Light-regulated growth from dynamic swollen substrates for making rough surfaces

Natural organic structures form via a growth mode in which nutrients are absorbed, transported, and integrated. In contrast, synthetic architectures are constructed through fundamentally different methods, such as assembling, molding, cutting, and printing. Here, we report a photoinduced strategy for regulating the localized growth of microstructures from the surface of a swollen dynamic substrate, by coupling photolysis, photopolymerization, and transesterification together. Photolysis is used to generate dissociable ionic groups to enhance the swelling ability that drives nutrient solutions containing polymerizable components into the irradiated region, photopolymerization converts polymerizable components into polymers, and transesterification incorporates newly formed polymers into the original network structure. Such light-regulated growth is spatially controllable and dose-dependent and allows fine modulation of the size, composition, and mechanical properties of the grown structures. We also demonstrate the application of this process in the preparation of microstructures on a surface and the restoration of large-scale surface damage.


Supplementary Note 3: photostability of NBA under blue light
As o-nitrobenzyl-ester-based monomers could be photocleaved when exposed to UV light, we should find a suitable light source for substrate curing without inducing photolysis of the NBA. Since onitrobenzyl-ester-based products have high adsorption in the wavelength (λ) of 250-350 nm, we chose the blue light (460 nm) with an intensity of 0.6 mW·cm^-2 as the light source to trigger the polymerization. UV-Vis and 1 H NMR spectroscopies had been utilized to detect the stability of NBA exposed to blue light (Supplementary Figure 2). The enclosed as-prepared NBA solutions (in acetonitrile and DMSO-d6 for UV-Vis spectrum and NMR spectra, respectively) were put directly under blue light irradiation to detect its photostability in this wavelength. Wswollen is the weight of the swollen sample and Wdry is the weight of the dry sample.
Supplementary Figure 4 shows the swelling curves of the seed-20% film before and after UV irradiation. Figure 4. Swelling curves of a seed-20% film before and after UV irradiation in nutrient solutions containing HBA, HDDA, I-819, and BZSA. The film was fully exposed to the UV light (10 mW·cm^-2) for 30 min before measurement. Four kinds of control samples were subjected to UV irradiation to study the possibility of UV-triggered partial chain scission: seed-20%, swollen seed-20% containing non-polymerizable liquids, seed-0%, and swollen seed-0% containing non-polymerizable liquids. As shown in Supplementary Figure 6, after 30 min UV irradiation, the compression moduli of samples with NBA units (seed-20% sample and swollen seed-20% containing non-polymerizable liquids) slightly increase while those of non-photorespinsive samples (seed-0% sample and swollen seed-0% containing non-polymerizable liquids) do not change.

Supplementary
These results indicated that possible UV-triggered partial chain scission was rare and its contribution to the mechanical properties was negligible. As shown in Supplementary Figure 12a, the copolymers contain HBA and promoters both, and the molar ratio of HBA was 22% calculated from the integration of peak c', f' and d-g , which was very accordance with the raw ratio before the polymerization. The molecular weight of the copolymers was 8500 with a PDI of 1.16 (GPC, Supplementary Figure 12b).
Supplementary Figure 13  In order to better observe the liquid transportation, the seed-20% was swelled in a solution consisting of HB acetate (97 wt%), I-819 (1 wt%) and BZSA (2 wt%). The swollen seed-20% was subjected to UV irradiation (10 mW·cm^-2). After 30 min irradiation, an obviously bulging was observed in the irradiation region (Supplementary Figure 15a). The surface profile of the irradiated sample was collected immediately (Figure 2d in the main text). After the sample was being stored in dark for 10 hours, the

Supplementary Note 16: calculation of mass transport rate
We evaluated the rate of mass transport in the growth by measuring the diffusion rate of a monomer analogue (4-hydroxylbutyl acetate) under irradiation condition. An analogue was used because HBA monomer will undergo polymerization during irradiation even without any initiator. The polymerization will change the composition of the diffusing liquid. To measure the diffusion rate, a fresh seed-20% sample with a thickness of 500 µm was immersed into 4-hydroxylbutyl acetate and its weight was recorded at different times. The diffusion rate can be determined using the following supplementary equation (2): 4, 5 where F is the rate of diffusion per area; K is a swelling constant, t is the time (s), n is a swelling exponent; Mt and M0 are the weight of the swollen and dry sample at time t, respectively. From supplementary equation (2), we know that = ln + ln We plotted ln F versus ln t (Supplementary Figure 16) by using the kinetic of swelling yields straight lines up to almost 60% increase in the mass of the swollen sample. 6,7 The swelling exponents n and the swelling constant K were calculated from the slopes and intercept of the lines. The intercept K value was used for determination of the diffusion coefficient D: where D is the diffusion coefficient (cm^2·s^-1), r is the radius of the cylindrical seed-20% sample (cm).
Combing with the supplementary equation (3), (4) and the plot from Supplementary Figure 16, the diffusion coefficient was 4.7×10 -5 cm^2·s^-1. As for the control without irradiation, the diffusion coefficient was 4.9×10 -6 cm^2·s^-1. Growth with different crosslinking degree in seed The protocol was the same as that used for poly(HBA-co-NBA) but varying crosslinking fraction. Briefly, precursor solutions containing different HDDA concentrations (0.2 wt%, or 2 wt%, or 5 wt%, or 10 wt%) were used for preparing the seeds. The obtained seeds were immersed in a nutrient solution containing HBA (96 wt%), HDDA (1 wt%), I-819 (1 wt%) and BZSA (2 wt%) to get the swollen seed network for growth. Tensile test was used to measure the E-moduli of the bulky samples with different crosslinker fraction and profilometer was applied to measure the height of the structure. Growth with different photomasks Photomasks with various scales (round shapes with diameter from 266 μm to 5000 μm) were utilized to control the irradiation area. After exposed to UV light until the samples did not grow anymore, the grown heights of the structures were measured from profiles. Poly(PEGA-co-NBA) systems

Supplementary
The protocol was the same as that used for poly(HBA-co-NBA) but replacing HBA with PEGA. Briefly, PEGA (80% molar ratio), NBA (20% molarl ratio), HDDA (1 wt%) and I-819 (1 wt%) are mixed together and used for preparing the seeds. This obtained seed network was immersed in a nutrient solution containing PEGA, HDDA, I-819 and BZSA to get the swollen PEGA based seed network for growth.
Profilometer was applied to measure the height of the structure and indentation was used to measure the E-moduli of the materials. Figure 30. E-moduli of PEGA based seed before and after growth.

Supplementary
Poly(BA-co-NBA) systems The protocol was the same as that used for poly(HBA-co-NBA) but replacing HBA with BA.
Profilometer was applied to measure the height of the structure and indentation was used to measure the E-moduli of the materials. Figure 31. E-moduli of BA based seed before and after growth.

Hybrid systems
A PEGA-based seed 20% was used for growth. It was immersed in a nutrient solution containing HBA, HDDA, I-819, and BZSA to get the swollen seed. Then UV 365 nm light with an intensity of 10 mW·cm^-2 was used to trigger the growth of the hybrid structures. Profilometer was applied to measure the height of the structure and indentation was used to measure the E-moduli of the materials. Figure 32. E-moduli of hybrid PPEGA-PHBA based materials before and after growth.

Sequential growth of structures on material surfaces
A HBA-based seed-20% was used. A big structure (a round shape with a diameter of 5000 μm) was grown out from the swollen sample under UV irradiation for 20 min. After the growth, the sample was immersed in ethanol solution to remove the unreacted monomers, and dried in air. This sample obtained was re-swelled in the nutrient solution to obtain the second swollen seed-20%. Finally, the second swollen seed networks were irradiated with UV light through a photomask with a diameter of 1250 μm for 30 min.