Printable Conducting Inks for Bioresorbable Electronics through Electrochemically Induced Sintering of Zinc Microparticles

Among the biodegradable metals, Zn is attractive for the development of printable conducting inks due to its low activation energy for atomic self-diffusion. When exposed to ambient conditions, Zn spontaneously forms a native oxide layer (thickness of ZnO: tens of nanometer), which is insulting in nature, possess a high melting point (~1975 °C) and low diffusivity. The presence of the native oxide layer on Zn poses difficulty in sintering and limits the use of Zn microparticles for the development of printable conducting inks.

Researchers at University of Illinois at Urbana-Champaign, USA, Kwangwoon University, Republic of Korea and Northwestern University, USA have described a process that enables a dilute acid-induced  dissolution of the native oxide layers on Zn followed by an electrochemical self-exchange reaction between Zn and Zn²⁺ ions that promotes rapid sintering of Zn metal particles under ambient conditions, without any heating or mechanical loading.

An aqueous solution of acetic acid (H₂O:CH₃COOH = 10:1 by volume, pH 2.3) is used to dissolve the native passive oxide layer on Zn. This dissolution promotes self-exchange between Zn and Zn²⁺ ions at the Zn/H₂O interfaces between the particles and enables cold welding of the Zn particles, resulting in the formation of a conductive network. The acetate anion (CH₃COO⁻(ac), pK_a = 4.8) serves as the buffer until the ink is dried and at this stage, the welded compact solid is covered with a new passivation layer (Zn(ac)₂). (Fig. 1)

Fig. 1 Electrochemical sintering of Zn microparticles in CH₃COOH/H₂O

The change in morphological features of Zn particles before and after immersion in H₂O:CH₃COOH (10:1 by volume, pH 2.3) for < 1 min at room temperature and ambient conditions is shown in Fig. 2. Before immersion the Zn particle remain intact (Figs. 2 (a) and 2(d)). In contrast, after immersion, formation of necks at points of near contact between the particles, that corresponds to regions of high local concentration of Zn is evident (Figs. 2(b) and 2(e)). When the interparticle distances becomes sufficiently short, the sintered particles are transformed into a solid compact covered with a thick passivation layer of Zn(ac)₂ (Figs. 2(c) and 2(f)).

Fig. 2 Morphological features of Zn particles before and after exposure to CH₃COOH/H₂O (10:1 by volume, pH 2.3) for < 1 min at room temperature

The Zn ink is mixed with polyvinylpyrrolidone (PVP) as a binder in isopropyl alcohol (IPA) (Zn:PVP:IPA = 30:1:10 by weight) to facilitate printing using a stencil mask while attachment of Au contacts enables measurement of resistance (Fig. 3(a)). Patterns generated with a 1 mm wide, 6 cm long, and ≈50 μm thick lines exhibit a decrease in resistance from >10 MOhm to <10 Ohm in < 1 min following treatment using <100 μL of H₂O:CH₃COOH (10:1 by volume, pH 2.3). Use of dilute HCl and HNO₃ has lead a decrease in resistance during the initial period following the removal of native oxide layer. Nevertheless, the resistance is increased again after drying due to the reformation of the oxide layer. Use of IPA in place of H₂O is not found to be effective in decreasing the resistance, due to a low rate of self-exchange of Zn²⁺/Zn and/or a low solubility of Zn(ac)₂ in IPA (Fig. 3(b)).

Fig. 3 (a) Schematic of the stencil mask and printing using zinc ink using PVP and Au contact arrangement; (b) Change in resistance after treatment with CH₃COOH, HCl, and HNO₃ (pH 2.3).

A near-field communication (NFC) device is screen printed using the Zn ink (800 μm line width). A flexible sheet of biodegradable (poly lactic-co-glycolic acid (PLGA)) prepared by drop casting of PLGA (20 w/v% in ethyl acetate) on a glass substrate served as the base (Fig. 4(a) (i)). It was screen printed with the Zn ink formulation (Zn:PVP:IPA = 3:0.1:1 by weight). The white printed lines correspond to the Zn ink in its high resistance state (Fig. 4(a) (ii)), which becomes conductive after treatment with a solution of water:CH₃COOH:PVP = 10:0.5:2 w/v% (Fig. 4(a) (iii)). Interconnecting the two terminals of the antenna, mounting an NFC chip and a light-emitting diode (LED), and drop-casting PLGA (20 w/v% in ethyl acetate, ≈100 μm) on top as an encapsulation layer complete the device (Fig. 4(a) (iv)). The validity of the circuit is verified by the glowing LED using a wireless power transfer through the RF antenna (Fig. 4(b)). The device is highly flexible and degradable (Fig. 4(c)). Upon immersion in water, the device remains functional for several hours due to the slow degradation of the 100 μm thick PLGA coating (Fig. 4(d)). Degradation of Zn is accompanied with the evolution of hydrogen (Fig. 4(e)) after several days.

Fig. 4 (a) Schematic illustration of the fabrication process of NFC device with Zn ink; (b) flexibility of the fabricated NFC device; (c) LED operated by wireless power transfer through the RF antenna; (d, e) stability of the device: (d) slow degradation of PLGA coating during the initial periods of immersion in water; and (e) degradation of Zn accompanied with the evolution of H₂.

The electrically conductivity, ability to print, and degradability of Zn ink  could find applications in environmentally sustainable electronic devices and resorbable biomedical implants.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Yoon Kyeung Le et al., Room Temperature Electrochemical Sintering of Zn Microparticles and Its Use in Printable Conducting Inks for Bioresorbable Electronics, Adv. Mater. 2017, 1702665, DOI: 10.1002/adma.201702665

Growth-accommodating implants – laying the foundation for a new paradigm of paediatric device development

Medical implants are often available in fixed size. The inability of such devices to accommodate with normal tissue growth remains a challenge, particularly in case of children. Hence, development of implant devices that could correct themselves in accordance with anatomical deformities and easily accommodate with tissue growth is highly warranted.

Researchers at the Harvard Medical School, USA and University College Dublin, Ireland have developed a growth-accommodating device that consists of a tubular braided sleeve and a biodegradable polymer core (Eric N. Feins et al., A growth-accommodating implant for paediatric applications, Nature Biomedical Engineering, 1 (2017) 818–825).

Fig. 1 (a) Schematic of a degradable polymer core (dark blue) placed inside a braided sleeve to control sleeve diameter, coupling inner polymer degradation to braided sleeve (and overall device) elongation; (b) A dissolvable spherical sucrose core (red) inside a nitinol biaxial braid acts as a degradable polymer surrogate. Upon immersion in water, the sucrose core gradually dissolves leading to a gradual decrease in the braided sleeve diameter along with a concomitant autonomous elongation; (c) Variation in length and diameter of the braided sleeve during core degradation.

A hydrophobic surface-eroding, biodegradable and biocompatible polymer poly(glycerol sebacate) (PGS) was used as the base material. The rationale behind the choice of PGS was justified based on its minimal swelling in water, ability to offer the requisite mechanical properties to resist compressive forces from the braided sleeve and capability to maintain structural integrity throughout degradation. To minimize the stretching of PGS to less than 5%, it was treated at 155 °C for 86 h in vacuum, which maximizes its cross-linking leading to the formation of extra-stiff PGS (ESPGS). The ESPGS was used as the polymer core while the braided sleeve was made of nitinol alloy.

The concept behind the development involves coupling the degradation of a surface-eroding polymer core to the braid length and overall device elongation. After implantation, once the polymer core starts to degrade, the braided sleeve begins to thin out and elongates in response to surrounding tissue growth (Figs. 1(a) and 1(b)), without the necessity for any additional interventions. Since the sleeve length and diameter of the braid are inversely related,  thinning of the sleeve results in its elongation (Fig. 1(c)).

The flexible nature of the braided sleeve and polymer contributes to the durability of the device and no evidence of fatigue failure of either the braided sleeve or the ESPGS core could be observed during in vivo studies using animal models.

By altering the number and thickness of braid fibres, the braid geometry and rate of degradation of the polymer core, it would be possible to modify the device elongation profile to match a wide spectrum of clinical applications.

Variability in the polymer erosion rate is the current limitation of the proposed device and achieving uniform degradation of polymer core will be the focus of future work.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Eric N. Feins et al., A growth-accommodating implant for paediatric applications, Nature Biomedical Engineering, 1 (2017) 818–825.