Figure 3 Bonding strength of CS-MA to four types of hydrogels. (A)
Process of hydrogel adhesion. (B-C) Adhesion energy (Force/width, F/w)
of PAAm hydrogels varies with the coating and integrative time.
*p<0.05 when compared with other groups. (D) Maximum adhesion
energy of various hydrogels at optimized coating and integrative time.
Small image is the T-peeling test of PAAm hydrogels.
To further confirm the stitching stability under deformation, the
adhered PAAm hydrogels were stretched and compressed on an automated
materials testing system. During the stretching, the hydrogels were
broken before debonding (Figure S2), indicating strong adhesion.
Moreover, the intact and stitched PAAm hydrogels withstood similar
compressive stress of > 2.7 MPa (Figure S2), indicating
that the mechanical strength was not weakened after suture.
Autoclaving is a commonly used and effective sterilization method but
may sometimes damage the polymer network by inducing phase separation
and aggregation 25. Therefore, the resistance of
hydrogels to autoclaving is a crucial concern for their applications in
cell culture and tissue engineering. To examine the alteration of
adhesion energy before and after autoclaving, the adhered PAAm hydrogels
were sterilized at 121°C for 30 min. As shown in Figure S3, the adhesion
energy of PAAm hydrogels was not impaired by autoclaving, although the
mechanical strength was reduced due to swelling. Since CS is known as a
thermal stable polymer 26, the stable adhesion by
CS-MA is likely due to the covalently crosslinked CS chains that were
modified by N-acylation reaction 21. In contrast, the
adhesion by other hydrogel adhesives, such as pristine chitosan27 and poly(acrylic acid)/Fe3+28, was unstable under varying conditions due to the
ionic bonds of adhesive chains, which is unsuitable for sealing
hydrogel-based microfluidic chips.
3.3 Mechanism of the adhesion between
hydrogels viaCS-MA
The strong adhesion between the adhesive chains and the hydrogels’
network could be attributed to their topological entanglement11. To investigate the interface of bonded PAAm
hydrogels, SEM was used and it revealed a dense interfacial zone with a
thickness of 30-50 µm between two sheets after stitching (Figure 4A).
This suggested that the CS-MA diffused into the porous hydrogels and was
crosslinked inside the pores. In contrast, hydrogels without CS-MA
treatment showed a clear interface (Figure 4B) and could be easily
peeled along the interface, resulting in a low adhesion energy of
~1 N/m. The CS-MA diffusion into hydrogel was also
visibly monitored using fluorescent FITC labeled CS-MA (Figure 4C and
4D). Since the CS-MA has lost the pH responsibility of CS (Figure 2A),
it could diffuse into the hydrogel freely. Consequently, the CS-MA rich
layer gradually thickened during coating but diffused away from the
interface over the integrative time. The well permeation of CS-MA might
be able to interlock the two hydrogels after polymerization and ensure
strong adhesion 27. This explained why extending the
coating time facilitated adhesion (Figure 3).