Nature - USA (2020-08-20)

388 | Nature | Vol 584 | 20 August 2020

Article

during the sol–gel transition (Extended Data Fig. 3a). The ink has
a shelf life of more than 20 days (Extended Data Fig. 2e, f ). During
printing, the ink flows easily through the nozzle because of shear
thinning, but the filament retains its shape after printing because
of the rapid viscosity increase in the absence of shear. Objects have
been printed with filament and nozzle diameters as low as 100 μm
(Extended Data Fig. 3b). Smaller diameters should be feasible, given
the particle size of the aerogel powder (Fig. 1f), provided the printing
system can operate at sufficiently high pressure. A silica sol, incor-
porated in the ink before printing, is activated with ammonia vapour
after the object has been printed to bind the aerogel particles and fill
the interstitial voids with silica gel. The printed gel may optionally be
hydrophobized before the solvent is removed by supercritical CO 2

drying. MnO 2 (ramsdellite) microspheres are mixed into some of the

inks to illustrate the ease of functionalization (light absorption or

photothermal catalysis)^28 ,^29.

We printed various aerogel objects with high shape fidelity and preci-

sion (Fig. 2a–c, e), including honeycombs, 3D lattices and multi-layered

continuous membranes. The printed filaments retain a circular

cross-section with a well-defined diameter (for example, 327 ± 6 μm).

The rheology of the ink can be adapted to the application: higher vis-

cosity for open structures with large overhangs (up to 45°) and wide

spans (for example, 10 mm for a filament with a diameter of 400 μm)

(Fig. 2d); lower viscosity to enable filaments to merge into continuous

membranes without voids (Fig. 2e). The original aerogel grains are

embedded in, and infused by, a low-density aerogel matrix derived from

G′

G′′

G′

, G

′′ (Pa)

Shear stress (Pa)

100 101 102 10 –3

10 –2 10 –1

100

101

102

103

104

105

10 –1

100

101

102

103

104

105

106

10 –2 10 –1 100 101 102 103 10 –4 10 –3 10 –2 10 –1 100 101 102 103 104

0

2

4

6

8

10

12

Volume (%

)

Particle size (μm)

In air ow

In isopropyl alcohol

NH 3 vapour

Direct ink writing

Ink

Conical

nozzle

Gelation

Supercritical

CO 2 drying

SiO 2 aerogel particles MnO 2 spheres SiO 2 colloids

a

b

f

c

g h

d e

Apparent viscosity (Pa s)

Shear rate (s–1)

SP1.4

SP1.6

SP2.5

SP1.3M0.9

pH < 3 pH > 4

Fig. 1 | Additive manufacture of silica aerogel by direct ink writing.
a, Scheme for direct ink writing of silica aerogels. The inks, either neat (blue) or
functionalized with, for example, MnO 2 nanoparticles (gold), are printed
pneumatically through micronozzles. The printed objects are gelled by a
vapour-based pH change, and dried from supercritical CO 2. b, 3D lotus f lower
of silica gel printed from ink SP2.5 (Extended Data Table 1) through a conical
nozzle with an inner diameter of 410 μm, with a f low rate of 15 mm s−1.

The f lower is printed using 38 layers; printing took 26 min. Supplementary

Video 1 shows a high-speed version of the printing process. c–e, Photographs

of the hydrogel from b before solidification (c), after ammonia-vapour-induced

gelation (d) and after drying (e). f, Particle size distribution for the silica

aerogel. g, Shear-thinning behaviour of different inks. h, Storage (G′) and

loss (G′′) modulus versus shear stress for the different inks.