Synthetic protocells to mimic and test cell function

Abstract

Synthetic protocells provide a new means to probe, mimic and deconstruct cell behavior; they are a powerful tool to quantify cell behavior and a useful platform to explore nanomedicine. Protocells are not simple particles; they mimic cell design and typically consist of a stabilized lipid bilayer with membrane proteins. With a finite number of well characterized components, protocells can be designed to maximize useful outputs. Energy conversion in cells is an intriguing output; many natural cells convert transmembrane ion gradients into electricity by membrane-protein regulated ion transport. Here, a synthetic cell system comprising two droplets separated by a lipid bilayer is described that functions as a biological battery. The factors that affect its electrogenic performance are explained and predicted by coupling equations of the electrodes, transport proteins and membrane behavior. We show that the output of such biological batteries can reach an energy density of 6.9 x 10(6) J m(-3), which is approximately 5% of the volumetric energy density of a lead-acid battery. The configuration with maximum power density has an energy conversion efficiency of 10%.

Figure 1

Schematic and performance of a DC biobattery. (A) Device based on cation selective α-HL, adapted from Ref [42], not drawn to scale. (B) Voltage output, volumes and ion concentrations of droplets over time

Figure 1

Schematic and performance of a DC biobattery. (A) Device based on cation selective α-HL, adapted from Ref [42], not drawn to scale. (B) Voltage output, volumes and ion concentrations of droplets over time

Figure 2

Performance of a DC biobattery with cation and anion selective α-HL mutants. (A) The cation selective channel (MTSES G133C) uses parameters based on published MTSES treated G133C α-HL mutants. For comparison, calculations using two anion-selective channels are shown: a hypothetical anion selective channel that is as strongly anion specific as the MTSES G133C is cation specific and parameters based on published N123R channels[42]. For each case, the total potential (E_t) is shown along with the contributions due to the channels (E_r) and the electrodes (E_c). (B) The output energy density (W) as a function of ion selectivity (a₁) of the membrane proteins for both cation and anion specific channels. The selectivity reported for specific mutants, MTSES treated G133C and N123R, is highlighted with arrows.

Figure 2

Performance of a DC biobattery with cation and anion selective α-HL mutants. (A) The cation selective channel (MTSES G133C) uses parameters based on published MTSES treated G133C α-HL mutants. For comparison, calculations using two anion-selective channels are shown: a hypothetical anion selective channel that is as strongly anion specific as the MTSES G133C is cation specific and parameters based on published N123R channels[42]. For each case, the total potential (E_t) is shown along with the contributions due to the channels (E_r) and the electrodes (E_c). (B) The output energy density (W) as a function of ion selectivity (a₁) of the membrane proteins for both cation and anion specific channels. The selectivity reported for specific mutants, MTSES treated G133C and N123R, is highlighted with arrows.

Figure 3

The output energy density (W) as a function of device parameters. (A) Output energy density as a function of α-HL channel density (n_α-HL), external resistance (R_ext) and the area of the bilayer (A₁). (B) The output voltage and energy density as a function of external impedance for a devices based on anion and cation specific channels and with low number of channels (≈ 100) and low channel density (≈ 0.003 channels· m⁻²). This channel density is comparable to that in the reported experimental setup [42], but is much lower than the values we report in Table 1. (C) Local maximum of energy density as a function α-HL channel density (n_α-HL). (D) Output energy density as a function of starting K⁺ concentration in the lower-concentration droplet (droplet A). At every given K⁺ concentration, the maximum energy output density was found by Nelder-Mead optimization[65]. The other parameters are the same as Figure 1.

Figure 3

The output energy density (W) as a function of device parameters. (A) Output energy density as a function of α-HL channel density (n_α-HL), external resistance (R_ext) and the area of the bilayer (A₁). (B) The output voltage and energy density as a function of external impedance for a devices based on anion and cation specific channels and with low number of channels (≈ 100) and low channel density (≈ 0.003 channels· m⁻²). This channel density is comparable to that in the reported experimental setup [42], but is much lower than the values we report in Table 1. (C) Local maximum of energy density as a function α-HL channel density (n_α-HL). (D) Output energy density as a function of starting K⁺ concentration in the lower-concentration droplet (droplet A). At every given K⁺ concentration, the maximum energy output density was found by Nelder-Mead optimization[65]. The other parameters are the same as Figure 1.

Figure 3

The output energy density (W) as a function of device parameters. (A) Output energy density as a function of α-HL channel density (n_α-HL), external resistance (R_ext) and the area of the bilayer (A₁). (B) The output voltage and energy density as a function of external impedance for a devices based on anion and cation specific channels and with low number of channels (≈ 100) and low channel density (≈ 0.003 channels· m⁻²). This channel density is comparable to that in the reported experimental setup [42], but is much lower than the values we report in Table 1. (C) Local maximum of energy density as a function α-HL channel density (n_α-HL). (D) Output energy density as a function of starting K⁺ concentration in the lower-concentration droplet (droplet A). At every given K⁺ concentration, the maximum energy output density was found by Nelder-Mead optimization[65]. The other parameters are the same as Figure 1.

Figure 3

The output energy density (W) as a function of device parameters. (A) Output energy density as a function of α-HL channel density (n_α-HL), external resistance (R_ext) and the area of the bilayer (A₁). (B) The output voltage and energy density as a function of external impedance for a devices based on anion and cation specific channels and with low number of channels (≈ 100) and low channel density (≈ 0.003 channels· m⁻²). This channel density is comparable to that in the reported experimental setup [42], but is much lower than the values we report in Table 1. (C) Local maximum of energy density as a function α-HL channel density (n_α-HL). (D) Output energy density as a function of starting K⁺ concentration in the lower-concentration droplet (droplet A). At every given K⁺ concentration, the maximum energy output density was found by Nelder-Mead optimization[65]. The other parameters are the same as Figure 1.