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Stretchable semiconducting polymer is fully degradable

By Kendra Redmond February 18, 2020
This stretchable semiconductor consists of degradable semiconductor fibers (dark green) embedded within a rubbery, biodegradable polymer (illustrated as the light green background). Credit: Adapted from ACS Central Science 2019, doi:10.1021/acscentsci.9b00850.

Most modern electronic components are rigid and nondegradable. Skin-like electronics that are elastic, self-healing, soft, and fully degradable could have transformative applications in diagnostic and therapeutic devices, environmental monitoring systems, consumer products, and other areas. As reported in ACS Central Science, a Stanford University research team has achieved an important milestone in the development of such electronics—synthesis of an intrinsically stretchable, fully biodegradable, organic semiconducting material.

Realizing skin-inspired electronics requires fabricating materials with a challenging combination of properties. Researchers have created electronics that are fully degradable, but the designs are not conducive to stretching. Semiconducting polymers are stretchable, but most of them have semi-crystalline structures that resist hydrolysis and electronic performance that varies with strain. However, the Stanford team notes that polymer-based electronics are still an appealing option, in part because they can easily be modified for self-healing, adhesion, or other functionalities.

“There are contrasting molecular design constraints imposed on a polymer that is concurrently electron conducting, stretchable, and fully degradable,” according to Helen Tran, a postdoctoral fellow at Stanford University working with lead researcher Zhenan Bao. “Imparting degradability to electronically conducting polymers presents a particular challenge,” she says.

To overcome this challenge, Tran and Bao led a project to decouple stretchability and transience with a two-component polymer design. First, the research team synthesized a semiconducting polymer from a diketopyrrolopyrrole-based polymer (DPP) and the polymer precursor p-phenyldiamine (PPD). The chemical structure of the resulting polymer, p(DPP-PPD), featured a backbone containing carbon-nitrogen double bonds (imine bonds). These bonds support electrical conductivity yet are degradable in aqueous solutions. The structure also has a side chain to which functional groups can be attached.

The team then mixed p(DPP-PPD) with an elastic, urethane-based polymer containing the biodegradable material polycaprolactone (E-PCL). The two polymers self-assembled to form aggregates of semiconducting nanofibers confined within an elastic matrix of E-PCL, termed nanoconfined p(DPP-PPD). Atomic force microscopy (AFM) revealed that nanofiber morphology varied with the blending ratio of the two polymers. Optimum stretchability occurred at 70% E-PCL.

In order to fabricate thin film electronic devices from p(DPP-PPD), the team spin-coated a solution of E-PCL and p(DPP-PPD) onto a substrate. Spin-coating required the p(DPP-PPD) to be soluble in chlorobenzene and free from macroscopic aggregates. The researchers were able to satisfy these conditions by attaching the alkyl functional group C4−C10C12 to the side chain of the polymer

The researchers created thin films of both nanoconfined p(DPP-PPD), which contained 70% E-PCL, and p(DPP-PPD) alone. Characterization revealed that even under 100% strain, thin films of nanoconfined p(DPP-PPD) showed no cracks. In contrast, thin films of p(DPP-PPD) alone displayed cracks at strains over 25%. AFM and x-ray photoelectron spectroscopy (XPS) provided insights into the morphology of the nanoconfined system, suggesting that nanofibers were more densely packed at the top and bottom interfaces of the film, with a higher concentration of E-PCL in the center region.

The researchers measured a mobility around 0.05 cm2/V·s in thin-film transistors that used the nanoconfined p(DPP-PPD) semiconductors. This value remained constant even under strains up to 100%, applied both in parallel and perpendicular to the channel direction. The researchers credit connectivity between the nanofiber aggregates for enabling this strain-independent charge transport. Transistors made with p(DPP-PPD) alone, on the other hand, showed a slightly larger mobility under zero strain, but visible cracks formed and mobility decreased under strain. At 100% strain, the mobility was around 2 orders of magnitude lower than under zero strain.

The nanoconfined p(DPP-PPD) also displayed full degradability, breaking down into monomers over about 10 days in a solution of 1 M trifluoroacetic acid in water. The trifluoroacetic acid was used to accelerate the time scale, but the researchers say that p(DPP-PPD) should degrade in any aqueous solution, including the human body. In addition, human embryonic kidney cells that were seeded onto thin films of nanoconfined p(DPP-PPD) thrived, indicative of biocompatibility.

“This work presents a significant contribution to the next frontier in materials science: developing compositions and structures that optimize performance over the required life cycle,” says Gordon Wallace, director of the ARC Centre of Excellence for Electromaterials Science (ACES) in Australia. “[It] requires a shift in thinking to design systems with the temporal dimension in mind rather than just for required performance at t = 0. Achieving this in highly functional materials as required in electronic devices is no trivial task,” he says. Wallace specializes in developing new materials for energy and health applications and was not associated with this research.

Moving forward, the Stanford researchers hope to improve the electronic performance of their nanoconfined p(DPP-PPD) thin films. They are also working to integrate the material into more complex devices and analyze the degradation by-products and toxicity in more detail. “There is still a lot of fundamental work in biodegradable and stretchable electronics ahead of us,” Tran says.

Read the article in ACS Central Science.