Macroscopic hybrid polymer-DNA coated colloid gels

As described in section 1, colloids of all sizes are widely used to create materials with unique mechanical properties,e.g. they can be used to vary the elastic response of rubbers [129]

but are also capable of tailoring the viscosity and the non-linear response of liquids [130].

Here, hybrid polymer-DNA-colloid gels were investigated together with Henry Dehne. These consist of a hydrogel matrix that is formed by PAM (polyacrylamide) of different

concen-4. Static Structure Formation by DNA coated colloids Figure 4.11:Elasticity of hybrid polymer-DNA coated colloid gels. The storage modulus G_max^′ , measured after 20 min of PAM polymerization, increases monotonously with an increase in PAM concentration. While the colloids them-selves lead to an increase of nearly one order of magnitude, the explicit structure of the colloids (monodispers/gelated) does not exhibit significant differences.

trations and of the binary DNA coated colloid system (XAB = 1) as it has been introduced in section 4.1.1. This hybdrid system offers the opportunity to control colloidal structure formation and hydrogel formation independently and thus to investigate the influence of internal colloidal structure formation on the mechanical properties of gels. To ensure that the hydrogel formation and the colloidal gelation are seperated in time, a light sensitive cat-alyst for PAM polymerization is used (see section 3.1.3.1 for details). In short, the fractal growth of the DNAcc system at a concentration of ΦDNAcc = 4% is triggered by addition of a linker strand at first. After gelation, the sample is exposed to white light, resulting in PAM polymerization. As a control, the DNAcc system is incubated without the linker strand before illumination, yielding monodisperse colloids dispersed in the PAM matrix.

Measuring the G_max^′ of the samples for different PAM concentrations shows that the ad-dition of the colloids leads to a significant enhancement of the hydrogel elasticity (see figure 4.11). However, the structure of the hydrogel encapsulated colloidal gel does not seem to make a significant difference, as monodisperse and geled DNAcc result in the same G_max^′ . In stark contrast to the elasticity, the toughness of the hydrogel is significantly in-fluenced by the colloidal structure formation within the hydrogel. Recording a strain ramp protocol at ΦPAM = 10% on the monodisperse and the geled system shows that the PAM matrix containing the DNAcc gel is capable of bearing an order of magnitude more stress before rupturing (see figure 4.12A,B). Calculating the amount of energy needed to rupture the hydrogels shows that the hydrogel containing the colloidal gel can absorb ≈35×more energy than the monodisperse system. As the composition for both samples is identical, it can be assumed that this vast difference in toughness is solely caused by the changes in colloidal structure. This is especially interesting as onlyΦDNAcc = 4% are required to create this tunable hybrid gel.

As the transition from the fluid-like to the gel-like state of presented hybrid gels can be triggered by light exposure, they can also be used in other applications that profit from the explicit macroscopic geometry of the gel in combination with its DNA functionality. In 2D, the hybrid gel can be used as a printable fluid, that is fixated by concentrated ligth exposure

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4. Static Structure Formation by DNA coated colloids

Figure 4.12.: Toughness of hybdrid polymer-DNAcc gels. (A) Linear increasing strain ramps show that the hybrid gel containing monodisperse colloids are irreversibly damaged below a stress of 5 kPa.(B) In contrast to monodisperse colloids, the gelated DNAcc can bear stresses up to 25 kPa. (C) Detailed plot of the last stable stress-strain curve conducted in (A) and (B). While the inital elasticity is comparable, the hybrid gel containing gelated DNAcc can bear 5×more stress and can absorb up to 35×more energy that the ungelated sample (inset).

at the current position of the printing probe (see figure 4.13A). Moving the printing probe along a substrate creates thin lines of the hybrid gel, containing a significant amount of DNA that is exposed at the surface (see figure 4.13B). This approach therefore results in biochemical functional hydrogel that can be printed in arbitrary 2D shapes. Moreover, using a mold also 3D functional hybrid gels can be produced. Using a cubic mold, PAM blocks can be created. In order to guarantee surface functionalization, a layer of highly concentrated DNAcc is added on top of the yet unpolymerized PAM base (see figure 4.13C). To further increase density and viscosity of the DNAcc solution, 800 mM of sucrose is added. This prevents mixing with the PAM base. The subsequent light exposure leads to full polymer-ization of the macroscopic gel block that is equipped with a thin functional top layer (see figure 4.13D).

4. Static Structure Formation by DNA coated colloids

Figure 4.13.: Printing and 3D molding of hybrid polymer-DNA coated colloid gels. (A) As the polymerization PAM part of the hybrid gel can be triggered by light illumination, it can be used in ink-jet printing approach. Moving an optical fibre along the ink-jet tip leads to a quasi-instantaneous polymerization of hybrid gel. (B) Printed lines of the hybrid-gel, illuminated with UV light. (C) Molding of a cubic PAM gel equipped with a top layer of the hybrid gel. After filling the mold with conventional PAM gel, it is covered by an un-polymerized DNA coated colloid hybrid gel with high sucrose content. Due to the high density of the topping solution, the DNAcc stay in the top layer and are fixated by subsequent light exposure. Scale bar 2 cm.