic tolerance is a genuine difference in kind — a methacrylate backbone is indifferent to concentrated alkali, a polysaccharide is not. Cellulose media therefore differ widely in what they tolerate; for MonoCore™ the conditions are on the datasheet.

Cellulose brings what a synthetic backbone has to be given: abundant surface hydroxyl groups, straightforward derivatisation chemistry and weak non-specific adsorption (J. Chromatogr. A 2023, DOI 10.1016/j.chroma.2023.464202; review, PMID 35809519). The same argument is visible one unit operation upstream, where regenerated-cellulose filtration membranes are positioned against polyethersulfone and PVDF precisely because hydrophobic surfaces foul.

Methacrylate monoliths reach a comparable surface only through modification — hydrolysis of the epoxide to the diol, PEG-methacrylate grafting, zwitterionic coatings or polyethylenimine. These are routine and they work. The difference is where each material starts.

Cellulose media differ widely in how they tolerate caustic cleaning and flow. For MonoCore™, cleaning-in-place conditions are listed on the datasheet.

And one honest note on hydrophobicity: it is not a defect. Hydrophobic interaction is a chemistry people deliberately buy. In one viroid concentration study, a butyl monolith recovered up to 60 % where the anion exchanger managed 30 % (J. Chromatogr. A 2012, “A novel application of methacrylate based short monolithic columns: Concentrating Potato spindle tuber viroid from water samples”). The fair statement is not that hydrophobic surfaces are bad, but that a hydrophobic backbone is a default you have to modify away before you can put it back deliberately where you want it.

What the surface actually decides: getting the product off again

For viruses, viral vectors and fragile particles, the limiting property is usually not how much binds. It is how much comes off intact.

The best available evidence comes from lentiviral vector work on anion exchange membranes. Standard membranes lost 70 to 80 % of the vector at scale, through a time-dependent conformational change of the bound particle leading to irreversible multi-point attachment. Reducing the ligand density from 100 to 8 µmol/mL — a factor of 12.5 — raised particle recovery from about 30 % to roughly 90 %, with functional titre improving from 17–19 % to 50–73 % (Pamenter et al., Mol. Ther. Methods Clin. Dev. 33(3), 101533, 2025, DOI 10.1016/j.omtm.2025.101533).

Read that carefully, because it is not the result a media supplier would like to claim. The lever in that study was ligand density and surface architecture, not hydrophilicity. The general principle it supports is the one that matters: with large, fragile particles, the surface decides releasability, and releasability decides yield — while capacity, the number everyone compares, is the wrong figure of merit.

A hydrophilic backbone is therefore a starting point, not a result. It is an honest argument only in that form.

What a coarse channel is for

Channel size is chosen, not inherited, and it is chosen against the feed. Permeability grows with the square of the channel diameter, so a wider channel runs at a far lower pressure drop and tolerates material that would block a finer structure — a 15 µm channel has roughly fifty times the permeability of a 2 µm one. That headroom is the reason to pick a coarse channel, and it is worth most where the molecule is large or the feed is difficult.

Where that trade pays off is documented for large molecules. In a study on anion exchange monoliths at 2, 3 and 6 µm, the open-circular plasmid isoform was captured convectively in the narrower channels, causing yield loss and irreversible pressure build-up; channels larger than about 3.5 µm were required for plasmids up to 16 kbp (Electrophoresis, DOI 10.1002/elps.202300035). The common commercial standard is 2 µm. MonoCore runs 15 µm.

On sustainability, briefly and honestly

A cellulose backbone is renewable, and that is true but small. Life cycle assessments of biologics manufacturing put the dominant burdens on cleanroom energy and water; the mass of resin is negligible against the mass of water (Curr. Opin. Green Sustain. Chem. 2022, DOI 10.1016/j.cogsc.2022.100628). The LCA literature also warns explicitly that bio-based origin does not by itself reduce environmental impact.

The environmental argument that survives scrutiny is a process argument: fewer cycles, less buffer, less water, a smaller eluate to handle downstream. Those attack a documented hotspot. "Made from a renewable material" is a fair secondary note, not a headline — and no life cycle assessment compares chromatography media by backbone material at all.

MonoCore™ in one paragraph

Modified cellulose monolith, 15 µm standard channel size (other sizes on request, fixed within any one monolith), porosity around 80 %, hydrophilic backbone with low non-specific adsorption. Available as Q and S (strong and weak ion exchange), HIC (butyl and phenyl), Protein A and Protein G. Pre-assembled 1.8 mL and 5.4 mL capsules, UNF 10-32 coned ports, larger formats on request. Detailed performance data are available on request.

Frequently asked questions

Are all monoliths made of polymer?

We are not aware of a second commercial cellulose monolith — cellulose-based convective media in fibre and nonwoven form do exist, which is a different class.

What is the cauliflower structure in a methacrylate monolith?

The morphology that results from precipitation polymerisation: microglobules growing from nuclei and sticking together in clusters, with the pores being the gaps between them. It is a consequence of the manufacturing route rather than of the monomer.

Does a hydrophilic backbone improve recovery?

It starts you in a better place. The published evidence for recovery losses with fragile particles points at ligand density and multi-point binding rather than at hydrophobicity as such, so the honest formulation is that a hydrophilic surface is a favourable starting condition, not a guaranteed result.

Is cellulose stable in sodium hydroxide?

Cellulose media differ widely in how they tolerate caustic cleaning. For MonoCore™, cleaning-in-place conditions are listed on the datasheet.

What does channel size actually change?

Hydraulics and accessibility. Permeability scales with the square of the channel diameter, so a wider channel means a lower pressure drop at the same flow and more tolerance for feeds that would block a finer structure. It also decides which molecules can move through the structure freely at all — the reason channel size matters far more for a plasmid or a viral vector than for a small protein. Capacity figures for a given chemistry, with the conditions they were measured under, are on the datasheet.