Editor | On 08, Jun 2018

Darrell Mann

We delve down into the micro-scale world for our patent of the month this month, and another lovely looking patent from the University of California, this time in San Diego. US 9,698,708 was granted on Independence Day. The inventor is Professor Joseph Wang of the self-named Wang Lab. Research in the Lab falls within the field of nano-bioelectronics, focusing mostly on the design of wearable biosensors and nano- and micromotors for drug delivery and microchip diagnostics. The Wang group’s work on nano- and micromotors have advanced the field significantly towards the dream of autonomous machines capable of navigating the circulation to deliver drugs or test for the presence of disease biomarkers (à la Fantastic Voyage). Their previous designs have demonstrated the feasibility of nano- and micromotors that rely on local chemical fuel or ultrasound or magnetic actuation to travel rapidly and carry cargo (unlike the vast majority of the field until recently focused on peroxide-driven machines). Excitingly, they have introduced the first water-driven micromotor, in which water reacts with aluminum to produce hydrogen microbubbles, which move efficiently in human serum. Recent contributions to fuel-free nanomotors include the development of magnetically guided ultrasound-powered nanowires capable of towing cargo and, now, thanks to this latest patent grant, of magnetically-powered flexible nanowire motors capable of high-speed propulsion. Here’s what the patent document background description has to say about the problem the invention solves:

Micro/nano-scale propulsion in fluids can be challenging due to the absence of the inertial forces exploited by biological organisms on macroscopic scales. The difficulties are summarized by E. M. Purcell’s “scallop theorem”, which states that a reciprocal motion (a deformation with time-reversal symmetry) cannot lead to any net propulsion at low Reynolds numbers. The Reynolds number, Re=.rho.UL/.mu., measures the relative importance of inertial to viscous forces, where .rho. and .mu. are the density and shear viscosity of the fluid, while U and L are the characteristic velocity and length scales of the self-propelling body. Natural microorganisms can inhabit a world where Re.about.10.sup.-5 (e.g., flagellated bacteria) to 10.sup.-2 (e.g., spermatozoa), and they achieve their propulsion by propagating traveling waves along their flagella (or rotating them) to break the time-reversibility requirement, and hence escape the constraints of the scallop theorem. Yet, there is a formidable challenge in engineering nanoscale, complex objects and systems capable of locomotion in fluids, which can be due to the combination of low Reynolds numbers and Brownian motion. Overcoming the challenges and limitations of micro/nano-scale propulsion in fluids can hold important implications.

At the macro-level, the basic contradiction relates to parallel needs to have a prime mover that is so small there is insufficient space for fuel. Here’s how we might map that problem on to the Contradiction Matrix:

And here’s a description of the main inventive steps contained in the solution:

…a nanostructure is configured as a nanowire diode formed of two or more segments [Principles 1, 15] of different electrically conducting materials [Principle 3] . A container contains a fluid surrounding the nanostructure [Principle 35a] . A mechanism produces an electric field in the fluid [Principle 28] , such that the electric field drives the nanostructure to locomote in the fluid…

… the system can include a mechanism for producing an electric field that includes electrodes and an AC signal source coupled to the electrodes. The system can include an electric field that can be an alternating electric field [Principle 19] , such as a uniform alternating electric field or a non-uniform [Principle 3] alternating electric field.

As the S-Field tool within TRIZ tells us, every system requires a source of energy (‘field’), what Professor Wang’s invention shows us is that the ‘field’ doesn’t have to be contained on-board the system (the classical-TRIZ community will probably admire the deployment of a field that is magnetic!) Plus, of course, it’s a very elegant and highly practical solution to an important problem. That’s one of the reasons we keep a close eye on the new work emerging from the Lab and its practical deployment. For example, they have shown that micromachines are capable of capturing and transporting cancer cells in biological media, which allow rapid, sensitive detection of circulating tumor cells in blood to improve the accuracy of cancer staging. In addition, their incorporation of tubular micromotors into a microchip enabled quantification of a specific protein, and has lead to on-chip immunoassays with no external power requirement.