Hard and soft micro- and nanofabrication: An integrated approach to hydrogel-based biosensing and drug delivery

Abstract

We review efforts to produce microfabricated glucose sensors and closed-loop insulin delivery systems. These devices function due to the swelling and shrinking of glucose-sensitive microgels that are incorporated into silicon-based microdevices. The glucose response of the hydrogel is due to incorporated phenylboronic acid (PBA) side chains. It is shown that in the presence of glucose, these polymers alter their swelling properties, either by ionization or by formation of glucose-mediated reversible crosslinks. Swelling pressures impinge on microdevice structures, leading either to a change in resonant frequency of a microcircuit, or valving action. Potential areas for future development and improvement are described. Finally, an asymmetric nano-microporous membrane, which may be integrated with the glucose-sensitive devices, is described. This membrane, formed using photolithography and block polymer assembly techniques, can be functionalized to enhance its biocompatibility and solute size selectivity. The work described here features the interplay of design considerations at the supramolecular, nano, and micro scales.

Figure 1

Schematic of forces influencing polyelectrolyte hydrogel swelling.

Figure 2

Conversion of trigonal (Lewis acid) form of phenylboronic acid moiety to the tetrahedral (Lewis base) form in the presence of OH^-, followed by bidentate condensation of cis-diol bearing ligands (sugars in the present case). Not shown is a relatively unimportant pathway in which sugar binds first to trigonal form. After Kataoka et al. [25].

Figure 3

Swelling (dilation factor with respect to diameter at synthesis) of p(MPBA-co-AAM) hydrogels. a) Effect of pH on swelling for hydrogels with following mol% PBA in polymer: (■) 0%; (●) 5%; (▲) 10%; (▼) 15%; (◆) 20%. b) Effect of sugar concentration at pH 7.4: (■) fructose; (●) glucose. c) Effect of sugar concentration at pH 10.0: (■) fructose; (●) glucose; (▲) glucose+0.2mM fructose. Error bars are standard deviations.

Figure 3

Swelling (dilation factor with respect to diameter at synthesis) of p(MPBA-co-AAM) hydrogels. a) Effect of pH on swelling for hydrogels with following mol% PBA in polymer: (■) 0%; (●) 5%; (▲) 10%; (▼) 15%; (◆) 20%. b) Effect of sugar concentration at pH 7.4: (■) fructose; (●) glucose. c) Effect of sugar concentration at pH 10.0: (■) fructose; (●) glucose; (▲) glucose+0.2mM fructose. Error bars are standard deviations.

Figure 4

Interactions of glucose and fructose with PBA units. A single glucose molecule can bind to one (monobidentate) or two (bisbidentate) PBA units at low and intermediate concentrations. Bisbidentate interactions lead to reversible crosslinks between polymer chains. At higher concentrations free glucose competes with bisbidentate glucose for PBA sites, reducing bisbidentate interactions. Fructose is only capable of monobidentate binding, and it inhibits bisbidentate crosslink formation.

Figure 5

Shear modulus of 20% MPBA hydrogel as function of glucose concentration at pH 10. Values are normalized by shear modulus of pH 10 solutions with no glucose added. (■) observed; (□) predicted based on data of Fig. 3c and assuming no change in crosslink density, i.e. G / G(0) = d(0) / d. Error bars are standard deviations.

Figure 6

a) Schematic of integrated implantable glucose sensor featuring a glucose-sensitive hydrogel sandwiched between a rigid nanoporous membrane and a semiflexible micrcapacitor, which is connected to a microinductor coil. b) Top view of microsensor indicating size. Microcapacitor in center is scored with cutting lines to reduce eddy currents, and from top vantage point appears to be “framed” by microinductor coil. c) Side view of microcoil, which is deposited into trenches in bottom silicon piece. Panels a,c reproduced from Ref. 45 with permission. Panel b reproduced from Ref. 39 with permission.

Figure 7

a) Example of frequency response (impedance) of LC microresonator, highlighting sharper resonance when cutting lines are included in microcapacitor. b) Resonant frequency, (f_r), of microsensor as a function of external glucose concentration in pH 7.4 PBS, at equilibrium. c) Kinetics of response of microdevice to changes in external glucose concentration. Panel a reproduced from Ref. 39 with permission. Panels b,c reproduced from Ref. 45 with permission.

Figure 7

a) Example of frequency response (impedance) of LC microresonator, highlighting sharper resonance when cutting lines are included in microcapacitor. b) Resonant frequency, (f_r), of microsensor as a function of external glucose concentration in pH 7.4 PBS, at equilibrium. c) Kinetics of response of microdevice to changes in external glucose concentration. Panel a reproduced from Ref. 39 with permission. Panels b,c reproduced from Ref. 45 with permission.

Figure 8

a) Schematic of microvalve which opens and shuts due to swelling and shrinking of glucose sensitive hydrogel. b) Flow response of microvalve connected via tubing to a water column, with valve switched between PBS (pH 7.4) solutions containing 0 mM and 20 mM glucose. Panel a adapted and panel b reproduced from Ref. 30 with permission.

Figure 9

a) Chemical structure of PS-PI-PLA block polymer. b) Self-assembly of PS-PI-PLA on Si₃N₄, which in turn is on top of a microporous silicon support membrane. “Test tube structures” of PLA cylindrical dots form a hexagonal array. Here only a cross-sectional slice of the array is shown. Layers are not drawn to scale since micro- and nanofeatures are of vastly different size. c) Same as b but with PLA removed. d) Removal of Si₃N₃ and wetting PS layer yields continuous nano-microporous membrane. Apparently “floating” nanostructures form a tubular hexagonal lattice in 3D.

Figure 10

Electron micrograph of microporous Si array carpeted with nanoporous block polymer membrane. Inset: tapping AFM of nanoporous membrane. Blowup is not literal, as two images were taken separately.

Figure 11

a) Transport of a small molecule (methyl orange, MO, 327 g/mol) and a large macromolecule, dextran blue, DEX, 2×10⁶ g/mol) through micro-nanoporous hybrid membrane. b) Similar comparison of transport of MO and DEX through a control microporous membrane. Differences is slopes here are due to size-dependent (Stokes-Einstein) diffusion coefficients of MO and DEX. Comparing a and b results, it is seen that hybrid provides greater size selectivity. c) Transport of MO across the hybrid membrane before and after removal of PLA block by spiking NaOH into receiver solution. Reproduced from Ref. 53 with permission.